Fixes

README.md

@@ -62,12 +62,12 @@ content selecting on `#minecraft:is_overworld` and friends.
 ## Install
 
 **Plugin (Paper/Purpur/Leaf/Canvas/Folia/Spigot):** drop the plugin jar into `plugins/` and start
-the server. On first boot Iris downloads the default `overworld` pack automatically.
+the server. On first boot Iris downloads the managed `overworld` and `underworld` beta packs automatically.
 
 **Mod (Fabric/Forge/NeoForge):** drop the mod jar into `mods/` and start the server. The jar is
 self-contained (core, SPI, and required Fabric API modules are bundled). On first boot Iris
-downloads the default `overworld` pack before the worldgen datapack is written, so the default
-pack is fully active immediately. Packs installed later register their custom dimension types
+prefetches the managed `overworld` and `underworld` beta packs and rebuilds the worldgen datapack;
+restart once if startup reports that registry-visible pack data was installed too late for that boot. Packs installed later register their custom dimension types
 (height ranges) and custom biomes through the forced datapack at server start - restart once after
 adding a pack so worlds get its full heights and biomes; worlds created before that restart run
 with fallback heights.

adapters/bukkit/plugin/src/main/java/art/arcane/iris/IrisBootstrap.java

@@ -1,9 +1,15 @@
 package art.arcane.iris;
 
+import art.arcane.iris.core.lifecycle.BukkitStartupPaths;
+import art.arcane.iris.core.lifecycle.WorldReplacementBootstrap;
+import art.arcane.iris.core.lifecycle.WorldReplacementBootstrapMarker;
 import art.arcane.iris.core.pack.DefaultPackBootstrapProvisioner;
 import art.arcane.iris.core.pack.DefaultPackBootstrapProvisioner.ProvisionResult;
 import io.papermc.paper.plugin.bootstrap.BootstrapContext;
 import io.papermc.paper.plugin.bootstrap.PluginBootstrap;
+import io.papermc.paper.plugin.lifecycle.event.LifecycleEvent;
+import io.papermc.paper.plugin.lifecycle.event.types.LifecycleEventType;
+import io.papermc.paper.plugin.lifecycle.event.types.LifecycleEvents;
 
 import java.io.IOException;
 import java.nio.file.Path;
@@ -12,16 +18,75 @@ import java.nio.file.Path;
 public final class IrisBootstrap implements PluginBootstrap {
     @Override
     public void bootstrap(BootstrapContext context) {
-        ProvisionResult provisioned = provision(context);
-        Path datapackRoot = provisioned.datapackRoot();
-        context.getLogger().info("Iris startup datapack is {} at {}", provisioned.status(), datapackRoot);
+        WorldReplacementBootstrapMarker.markBootstrapped();
+        try {
+            BukkitStartupPaths startupPaths = BukkitStartupPaths.resolveCurrent();
+            reconcilePendingWorldReplacements(context, startupPaths);
+            ProvisionResult provisioned = provision(context, startupPaths);
+            Path datapackRoot = provisioned.datapackRoot();
+            context.getLogger().info("Iris startup datapack is {} at {}", provisioned.status(), datapackRoot);
+        } catch (Throwable failure) {
+            armStartupFailure(context, failure);
+        }
     }
 
-    private static ProvisionResult provision(BootstrapContext context) {
+    private static void reconcilePendingWorldReplacements(
+            BootstrapContext context,
+            BukkitStartupPaths startupPaths
+    ) {
+        try {
+            WorldReplacementBootstrap.ReconcileResult result = WorldReplacementBootstrap.reconcile(
+                    context.getDataDirectory(),
+                    startupPaths.levelRoot(),
+                    startupPaths.bukkitConfiguration(),
+                    message -> context.getLogger().info(message)
+            );
+            if (result.transactions() > 0) {
+                context.getLogger().info(
+                        "Reconciled {} pending Iris world replacement(s): {} published, {} rolled back, {} retained",
+                        result.transactions(),
+                        result.published(),
+                        result.rolledBack(),
+                        result.retained()
+                );
+            }
+        } catch (IOException failure) {
+            throw new IllegalStateException(
+                    "Unable to reconcile pending Iris world replacements before registry bootstrap",
+                    failure
+            );
+        }
+    }
+
+    static void armStartupFailure(BootstrapContext context, Throwable failure) {
+        armStartupFailure(context, failure, LifecycleEvents.DATAPACK_DISCOVERY);
+    }
+
+    static <E extends LifecycleEvent> void armStartupFailure(
+            BootstrapContext context,
+            Throwable failure,
+            LifecycleEventType<? super BootstrapContext, ? extends E, ?> eventType
+    ) {
+        context.getLifecycleManager().registerEventHandler(eventType, event -> {
+            throw new IllegalStateException("Iris bootstrap did not establish safe world-generation state.", failure);
+        });
+        try {
+            context.getLogger().error(
+                    "Iris bootstrap failed; registry and world startup will be stopped at datapack discovery.",
+                    failure
+            );
+        } catch (Throwable loggingFailure) {
+            failure.addSuppressed(loggingFailure);
+            failure.printStackTrace(System.err);
+        }
+    }
+
+    private static ProvisionResult provision(BootstrapContext context, BukkitStartupPaths startupPaths) {
         try {
             return DefaultPackBootstrapProvisioner.provision(
                     context.getDataDirectory(),
-                    message -> context.getLogger().info(message)
+                    message -> context.getLogger().info(message),
+                    startupPaths
             );
         } catch (IOException e) {
             throw new IllegalStateException("Unable to provision the Iris startup datapack", e);

adapters/bukkit/plugin/src/main/java/art/arcane/iris/core/PendingWorldReplacementManager.java

@@ -7,6 +7,7 @@ import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.GeneratorReplacem
 import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.WorldGeneratorSnapshot;
 import art.arcane.iris.core.lifecycle.LifecycleOperationCoordinator;
 import art.arcane.iris.core.lifecycle.WorldReplacementBootstrap;
+import art.arcane.iris.core.lifecycle.WorldReplacementBootstrapMarker;
 import art.arcane.iris.core.lifecycle.WorldReplacementFilesystem;
 import art.arcane.iris.core.lifecycle.WorldReplacementFilesystem.ReplacementPaths;
 import art.arcane.iris.core.lifecycle.WorldReplacementJournal;
@@ -68,7 +69,7 @@ public final class PendingWorldReplacementManager implements Listener {
         if (worldKey == null) {
             throw new IllegalArgumentException("World identifier is invalid: " + requestedName);
         }
-        ExactWorldSlotPathPolicy.resolve(IrisWorldStorage.levelRoot().toPath(), worldKey);
+        ExactWorldSlotPathPolicy.resolve(IrisWorldStorage.levelRoot().toPath(), toWorldSlotKey(worldKey));
         return worldKey;
     }
 
@@ -80,8 +81,12 @@ public final class PendingWorldReplacementManager implements Listener {
     ) throws IOException {
         VolmitSender requiredSender = Objects.requireNonNull(sender, "sender");
         NamespacedKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
+        WorldSlotKey requiredWorldSlotKey = toWorldSlotKey(requiredWorldKey);
         IrisDimension requiredDimension = Objects.requireNonNull(dimension, "dimension");
         IrisStartupValidation.requireWorldCreationReady();
+        if (!WorldReplacementBootstrapMarker.wasBootstrappedThisProcess()) {
+            throw new IOException("Exact world replacement requires a full Paper-family startup bootstrap.");
+        }
         PackValidationRegistry.requireLoadable(requiredDimension.getLoader().getDataFolder().getName());
         LifecycleOperationCoordinator coordinator = LifecycleOperationCoordinator.get();
         try (LifecycleOperationCoordinator.Lease ignored = coordinator.acquire(
@@ -89,13 +94,13 @@ public final class PendingWorldReplacementManager implements Listener {
                 LifecycleOperationCoordinator.OperationKind.WORLD_REPLACE,
                 requiredWorldKey.toString()
         )) {
-            if (findTransaction(requiredWorldKey) != null) {
+            if (findTransaction(requiredWorldSlotKey) != null) {
                 throw new IOException("A replacement is already pending for " + requiredWorldKey + ".");
             }
-            ExactWorldSlotPathPolicy.Target target = prepareTarget(requiredWorldKey);
+            ExactWorldSlotPathPolicy.Target target = prepareTarget(requiredWorldSlotKey);
             requireCompatibleEnvironment(target.slotKind(), requiredDimension.getEnvironment());
             long effectiveSeed = resolveEffectiveSeed(target.slotKind(), seed);
-            String worldName = WorldReplacementJournal.logicalWorldName(target.levelRoot(), requiredWorldKey);
+            String worldName = WorldReplacementJournal.logicalWorldName(target.levelRoot(), requiredWorldSlotKey);
             DatapackInstallResult datapacks = ServerConfigurator.installDataPacksIfChanged(true);
             if (!datapacks.succeeded()) {
                 throw new IOException("Iris could not compile the dimension datapacks.");
@@ -103,7 +108,8 @@ public final class PendingWorldReplacementManager implements Listener {
 
             UUID transactionId = UUID.randomUUID();
             ReplacementPaths paths = WorldReplacementFilesystem.paths(target, transactionId);
-            boolean targetPresent = Files.exists(paths.target(), LinkOption.NOFOLLOW_LINKS);
+            WorldReplacementFilesystem.requireExistingTarget(paths);
+            boolean targetPresent = true;
             WorldGeneratorSnapshot originalConfiguration = BukkitWorldConfiguration.snapshot(
                     ServerProperties.BUKKIT_YML,
                     worldName
@@ -127,7 +133,7 @@ public final class PendingWorldReplacementManager implements Listener {
                 String packFingerprint = WorldReplacementFilesystem.fingerprintPack(stagedPack.toPath());
                 transaction = new Transaction(
                         transactionId,
-                        requiredWorldKey,
+                        requiredWorldSlotKey,
                         worldName,
                         target.levelRoot(),
                         installed.getLoadKey(),
@@ -163,7 +169,7 @@ public final class PendingWorldReplacementManager implements Listener {
             } catch (Throwable failure) {
                 if (configurationApplied && transaction != null) {
                     try {
-                        WorldGeneratorSnapshot replacement = replacementSnapshot(transaction);
+                        WorldGeneratorSnapshot replacement = WorldReplacementBootstrap.replacementSnapshot(transaction);
                         if (!BukkitWorldConfiguration.restoreIfMatching(
                                 ServerProperties.BUKKIT_YML,
                                 worldName,
@@ -236,7 +242,7 @@ public final class PendingWorldReplacementManager implements Listener {
                 if (transaction.phase() == Phase.CLEANUP_PENDING) {
                     scheduleCommittedCleanup(transaction);
                 } else if (transaction.phase() == Phase.PUBLISHED) {
-                    WorldIdentity.resolve(transaction.worldKey())
+                    WorldIdentity.resolve(toNamespacedKey(transaction.worldKey()))
                             .ifPresent(world -> verifyPublishedWorld(world, transaction));
                 }
             }
@@ -266,7 +272,7 @@ public final class PendingWorldReplacementManager implements Listener {
 
     private void verifyPublishedWorld(World world, Transaction transaction) {
         try {
-            if (!transaction.worldKey().equals(WorldIdentity.key(world))) {
+            if (!transaction.worldKey().equals(toWorldSlotKey(WorldIdentity.key(world)))) {
                 throw new IOException("Loaded world identity does not match the replacement journal.");
             }
             if (!IrisToolbelt.isIrisWorld(world)) {
@@ -295,6 +301,13 @@ public final class PendingWorldReplacementManager implements Listener {
             if (!transaction.packFingerprint().equals(fingerprint)) {
                 throw new IOException("The replacement pack changed before runtime verification.");
             }
+            WorldGeneratorSnapshot configured = BukkitWorldConfiguration.snapshot(
+                    ServerProperties.BUKKIT_YML,
+                    transaction.worldName()
+            );
+            if (!configured.matchesGeneratorAndSeed(WorldReplacementBootstrap.replacementSnapshot(transaction))) {
+                throw new IOException("bukkit.yml changed before the replacement could be committed.");
+            }
         } catch (Throwable failure) {
             initiateRollback(transaction, failure);
             return;
@@ -317,13 +330,14 @@ public final class PendingWorldReplacementManager implements Listener {
         try {
             Transaction rollback = transaction.withPhase(Phase.ROLLBACK_PENDING);
             writeTransaction(rollback);
-            ServerConfigurator.restart("An Iris world replacement failed verification and will be rolled back.");
         } catch (Throwable rollbackFailure) {
             failure.addSuppressed(rollbackFailure);
             IrisStartupValidation.markPacksInvalid(List.of(
                     "Iris could not arm rollback for " + transaction.worldKey() + ": " + detail(rollbackFailure)));
             Iris.reportError("Failed to arm Iris world replacement rollback for "
                     + transaction.worldKey() + ". Stop the server and preserve the replacement artifacts.", rollbackFailure);
+        } finally {
+            ServerConfigurator.restart("An Iris world replacement failed verification and requires a cold restart.");
         }
     }
 
@@ -348,14 +362,14 @@ public final class PendingWorldReplacementManager implements Listener {
             throw new RestartBoundaryRequired("The world-storage transaction has not reached its cold bootstrap.");
         }
         if (transaction.phase() == Phase.PREPARED) {
-            if (current.matchesGeneratorAndSeed(replacement)) {
-                writeTransaction(transaction.withPhase(Phase.ARMED));
-                throw new RestartBoundaryRequired("The replacement was armed before its journal phase was durable.");
-            } else if (current.matchesGeneratorAndSeed(transaction.originalConfiguration())) {
+            if (current.matchesGeneratorAndSeed(transaction.originalConfiguration())) {
                 WorldReplacementFilesystem.discardStage(paths);
                 deleteJournal(transaction.id());
                 Iris.warn("Cancelled incomplete Iris world replacement for " + transaction.worldKey() + ".");
                 return;
+            } else if (current.matchesGeneratorAndSeed(replacement)) {
+                writeTransaction(transaction.withPhase(Phase.ARMED));
+                throw new RestartBoundaryRequired("The replacement was armed before its journal phase was durable.");
             } else {
                 throw new IOException("bukkit.yml does not match the prepared replacement or its original state.");
             }
@@ -376,16 +390,38 @@ public final class PendingWorldReplacementManager implements Listener {
         if (!cleanupInFlight.add(transaction.id())) {
             return;
         }
-        J.a(() -> cleanupCommittedReplacement(transaction));
+        try {
+            J.a(() -> cleanupCommittedReplacement(transaction));
+        } catch (Throwable failure) {
+            cleanupInFlight.remove(transaction.id());
+            Iris.reportError("Could not schedule retained-backup cleanup for " + transaction.worldKey()
+                    + "; cleanup will retry without rolling back the verified world.", failure);
+        }
     }
 
     private void cleanupCommittedReplacement(Transaction transaction) {
         try {
-            ExactWorldSlotPathPolicy.Target target = resolveTransactionTarget(transaction);
-            ReplacementPaths paths = WorldReplacementFilesystem.paths(target, transaction.id());
-            WorldReplacementFilesystem.cleanupBackup(paths);
-            deleteJournal(transaction.id());
-            Iris.success("Removed the retained backup for " + transaction.worldKey() + ".");
+            synchronized (this) {
+                Transaction current = findTransaction(transaction.worldKey());
+                if (current == null
+                        || !current.id().equals(transaction.id())
+                        || current.phase() != Phase.CLEANUP_PENDING) {
+                    return;
+                }
+                WorldGeneratorSnapshot configured = BukkitWorldConfiguration.snapshot(
+                        ServerProperties.BUKKIT_YML,
+                        current.worldName()
+                );
+                if (!configured.matchesGeneratorAndSeed(WorldReplacementBootstrap.replacementSnapshot(current))) {
+                    throw new IOException("bukkit.yml changed before the retained backup could be removed.");
+                }
+                ExactWorldSlotPathPolicy.Target target = resolveTransactionTarget(current);
+                ReplacementPaths paths = WorldReplacementFilesystem.paths(target, current.id());
+                WorldReplacementFilesystem.validateCommittedTarget(paths, current.packFingerprint());
+                WorldReplacementFilesystem.cleanupBackup(paths);
+                deleteJournal(current.id());
+                Iris.success("Removed the retained backup for " + current.worldKey() + ".");
+            }
         } catch (Throwable failure) {
             Iris.reportError("Could not clean the retained backup for " + transaction.worldKey()
                     + "; cleanup will retry without rolling back the verified world.", failure);
@@ -396,7 +432,7 @@ public final class PendingWorldReplacementManager implements Listener {
         }
     }
 
-    private ExactWorldSlotPathPolicy.Target prepareTarget(NamespacedKey worldKey) throws IOException {
+    private ExactWorldSlotPathPolicy.Target prepareTarget(WorldSlotKey worldKey) throws IOException {
         Path levelRoot = IrisWorldStorage.levelRoot().toPath();
         ExactWorldSlotPathPolicy.Target target = ExactWorldSlotPathPolicy.resolve(levelRoot, worldKey);
         Path dimensions = target.levelRoot().resolve("dimensions");
@@ -416,6 +452,10 @@ public final class PendingWorldReplacementManager implements Listener {
     }
 
     private Transaction findTransaction(NamespacedKey worldKey) throws IOException {
+        return findTransaction(toWorldSlotKey(worldKey));
+    }
+
+    private Transaction findTransaction(WorldSlotKey worldKey) throws IOException {
         for (Transaction transaction : loadTransactions()) {
             if (transaction.worldKey().equals(worldKey)) {
                 return transaction;
@@ -425,159 +465,23 @@ public final class PendingWorldReplacementManager implements Listener {
     }
 
     private List<Transaction> loadTransactions() throws IOException {
-        Path directory = journalDirectory(false);
-        if (directory == null) {
-            return List.of();
-        }
-        ArrayList<Transaction> transactions = new ArrayList<>();
-        try (DirectoryStream<Path> files = Files.newDirectoryStream(directory, "*" + JOURNAL_SUFFIX)) {
-            for (Path file : files) {
-                if (Files.isSymbolicLink(file) || !Files.isRegularFile(file, LinkOption.NOFOLLOW_LINKS)) {
-                    throw new IOException("Replacement journal entry is unsafe: " + file);
-                }
-                transactions.add(readTransaction(file));
-            }
-        }
-        transactions.sort(Comparator.comparing(transaction -> transaction.id().toString()));
-        return List.copyOf(transactions);
-    }
-
-    private Transaction readTransaction(Path file) throws IOException {
-        Properties properties = new Properties();
-        try (InputStream input = Files.newInputStream(file)) {
-            properties.load(input);
-        }
-        UUID id = UUID.fromString(required(properties, "id"));
-        if (!file.getFileName().toString().equals(id + JOURNAL_SUFFIX)) {
-            throw new IOException("Replacement journal filename does not match its transaction id.");
-        }
-        NamespacedKey worldKey = NamespacedKey.fromString(required(properties, "worldKey"));
-        if (worldKey == null) {
-            throw new IOException("Replacement journal contains an invalid world key.");
-        }
-        ExactWorldSlotPathPolicy.resolve(IrisWorldStorage.levelRoot().toPath(), worldKey);
-        String dimension = required(properties, "dimension");
-        if (!safeDimension(dimension)) {
-            throw new IOException("Replacement journal contains an invalid dimension key.");
-        }
-        long seed = parseLong(properties, "seed");
-        String packFingerprint = required(properties, "packFingerprint");
-        if (!packFingerprint.matches("[0-9a-f]{64}")) {
-            throw new IOException("Replacement journal contains an invalid pack fingerprint.");
-        }
-        WorldGeneratorSnapshot original = readSnapshot(properties, "original.");
-        boolean originalTargetPresent = parseBoolean(properties, "originalTargetPresent");
-        Phase phase;
-        try {
-            phase = Phase.valueOf(required(properties, "phase"));
-        } catch (IllegalArgumentException failure) {
-            throw new IOException("Replacement journal contains an invalid phase.", failure);
-        }
-        return new Transaction(
-                id,
-                worldKey,
-                dimension,
-                seed,
-                packFingerprint,
-                original,
-                originalTargetPresent,
-                phase
-        );
+        return WorldReplacementJournal.load(dataDirectory(), IrisWorldStorage.levelRoot().toPath());
     }
 
     private void writeTransaction(Transaction transaction) throws IOException {
-        Path directory = Objects.requireNonNull(journalDirectory(true));
-        Path target = directory.resolve(transaction.id() + JOURNAL_SUFFIX);
-        Properties properties = new Properties();
-        properties.setProperty("id", transaction.id().toString());
-        properties.setProperty("worldKey", transaction.worldKey().toString());
-        properties.setProperty("dimension", transaction.dimension());
-        properties.setProperty("seed", Long.toString(transaction.seed()));
-        properties.setProperty("packFingerprint", transaction.packFingerprint());
-        properties.setProperty("originalTargetPresent", Boolean.toString(transaction.originalTargetPresent()));
-        properties.setProperty("phase", transaction.phase().name());
-        writeSnapshot(properties, "original.", transaction.originalConfiguration());
-        ByteArrayOutputStream output = new ByteArrayOutputStream();
-        properties.store(output, null);
-        writeAtomic(target, output.toByteArray());
+        WorldReplacementJournal.write(dataDirectory(), transaction);
     }
 
     private void deleteJournal(UUID id) throws IOException {
-        Path directory = journalDirectory(false);
-        if (directory == null) {
-            return;
-        }
-        Files.deleteIfExists(directory.resolve(id + JOURNAL_SUFFIX));
-        forceDirectory(directory);
+        WorldReplacementJournal.delete(dataDirectory(), id);
     }
 
-    private Path journalDirectory(boolean create) throws IOException {
-        Path directory = plugin.getDataFile(JOURNAL_DIRECTORY).toPath().toAbsolutePath().normalize();
-        if (!Files.exists(directory, LinkOption.NOFOLLOW_LINKS)) {
-            if (!create) {
-                return null;
-            }
-            Files.createDirectories(directory);
-        }
-        if (Files.isSymbolicLink(directory) || !Files.isDirectory(directory, LinkOption.NOFOLLOW_LINKS)) {
-            throw new IOException("Replacement journal storage is unsafe: " + directory);
-        }
-        return directory;
+    private Path dataDirectory() {
+        return plugin.getDataFolder().toPath().toAbsolutePath().normalize();
     }
 
-    private static void writeAtomic(Path target, byte[] content) throws IOException {
-        Path parent = Objects.requireNonNull(target.getParent(), "journal parent");
-        Path temporary = parent.resolve("." + target.getFileName() + ".tmp-" + UUID.randomUUID());
-        try {
-            try (FileChannel channel = FileChannel.open(
-                    temporary,
-                    StandardOpenOption.CREATE_NEW,
-                    StandardOpenOption.WRITE
-            )) {
-                ByteBuffer buffer = ByteBuffer.wrap(content);
-                while (buffer.hasRemaining()) {
-                    channel.write(buffer);
-                }
-                channel.force(true);
-            }
-            try {
-                Files.move(temporary, target, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
-            } catch (AtomicMoveNotSupportedException failure) {
-                Files.move(temporary, target, StandardCopyOption.REPLACE_EXISTING);
-            }
-            forceDirectory(parent);
-        } finally {
-            Files.deleteIfExists(temporary);
-        }
-    }
-
-    private static void forceDirectory(Path directory) throws IOException {
-        try (FileChannel channel = FileChannel.open(directory, StandardOpenOption.READ)) {
-            channel.force(true);
-        }
-    }
-
-    private static ReplacementPaths replacementPaths(
-            ExactWorldSlotPathPolicy.Target target,
-            UUID id
-    ) {
-        String artifactBase = ".iris-replace-" + target.worldKey().getKey() + "-" + id;
-        return new ReplacementPaths(
-                target.worldDirectory(),
-                target.namespaceRoot().resolve(artifactBase + ".stage"),
-                target.namespaceRoot().resolve(artifactBase + ".backup")
-        );
-    }
-
-    private static WorldGeneratorSnapshot replacementSnapshot(Transaction transaction) {
-        return new WorldGeneratorSnapshot(
-                true,
-                true,
-                true,
-                "Iris:" + transaction.dimension(),
-                true,
-                transaction.seed()
-        );
+    private ExactWorldSlotPathPolicy.Target resolveTransactionTarget(Transaction transaction) throws IOException {
+        return WorldReplacementJournal.resolveTarget(transaction, IrisWorldStorage.levelRoot().toPath());
     }
 
     private static boolean configurationMatches(WorldGeneratorSnapshot expected, String worldName) {
@@ -633,91 +537,27 @@ public final class PendingWorldReplacementManager implements Listener {
         }
     }
 
-    private static World.Environment expectedEnvironment(NamespacedKey worldKey) {
-        if (NamespacedKey.minecraft("overworld").equals(worldKey)) {
+    private static World.Environment expectedEnvironment(WorldSlotKey worldKey) {
+        if (WorldSlotKey.minecraft("overworld").equals(worldKey)) {
             return World.Environment.NORMAL;
         }
-        if (NamespacedKey.minecraft("the_nether").equals(worldKey)) {
+        if (WorldSlotKey.minecraft("the_nether").equals(worldKey)) {
             return World.Environment.NETHER;
         }
-        if (NamespacedKey.minecraft("the_end").equals(worldKey)) {
+        if (WorldSlotKey.minecraft("the_end").equals(worldKey)) {
             return World.Environment.THE_END;
         }
         return null;
     }
 
-    private static void writeSnapshot(Properties properties, String prefix, WorldGeneratorSnapshot snapshot) {
-        properties.setProperty(prefix + "worldsSectionPresent", Boolean.toString(snapshot.worldsSectionPresent()));
-        properties.setProperty(prefix + "worldSectionPresent", Boolean.toString(snapshot.worldSectionPresent()));
-        properties.setProperty(prefix + "generatorPresent", Boolean.toString(snapshot.generatorPresent()));
-        if (snapshot.generatorPresent()) {
-            properties.setProperty(prefix + "generator", snapshot.generator());
-        }
-        properties.setProperty(prefix + "seedPresent", Boolean.toString(snapshot.seedPresent()));
-        if (snapshot.seedPresent()) {
-            properties.setProperty(prefix + "seed", Long.toString(snapshot.seed()));
-        }
+    private static WorldSlotKey toWorldSlotKey(NamespacedKey worldKey) {
+        NamespacedKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
+        return new WorldSlotKey(requiredWorldKey.getNamespace(), requiredWorldKey.getKey());
     }
 
-    private static WorldGeneratorSnapshot readSnapshot(Properties properties, String prefix) throws IOException {
-        boolean worldsPresent = parseBoolean(properties, prefix + "worldsSectionPresent");
-        boolean worldPresent = parseBoolean(properties, prefix + "worldSectionPresent");
-        boolean generatorPresent = parseBoolean(properties, prefix + "generatorPresent");
-        String generator = generatorPresent ? required(properties, prefix + "generator") : null;
-        boolean seedPresent = parseBoolean(properties, prefix + "seedPresent");
-        Long seed = seedPresent ? parseLong(properties, prefix + "seed") : null;
-        try {
-            return new WorldGeneratorSnapshot(
-                    worldsPresent,
-                    worldPresent,
-                    generatorPresent,
-                    generator,
-                    seedPresent,
-                    seed
-            );
-        } catch (IllegalArgumentException failure) {
-            throw new IOException("Replacement journal contains an invalid configuration snapshot.", failure);
-        }
-    }
-
-    private static boolean safeDimension(String value) {
-        if (value.isEmpty() || value.length() > 256 || value.startsWith(".") || value.contains("..")) {
-            return false;
-        }
-        String[] segments = value.split("/", -1);
-        if (segments.length > 16) {
-            return false;
-        }
-        for (String segment : segments) {
-            if (segment.isEmpty() || !segment.matches("[A-Za-z0-9_-]+")) {
-                return false;
-            }
-        }
-        return true;
-    }
-
-    private static String required(Properties properties, String key) throws IOException {
-        String value = properties.getProperty(key);
-        if (value == null || value.isBlank()) {
-            throw new IOException("Replacement journal is missing " + key + ".");
-        }
-        return value.trim();
-    }
-
-    private static boolean parseBoolean(Properties properties, String key) throws IOException {
-        String value = required(properties, key);
-        if (!"true".equals(value) && !"false".equals(value)) {
-            throw new IOException("Replacement journal contains an invalid boolean for " + key + ".");
-        }
-        return Boolean.parseBoolean(value);
-    }
-
-    private static long parseLong(Properties properties, String key) throws IOException {
-        try {
-            return Long.parseLong(required(properties, key));
-        } catch (NumberFormatException failure) {
-            throw new IOException("Replacement journal contains an invalid integer for " + key + ".", failure);
-        }
+    private static NamespacedKey toNamespacedKey(WorldSlotKey worldKey) {
+        WorldSlotKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
+        return new NamespacedKey(requiredWorldKey.namespace(), requiredWorldKey.key());
     }
 
     private static String detail(Throwable failure) {
@@ -740,50 +580,12 @@ public final class PendingWorldReplacementManager implements Listener {
         }
     }
 
-    private record Transaction(
-            UUID id,
-            NamespacedKey worldKey,
-            String dimension,
-            long seed,
-            String packFingerprint,
-            WorldGeneratorSnapshot originalConfiguration,
-            boolean originalTargetPresent,
-            Phase phase
-    ) {
-        private Transaction {
-            Objects.requireNonNull(id, "id");
-            Objects.requireNonNull(worldKey, "worldKey");
-            Objects.requireNonNull(dimension, "dimension");
-            Objects.requireNonNull(packFingerprint, "packFingerprint");
-            Objects.requireNonNull(originalConfiguration, "originalConfiguration");
-            Objects.requireNonNull(phase, "phase");
-        }
-
-        private String worldName() {
-            return IrisWorldStorage.logicalName(worldKey);
-        }
-
-        private Transaction withPhase(Phase nextPhase) {
-            return new Transaction(
-                    id,
-                    worldKey,
-                    dimension,
-                    seed,
-                    packFingerprint,
-                    originalConfiguration,
-                    originalTargetPresent,
-                    nextPhase
-            );
-        }
-    }
-
-    private enum Phase {
-        PREPARED,
-        ARMED,
-        PUBLISHED,
-        ROLLBACK_PENDING
-    }
-
     private record VanillaLevelContext(long seed, boolean allowNether, boolean allowEnd) {
     }
+
+    private static final class RestartBoundaryRequired extends IOException {
+        private RestartBoundaryRequired(String message) {
+            super(message);
+        }
+    }
 }

adapters/bukkit/plugin/src/main/java/art/arcane/iris/core/commands/CommandIris.java

@@ -710,9 +710,9 @@ public class CommandIris implements DirectorExecutor {
             @Param(name = "overwrite", description = "Whether or not to overwrite the pack with the downloaded one", descriptionKey = "iris.director.commandiris.param.whether_not_overwrite_pack_with_downloaded_one", aliases = "force", defaultValue = "false")
             boolean overwrite
     ) {
-        if (PackDownloader.isDefaultOverworld(pack)) {
+        if (PackDownloader.isManagedBetaPack(pack)) {
             sender().sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_IRIS_DOWNLOADING_PACK_BETA_RELEASE, MessageArgument.untrusted("pack", pack), MessageArgument.trusted("value", overwrite ? IrisLanguage.text(RuntimeUiMessages.DOWNLOAD_OVERWRITE_SUFFIX) : "")));
-            Iris.service(StudioSVC.class).downloadDefaultOverworld(sender(), overwrite);
+            Iris.service(StudioSVC.class).downloadManagedBeta(sender(), pack, overwrite);
         } else {
             sender().sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_IRIS_DOWNLOADING_PACK, MessageArgument.untrusted("pack", pack), MessageArgument.untrusted("branch", branch), MessageArgument.trusted("value", overwrite ? IrisLanguage.text(RuntimeUiMessages.DOWNLOAD_OVERWRITE_SUFFIX) : "")));
             Iris.service(StudioSVC.class).downloadSearch(sender(), "IrisDimensions/" + pack + "/" + branch, overwrite);

adapters/bukkit/plugin/src/test/java/art/arcane/iris/IrisBootstrapFailureContractTest.java

@@ -0,0 +1,65 @@
+package art.arcane.iris;
+
+import io.papermc.paper.plugin.bootstrap.BootstrapContext;
+import io.papermc.paper.plugin.lifecycle.event.LifecycleEvent;
+import io.papermc.paper.plugin.lifecycle.event.LifecycleEventManager;
+import io.papermc.paper.plugin.lifecycle.event.handler.LifecycleEventHandler;
+import io.papermc.paper.plugin.lifecycle.event.handler.configuration.LifecycleEventHandlerConfiguration;
+import io.papermc.paper.plugin.lifecycle.event.types.LifecycleEventType;
+import net.kyori.adventure.text.logger.slf4j.ComponentLogger;
+import org.junit.Test;
+
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisBootstrapFailureContractTest {
+    @Test
+    public void bootstrapFailureAbortsDatapackDiscoveryInsteadOfDisablingOnlyIris() {
+        BootstrapContext context = mock(BootstrapContext.class);
+        ComponentLogger logger = mock(ComponentLogger.class);
+        CapturingLifecycleManager manager = new CapturingLifecycleManager();
+        IllegalStateException cause = new IllegalStateException("unsafe transaction");
+        LifecycleEventType<BootstrapContext, LifecycleEvent, ?> eventType = lifecycleEventType();
+        when(context.getLogger()).thenReturn(logger);
+        when(context.getLifecycleManager()).thenReturn(manager);
+
+        IrisBootstrap.armStartupFailure(context, cause, eventType);
+
+        assertSame(eventType, manager.eventType);
+        IllegalStateException failure = assertThrows(IllegalStateException.class, manager::runCapturedHandler);
+        assertSame(cause, failure.getCause());
+    }
+
+    @SuppressWarnings("unchecked")
+    private static LifecycleEventType<BootstrapContext, LifecycleEvent, ?> lifecycleEventType() {
+        return (LifecycleEventType<BootstrapContext, LifecycleEvent, ?>) mock(LifecycleEventType.class);
+    }
+
+    private static final class CapturingLifecycleManager implements LifecycleEventManager<BootstrapContext> {
+        private LifecycleEventType<?, ?, ?> eventType;
+        private LifecycleEventHandler<?> handler;
+
+        @Override
+        public <E extends LifecycleEvent> void registerEventHandler(
+                LifecycleEventType<? super BootstrapContext, ? extends E, ?> eventType,
+                LifecycleEventHandler<? super E> eventHandler
+        ) {
+            this.eventType = eventType;
+            this.handler = eventHandler;
+        }
+
+        @Override
+        public void registerEventHandler(
+                LifecycleEventHandlerConfiguration<? super BootstrapContext> handlerConfiguration
+        ) {
+            throw new AssertionError("The direct handler overload must be used.");
+        }
+
+        @SuppressWarnings({"rawtypes", "unchecked"})
+        private void runCapturedHandler() {
+            ((LifecycleEventHandler) handler).run(null);
+        }
+    }
+}

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedPackInstaller.java

@@ -42,12 +42,14 @@ public final class ModdedPackInstaller {
     }
 
     public static boolean install(Path configDir, String pack, String branch,
-                                  boolean forceOverwrite, Consumer<String> feedback) {
+                                  boolean forceOverwrite, boolean refreshDatapack,
+                                  Consumer<String> feedback) {
         if (pack == null || !PACK_NAME.matcher(pack).matches()) {
             feedback.accept(IrisLanguage.plain(PackDownloadMessages.INVALID_PACK_NAME, MessageArgument.untrusted("pack", String.valueOf(pack))));
             return false;
         }
-        if (branch == null || !BRANCH_NAME.matcher(branch).matches()) {
+        boolean managedBeta = PackDownloader.isManagedBetaPack(pack);
+        if (!acceptsBranch(pack, branch)) {
             feedback.accept(IrisLanguage.plain(PackDownloadMessages.INVALID_BRANCH_NAME, MessageArgument.untrusted("branch", String.valueOf(branch))));
             return false;
         }
@@ -56,8 +58,8 @@ public final class ModdedPackInstaller {
         synchronized (installLock) {
             File packs = configDir.resolve("irisworldgen").resolve("packs").toFile();
             try {
-                PackDownloader.PackInstallResult result = PackDownloader.isDefaultOverworld(pack)
-                        ? PackDownloader.downloadDefaultOverworld(packs, forceOverwrite, feedback)
+                PackDownloader.PackInstallResult result = managedBeta
+                        ? PackDownloader.downloadManagedBeta(packs, pack, forceOverwrite, feedback)
                         : PackDownloader.download(
                                 packs,
                                 "IrisDimensions/" + pack,
@@ -67,7 +69,7 @@ public final class ModdedPackInstaller {
                                 pack,
                                 feedback);
                 boolean installed = result != null;
-                if (result != null && result.changed()) {
+                if (shouldRefreshDatapack(result, refreshDatapack)) {
                     // Pack-install completion is one of the four forced-datapack regeneration triggers; every
                     // install call site already runs off the server thread, so regenerate inline here. A
                     // regeneration failure must never turn a successful install into a failed one.
@@ -82,7 +84,8 @@ public final class ModdedPackInstaller {
                 }
                 return installed;
             } catch (IOException error) {
-                LOGGER.error("Iris pack download failed for IrisDimensions/{} ({})", pack, branch, error);
+                String source = managedBeta ? "beta release" : "branch " + branch;
+                LOGGER.error("Iris pack download failed for IrisDimensions/{} ({})", pack, source, error);
                 feedback.accept(IrisLanguage.plain(
                         PackDownloadMessages.DOWNLOAD_FAILED,
                         MessageArgument.untrusted("type", error.getClass().getSimpleName()),
@@ -92,4 +95,13 @@ public final class ModdedPackInstaller {
             }
         }
     }
+
+    static boolean acceptsBranch(String pack, String branch) {
+        return PackDownloader.isManagedBetaPack(pack)
+                || (branch != null && BRANCH_NAME.matcher(branch).matches());
+    }
+
+    static boolean shouldRefreshDatapack(PackDownloader.PackInstallResult result, boolean refreshDatapack) {
+        return result != null && result.changed() && refreshDatapack;
+    }
 }

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedStartup.java

@@ -37,6 +37,7 @@ import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.attribute.BasicFileAttributes;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -253,20 +254,39 @@ public final class ModdedStartup {
             if (!config.autoDownloadDefaultPack()) {
                 return;
             }
-            String pack = config.defaultPack();
             Path configDir = ModdedEngineBootstrap.loader().configDir();
-            File packFolder = new File(ModdedPackCommands.packsRoot(), pack);
-            if (new File(packFolder, "dimensions/" + pack + ".json").isFile()) {
-                return;
-            }
-            String source = "master branch";
-            LOGGER.info("Iris default pack '{}' missing; downloading IrisDimensions/{} ({})", pack, pack, source);
-            boolean installed = ModdedPackInstaller.install(
-                    configDir, pack, "master", false,
-                    (String line) -> LOGGER.info("Iris: {}", line));
-            if (!installed) {
-                LOGGER.warn("Iris default pack '{}' could not be downloaded; install it with /iris download {}", pack, pack);
+            File packsRoot = ModdedPackCommands.packsRoot();
+            for (String pack : startupPacks(config.defaultPack())) {
+                ensureStartupPack(configDir, packsRoot, pack);
             }
         }
     }
+
+    static List<String> startupPacks(String configuredDefault) {
+        LinkedHashSet<String> packs = new LinkedHashSet<>(PackDownloader.managedBetaPacks());
+        if (configuredDefault != null && !configuredDefault.isBlank()) {
+            packs.add(configuredDefault);
+        }
+        return List.copyOf(packs);
+    }
+
+    private static void ensureStartupPack(Path configDir, File packsRoot, String pack) {
+        boolean present = PackDownloader.isManagedBetaPack(pack)
+                ? PackDownloader.isManagedBetaPackPresent(packsRoot, pack)
+                : PackDownloader.isPackPresent(packsRoot, pack);
+        if (present) {
+            return;
+        }
+        boolean managedBeta = PackDownloader.isManagedBetaPack(pack);
+        String branch = managedBeta ? "beta" : "master";
+        String source = managedBeta ? "beta release" : "master branch";
+        String role = managedBeta ? "managed beta pack" : "default pack";
+        LOGGER.info("Iris {} '{}' missing; downloading IrisDimensions/{} ({})", role, pack, pack, source);
+        boolean installed = ModdedPackInstaller.install(
+                configDir, pack, branch, false, false,
+                (String line) -> LOGGER.info("Iris: {}", line));
+        if (!installed) {
+            LOGGER.warn("Iris {} '{}' could not be downloaded; install it with /iris download {}", role, pack, pack);
+        }
+    }
 }

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/IrisModdedCommands.java

@@ -258,8 +258,8 @@ public final class IrisModdedCommands {
 
     static int download(CommandSourceStack source, String pack,
                         String branch, boolean forceOverwrite) {
-        boolean defaultOverworld = PackDownloader.isDefaultOverworld(pack);
-        String baseDownloadSource = defaultOverworld ? "beta release" : "branch " + branch;
+        boolean managedBeta = PackDownloader.isManagedBetaPack(pack);
+        String baseDownloadSource = managedBeta ? "beta release" : "branch " + branch;
         String downloadSource = forceOverwrite
                 ? baseDownloadSource + IrisLanguage.plain(RuntimeUiMessages.DOWNLOAD_OVERWRITE_SUFFIX)
                 : baseDownloadSource;
@@ -274,7 +274,7 @@ public final class IrisModdedCommands {
         }
         scheduler.async(() -> {
             boolean installed = ModdedPackInstaller.install(
-                    ModdedEngineBootstrap.loader().configDir(), pack, branch, forceOverwrite,
+                    ModdedEngineBootstrap.loader().configDir(), pack, branch, forceOverwrite, true,
                     (String message) -> scheduler.global(() -> ok(source, message)));
             if (installed) {
                 scheduler.global(() -> ok(source, IrisLanguage.plain(ModdedCommandMessages.IRIS_MODDED_COMMANDS_PACK_INSTALLED_ITS_EXACT_DIMENSION_TYPES_CUSTOM_BIOMES_JOIN_FORCED, MessageArgument.untrusted("pack", pack))));

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/ModdedStudioCommands.java

@@ -383,7 +383,7 @@ public final class ModdedStudioCommands {
             File packFolder = new File(ModdedPackCommands.packsRoot(), pack);
             if (!new File(packFolder, "dimensions/" + pack + ".json").isFile()) {
                 server.execute(() -> IrisModdedCommands.ok(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_STUDIO_COMMANDS_PACK_MISSING_DOWNLOADING_IRISDIMENSIONS, MessageArgument.untrusted("pack", pack), MessageArgument.untrusted("pack2", pack))));
-                boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false,
+                boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false, true,
                         (String line) -> server.execute(() -> IrisModdedCommands.ok(source, line)));
                 if (!installed || !new File(packFolder, "dimensions/" + pack + ".json").isFile()) {
                     server.execute(() -> IrisModdedCommands.fail(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_STUDIO_COMMANDS_PACK_COULD_NOT_BE_DOWNLOADED_CHECK_NAME_TRY_IRIS_DOWNLOAD, MessageArgument.untrusted("pack", pack), MessageArgument.untrusted("pack2", pack))));
@@ -588,7 +588,7 @@ public final class ModdedStudioCommands {
                 File templateFolder = new File(packsRoot, template);
                 if (!new File(templateFolder, "dimensions/" + template + ".json").isFile()) {
                     server.execute(() -> IrisModdedCommands.ok(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_STUDIO_COMMANDS_TEMPLATE_IS_NOT_INSTALLED_DOWNLOADING_IRISDIMENSIONS, MessageArgument.untrusted("template", template), MessageArgument.untrusted("template2", template))));
-                    boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), template, "master", false,
+                    boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), template, "master", false, true,
                             (String line) -> server.execute(() -> IrisModdedCommands.ok(source, line)));
                     if (!installed || !new File(templateFolder, "dimensions/" + template + ".json").isFile()) {
                         server.execute(() -> IrisModdedCommands.fail(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_STUDIO_COMMANDS_TEMPLATE_COULD_NOT_BE_DOWNLOADED_INSTALL_PACK_WITH_DIMENSIONS_JSON, MessageArgument.untrusted("template", template), MessageArgument.untrusted("template2", template))));

adapters/modded-common/src/main/java/art/arcane/iris/modded/command/ModdedWorldCommands.java

@@ -177,7 +177,7 @@ public final class ModdedWorldCommands {
         }
         IrisModdedCommands.ok(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_WORLD_COMMANDS_PACK_IS_NOT_INSTALLED_DOWNLOADING_IRISDIMENSIONS, MessageArgument.untrusted("pack", pack), MessageArgument.untrusted("pack2", pack)));
         Thread thread = new Thread(() -> {
-            boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false,
+            boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false, true,
                     (String line) -> server.execute(() -> IrisModdedCommands.ok(source, line)));
             server.execute(() -> {
                 if (!installed || !packFolder.isDirectory()) {
@@ -280,7 +280,7 @@ public final class ModdedWorldCommands {
         }
         IrisModdedCommands.ok(source, IrisLanguage.plain(ModdedCommandMessages.MODDED_WORLD_COMMANDS_PACK_IS_NOT_INSTALLED_DOWNLOADING_IRISDIMENSIONS_2, MessageArgument.untrusted("pack", pack), MessageArgument.untrusted("pack2", pack)));
         Thread thread = new Thread(() -> {
-            boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false,
+            boolean installed = ModdedPackInstaller.install(ModdedEngineBootstrap.loader().configDir(), pack, "master", false, true,
                     (String line) -> server.execute(() -> IrisModdedCommands.ok(source, line)));
             server.execute(() -> {
                 if (!installed || !packFolder.isDirectory()) {

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedPackInstallerTest.java

@@ -0,0 +1,52 @@
+/*
+ * Iris is a World Generator for Minecraft Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.modded;
+
+import art.arcane.iris.core.pack.PackDownloader;
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class ModdedPackInstallerTest {
+    @Test
+    public void managedBetaPacksIgnoreTheRequestedBranch() {
+        assertTrue(ModdedPackInstaller.acceptsBranch("overworld", null));
+        assertTrue(ModdedPackInstaller.acceptsBranch("underworld", "feature/arbitrary"));
+    }
+
+    @Test
+    public void nonManagedPacksStillRequireAValidBranch() {
+        assertTrue(ModdedPackInstaller.acceptsBranch("custom", "stable"));
+        assertFalse(ModdedPackInstaller.acceptsBranch("custom", null));
+        assertFalse(ModdedPackInstaller.acceptsBranch("custom", "feature/arbitrary"));
+    }
+
+    @Test
+    public void startupBatchDefersDatapackRefresh() {
+        PackDownloader.PackInstallResult changed = new PackDownloader.PackInstallResult("underworld", true, true);
+
+        assertFalse(ModdedPackInstaller.shouldRefreshDatapack(changed, false));
+        assertTrue(ModdedPackInstaller.shouldRefreshDatapack(changed, true));
+        assertFalse(ModdedPackInstaller.shouldRefreshDatapack(
+                new PackDownloader.PackInstallResult("underworld", false, false),
+                true
+        ));
+    }
+}

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedStartupPackSelectionTest.java

@@ -0,0 +1,41 @@
+/*
+ * Iris is a World Generator for Minecraft Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.modded;
+
+import org.junit.Test;
+
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+
+public class ModdedStartupPackSelectionTest {
+    @Test
+    public void managedBetaPacksAreAlwaysInstalledInStableOrder() {
+        assertEquals(List.of("overworld", "underworld"), ModdedStartup.startupPacks("overworld"));
+        assertEquals(List.of("overworld", "underworld"), ModdedStartup.startupPacks("underworld"));
+    }
+
+    @Test
+    public void configuredNonManagedDefaultIsInstalledAfterManagedBetaPacks() {
+        assertEquals(
+                List.of("overworld", "underworld", "custom"),
+                ModdedStartup.startupPacks("custom")
+        );
+    }
+}

core/build.gradle

@@ -228,7 +228,7 @@ tasks.named('compileJava', JavaCompile).configure {
 
 tasks.named('test', Test).configure {
     jvmArgs('--add-modules', 'jdk.incubator.vector')
-    classpath += files('src/main/resources')
+    classpath = files('src/main/resources') + classpath
 }
 
 configurations.matching { it.name.startsWith('slim') }.all { }

core/src/main/java/art/arcane/iris/core/ExactWorldSlotPathPolicy.java

@@ -1,7 +1,5 @@
 package art.arcane.iris.core;
 
-import org.bukkit.NamespacedKey;
-
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.LinkOption;
@@ -17,13 +15,13 @@ public final class ExactWorldSlotPathPolicy {
     private ExactWorldSlotPathPolicy() {
     }
 
-    public static Target resolve(Path levelRoot, NamespacedKey worldKey) {
-        NamespacedKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
+    public static Target resolve(Path levelRoot, WorldSlotKey worldKey) {
+        WorldSlotKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
         Path canonicalLevelRoot = canonicalLevelRoot(levelRoot);
         SlotKind slotKind = classify(requiredWorldKey);
         Path dimensionsRoot = canonicalLevelRoot.resolve("dimensions");
-        Path namespaceRoot = dimensionsRoot.resolve(requiredWorldKey.getNamespace());
-        Path worldDirectory = namespaceRoot.resolve(requiredWorldKey.getKey()).normalize();
+        Path namespaceRoot = dimensionsRoot.resolve(requiredWorldKey.namespace());
+        Path worldDirectory = namespaceRoot.resolve(requiredWorldKey.key()).normalize();
         if (!Objects.equals(worldDirectory.getParent(), namespaceRoot)) {
             throw new Rejection(
                     RejectionReason.PATH_TRAVERSAL,
@@ -37,7 +35,7 @@ public final class ExactWorldSlotPathPolicy {
         return new Target(requiredWorldKey, slotKind, canonicalLevelRoot, namespaceRoot, worldDirectory);
     }
 
-    public static Target validate(Path levelRoot, NamespacedKey worldKey, Path candidate) {
+    public static Target validate(Path levelRoot, WorldSlotKey worldKey, Path candidate) {
         Path requiredCandidate = Objects.requireNonNull(candidate, "candidate");
         rejectTraversal(requiredCandidate, "World candidate");
         Target target = resolve(levelRoot, worldKey);
@@ -71,9 +69,9 @@ public final class ExactWorldSlotPathPolicy {
         }
     }
 
-    private static SlotKind classify(NamespacedKey worldKey) {
-        if ("iris".equals(worldKey.getNamespace())) {
-            if (!SAFE_IRIS_KEY.matcher(worldKey.getKey()).matches()) {
+    private static SlotKind classify(WorldSlotKey worldKey) {
+        if ("iris".equals(worldKey.namespace())) {
+            if (!SAFE_IRIS_KEY.matcher(worldKey.key()).matches()) {
                 throw new Rejection(
                         RejectionReason.INVALID_IRIS_KEY,
                         "Iris world keys must be safe single path segments."
@@ -81,13 +79,13 @@ public final class ExactWorldSlotPathPolicy {
             }
             return SlotKind.IRIS_MANAGED;
         }
-        if (!NamespacedKey.MINECRAFT.equals(worldKey.getNamespace())) {
+        if (!"minecraft".equals(worldKey.namespace())) {
             throw new Rejection(
                     RejectionReason.FOREIGN_NAMESPACE,
                     "Only Iris-managed and exact vanilla dimension slots can be replaced."
             );
         }
-        return switch (worldKey.getKey()) {
+        return switch (worldKey.key()) {
             case "overworld" -> SlotKind.VANILLA_OVERWORLD;
             case "the_nether" -> SlotKind.VANILLA_NETHER;
             case "the_end" -> SlotKind.VANILLA_END;
@@ -154,7 +152,7 @@ public final class ExactWorldSlotPathPolicy {
     }
 
     public record Target(
-            NamespacedKey worldKey,
+            WorldSlotKey worldKey,
             SlotKind slotKind,
             Path levelRoot,
             Path namespaceRoot,
@@ -167,8 +165,8 @@ public final class ExactWorldSlotPathPolicy {
             namespaceRoot = Objects.requireNonNull(namespaceRoot, "namespaceRoot").toAbsolutePath().normalize();
             worldDirectory = Objects.requireNonNull(worldDirectory, "worldDirectory").toAbsolutePath().normalize();
             SlotKind expectedSlotKind = classify(worldKey);
-            Path expectedNamespaceRoot = levelRoot.resolve("dimensions").resolve(worldKey.getNamespace());
-            Path expectedWorldDirectory = expectedNamespaceRoot.resolve(worldKey.getKey());
+            Path expectedNamespaceRoot = levelRoot.resolve("dimensions").resolve(worldKey.namespace());
+            Path expectedWorldDirectory = expectedNamespaceRoot.resolve(worldKey.key());
             if (slotKind != expectedSlotKind
                     || !namespaceRoot.equals(expectedNamespaceRoot)
                     || !worldDirectory.equals(expectedWorldDirectory)) {

core/src/main/java/art/arcane/iris/core/WorldSlotKey.java

@@ -0,0 +1,41 @@
+package art.arcane.iris.core;
+
+import java.util.Objects;
+import java.util.regex.Pattern;
+
+public record WorldSlotKey(String namespace, String key) {
+    private static final Pattern KEY_PATTERN = Pattern.compile("^[a-z0-9/._-]+$");
+    private static final Pattern NAMESPACE_PATTERN = Pattern.compile("^[a-z0-9._-]+$");
+
+    public WorldSlotKey {
+        namespace = Objects.requireNonNull(namespace, "namespace");
+        key = Objects.requireNonNull(key, "key");
+        if (!NAMESPACE_PATTERN.matcher(namespace).matches()) {
+            throw new IllegalArgumentException("World slot namespace is invalid: " + namespace);
+        }
+        if (!KEY_PATTERN.matcher(key).matches()) {
+            throw new IllegalArgumentException("World slot key is invalid: " + key);
+        }
+    }
+
+    public static WorldSlotKey parse(String value) {
+        String requiredValue = Objects.requireNonNull(value, "value");
+        int separator = requiredValue.indexOf(':');
+        if (separator <= 0 || separator != requiredValue.lastIndexOf(':') || separator == requiredValue.length() - 1) {
+            throw new IllegalArgumentException("World slot key must use namespace:key syntax.");
+        }
+        return new WorldSlotKey(
+                requiredValue.substring(0, separator),
+                requiredValue.substring(separator + 1)
+        );
+    }
+
+    public static WorldSlotKey minecraft(String key) {
+        return new WorldSlotKey("minecraft", key);
+    }
+
+    @Override
+    public String toString() {
+        return namespace + ":" + key;
+    }
+}

core/src/main/java/art/arcane/iris/core/lifecycle/BukkitStartupPaths.java

@@ -0,0 +1,370 @@
+package art.arcane.iris.core.lifecycle;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.nio.file.InvalidPathException;
+import java.nio.file.LinkOption;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Objects;
+import java.util.Properties;
+import java.util.Set;
+
+public final class BukkitStartupPaths {
+    private static final String DEFAULT_BUKKIT_CONFIGURATION = "bukkit.yml";
+    private static final String DEFAULT_LEVEL_NAME = "world";
+    private static final String DEFAULT_SERVER_PROPERTIES = "server.properties";
+    private static final String DEFAULT_WORLD_CONTAINER = ".";
+
+    private final Path bukkitConfiguration;
+    private final Path levelRoot;
+    private final String levelName;
+    private final Path serverProperties;
+    private final Path serverRoot;
+    private final Path worldContainer;
+
+    private BukkitStartupPaths(
+            Path serverRoot,
+            Path serverProperties,
+            Path bukkitConfiguration,
+            Path worldContainer,
+            String levelName,
+            Path levelRoot
+    ) {
+        this.serverRoot = Objects.requireNonNull(serverRoot, "serverRoot");
+        this.serverProperties = Objects.requireNonNull(serverProperties, "serverProperties");
+        this.bukkitConfiguration = Objects.requireNonNull(bukkitConfiguration, "bukkitConfiguration");
+        this.worldContainer = Objects.requireNonNull(worldContainer, "worldContainer");
+        this.levelName = Objects.requireNonNull(levelName, "levelName");
+        this.levelRoot = Objects.requireNonNull(levelRoot, "levelRoot");
+    }
+
+    public static BukkitStartupPaths resolveCurrent() throws IOException {
+        return resolve(Path.of(""), currentArguments());
+    }
+
+    public static BukkitStartupPaths resolve(Path serverWorkingDirectory) throws IOException {
+        return resolve(serverWorkingDirectory, currentArguments());
+    }
+
+    public static BukkitStartupPaths resolve(Path serverWorkingDirectory, String[] processArguments)
+            throws IOException {
+        Path serverRoot = canonicalDirectory(
+                Objects.requireNonNull(serverWorkingDirectory, "serverWorkingDirectory").toAbsolutePath().normalize(),
+                "Server working directory"
+        );
+        StartupArguments arguments = parseArguments(processArguments);
+        Path serverProperties = resolveAgainstServerRoot(
+                serverRoot,
+                arguments.serverProperties(),
+                DEFAULT_SERVER_PROPERTIES,
+                "server properties"
+        );
+        Path bukkitConfiguration = resolveAgainstServerRoot(
+                serverRoot,
+                arguments.bukkitConfiguration(),
+                DEFAULT_BUKKIT_CONFIGURATION,
+                "Bukkit configuration"
+        );
+        String levelName = effectiveLevelName(serverProperties, arguments.levelName());
+        Path worldContainer = canonicalPotentialDirectory(
+                resolveAgainstServerRoot(
+                        serverRoot,
+                        arguments.worldContainer() == null
+                                ? BukkitWorldConfiguration.readWorldContainer(bukkitConfiguration.toFile())
+                                : arguments.worldContainer(),
+                        DEFAULT_WORLD_CONTAINER,
+                        "world container"
+                ),
+                "Configured world container"
+        );
+        Path configuredLevel = parsePath(levelName, "level name");
+        Path levelRoot = canonicalPotentialDirectory(
+                configuredLevel.isAbsolute()
+                        ? configuredLevel
+                        : worldContainer.resolve(configuredLevel),
+                "Configured level root"
+        );
+        return new BukkitStartupPaths(
+                serverRoot,
+                serverProperties,
+                bukkitConfiguration,
+                worldContainer,
+                levelName,
+                levelRoot
+        );
+    }
+
+    public Path bukkitConfiguration() {
+        return bukkitConfiguration;
+    }
+
+    public Path levelRoot() {
+        return levelRoot;
+    }
+
+    public String levelName() {
+        return levelName;
+    }
+
+    public Path serverProperties() {
+        return serverProperties;
+    }
+
+    public Path serverRoot() {
+        return serverRoot;
+    }
+
+    public Path worldContainer() {
+        return worldContainer;
+    }
+
+    private static String[] currentArguments() {
+        return applicationArguments(ProcessHandle.current().info().arguments().orElse(new String[0]));
+    }
+
+    static String[] applicationArguments(String[] processArguments) {
+        String[] arguments = Objects.requireNonNull(processArguments, "processArguments");
+        for (int index = 0; index < arguments.length; index++) {
+            if (arguments[index].equals("-jar")
+                    || arguments[index].equals("-m")
+                    || arguments[index].equals("--module")) {
+                int applicationStart = Math.min(arguments.length, index + 2);
+                return Arrays.copyOfRange(arguments, applicationStart, arguments.length);
+            }
+        }
+        Set<String> optionsWithValues = Set.of(
+                "-cp",
+                "-classpath",
+                "--class-path",
+                "-p",
+                "--module-path",
+                "--upgrade-module-path",
+                "--add-modules",
+                "--enable-native-access",
+                "--limit-modules",
+                "--add-exports",
+                "--add-opens",
+                "--add-reads",
+                "--patch-module",
+                "--source"
+        );
+        for (int index = 0; index < arguments.length; index++) {
+            String argument = arguments[index];
+            if (optionsWithValues.contains(argument)) {
+                index++;
+                continue;
+            }
+            if (argument.startsWith("-") || argument.startsWith("@")) {
+                continue;
+            }
+            return Arrays.copyOfRange(arguments, index + 1, arguments.length);
+        }
+        return new String[0];
+    }
+
+    private static StartupArguments parseArguments(String[] processArguments) throws IOException {
+        String[] arguments = Objects.requireNonNull(processArguments, "processArguments");
+        String serverProperties = null;
+        String bukkitConfiguration = null;
+        String levelName = null;
+        String worldContainer = null;
+        for (int index = 0; index < arguments.length; index++) {
+            String argument = Objects.requireNonNull(arguments[index], "process argument");
+            if (argument.equals("--")) {
+                break;
+            }
+            ParsedArgument parsed = parseArgument(
+                    argument,
+                    index + 1 < arguments.length ? arguments[index + 1] : null
+            );
+            if (parsed == null) {
+                continue;
+            }
+            switch (parsed.kind()) {
+                case SERVER_PROPERTIES -> serverProperties = parsed.value();
+                case BUKKIT_CONFIGURATION -> bukkitConfiguration = parsed.value();
+                case LEVEL_NAME -> levelName = parsed.value();
+                case WORLD_CONTAINER -> worldContainer = parsed.value();
+            }
+            if (parsed.consumedFollowing()) {
+                index++;
+            }
+        }
+        return new StartupArguments(serverProperties, bukkitConfiguration, levelName, worldContainer);
+    }
+
+    private static ParsedArgument parseArgument(String argument, String following) throws IOException {
+        ParsedArgument parsed = parseArgument(
+                argument,
+                following,
+                ArgumentKind.SERVER_PROPERTIES,
+                "-c",
+                "--config"
+        );
+        if (parsed != null) {
+            return parsed;
+        }
+        parsed = parseArgument(
+                argument,
+                following,
+                ArgumentKind.BUKKIT_CONFIGURATION,
+                "-b",
+                "--bukkit-settings"
+        );
+        if (parsed != null) {
+            return parsed;
+        }
+        parsed = parseArgument(
+                argument,
+                following,
+                ArgumentKind.WORLD_CONTAINER,
+                "-W",
+                "--world-dir",
+                "--universe",
+                "--world-container"
+        );
+        if (parsed != null) {
+            return parsed;
+        }
+        return parseArgument(
+                argument,
+                following,
+                ArgumentKind.LEVEL_NAME,
+                "-w",
+                "--world",
+                "--level-name"
+        );
+    }
+
+    private static ParsedArgument parseArgument(
+            String argument,
+            String following,
+            ArgumentKind kind,
+            String... keys
+    ) throws IOException {
+        for (String key : keys) {
+            if (argument.equals(key)) {
+                if (following == null || following.isBlank()) {
+                    throw new IOException("Startup argument " + key + " requires a value");
+                }
+                return new ParsedArgument(kind, following, true);
+            }
+            if (key.length() == 2 && argument.startsWith(key) && argument.length() > key.length()) {
+                String value = argument.substring(key.length());
+                if (value.startsWith("=")) {
+                    value = value.substring(1);
+                }
+                if (value.isBlank()) {
+                    throw new IOException("Startup argument " + key + " requires a value");
+                }
+                return new ParsedArgument(kind, value, false);
+            }
+            String prefix = key + "=";
+            if (argument.startsWith(prefix)) {
+                String value = argument.substring(prefix.length());
+                if (value.isBlank()) {
+                    throw new IOException("Startup argument " + key + " requires a value");
+                }
+                return new ParsedArgument(kind, value, false);
+            }
+        }
+        return null;
+    }
+
+    private static Path resolveAgainstServerRoot(
+            Path serverRoot,
+            String configuredValue,
+            String defaultValue,
+            String label
+    ) throws IOException {
+        String value = configuredValue == null ? defaultValue : configuredValue;
+        Path configured = parsePath(value, label);
+        return configured.isAbsolute()
+                ? configured.normalize()
+                : serverRoot.resolve(configured).normalize();
+    }
+
+    private static Path parsePath(String value, String label) throws IOException {
+        if (value == null || value.isBlank()) {
+            throw new IOException("Configured " + label + " is empty");
+        }
+        try {
+            return Path.of(value);
+        } catch (InvalidPathException exception) {
+            throw new IOException("Configured " + label + " is invalid", exception);
+        }
+    }
+
+    private static String effectiveLevelName(Path serverProperties, String argumentOverride) throws IOException {
+        if (argumentOverride != null) {
+            return requireValue(argumentOverride, "level name");
+        }
+        if (!Files.exists(serverProperties, LinkOption.NOFOLLOW_LINKS)) {
+            return DEFAULT_LEVEL_NAME;
+        }
+        if (!Files.isRegularFile(serverProperties)) {
+            throw new IOException("Configured server properties is not a regular file: " + serverProperties);
+        }
+        Properties properties = new Properties();
+        try (InputStream input = Files.newInputStream(serverProperties)) {
+            properties.load(input);
+        }
+        return requireValue(properties.getProperty("level-name", DEFAULT_LEVEL_NAME), "level name");
+    }
+
+    private static String requireValue(String value, String label) throws IOException {
+        if (value == null || value.isBlank()) {
+            throw new IOException("Configured " + label + " is empty");
+        }
+        return value;
+    }
+
+    private static Path canonicalDirectory(Path path, String label) throws IOException {
+        Path canonical = path.toRealPath();
+        if (!Files.isDirectory(canonical)) {
+            throw new IOException(label + " is not a directory: " + path);
+        }
+        return canonical;
+    }
+
+    private static Path canonicalPotentialDirectory(Path path, String label) throws IOException {
+        Path normalized = path.toAbsolutePath().normalize();
+        ArrayList<Path> missing = new ArrayList<>();
+        Path existing = normalized;
+        while (!Files.exists(existing, LinkOption.NOFOLLOW_LINKS)) {
+            Path fileName = existing.getFileName();
+            Path parent = existing.getParent();
+            if (fileName == null || parent == null) {
+                throw new IOException(label + " has no existing filesystem ancestor: " + normalized);
+            }
+            missing.add(fileName);
+            existing = parent;
+        }
+        Path canonical = canonicalDirectory(existing, label);
+        for (int index = missing.size() - 1; index >= 0; index--) {
+            canonical = canonical.resolve(missing.get(index));
+        }
+        return canonical.normalize();
+    }
+
+    private enum ArgumentKind {
+        SERVER_PROPERTIES,
+        BUKKIT_CONFIGURATION,
+        LEVEL_NAME,
+        WORLD_CONTAINER
+    }
+
+    private record ParsedArgument(ArgumentKind kind, String value, boolean consumedFollowing) {
+    }
+
+    private record StartupArguments(
+            String serverProperties,
+            String bukkitConfiguration,
+            String levelName,
+            String worldContainer
+    ) {
+    }
+}

core/src/main/java/art/arcane/iris/core/lifecycle/BukkitWorldConfiguration.java

@@ -9,14 +9,17 @@ import java.io.IOException;
 import java.nio.channels.FileChannel;
 import java.nio.file.AtomicMoveNotSupportedException;
 import java.nio.file.Files;
+import java.nio.file.LinkOption;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 import java.nio.file.StandardOpenOption;
+import java.nio.file.attribute.BasicFileAttributes;
 import java.util.ArrayList;
 import java.util.Objects;
 import java.util.function.Predicate;
 
 public final class BukkitWorldConfiguration {
+    private static final String DEFAULT_WORLD_CONTAINER = ".";
     private static final Object MUTATION_LOCK = new Object();
 
     private BukkitWorldConfiguration() {
@@ -63,6 +66,31 @@ public final class BukkitWorldConfiguration {
         }
     }
 
+    public static String readWorldContainer(File configurationFile) throws IOException {
+        File requiredConfigurationFile = Objects.requireNonNull(configurationFile, "configurationFile");
+        Path configurationPath = requiredConfigurationFile.toPath();
+        if (!Files.exists(configurationPath, LinkOption.NOFOLLOW_LINKS)) {
+            return DEFAULT_WORLD_CONTAINER;
+        }
+        if (!Files.isRegularFile(configurationPath)) {
+            throw new IOException("Configured Bukkit configuration is not a regular file: " + configurationPath);
+        }
+        synchronized (MUTATION_LOCK) {
+            YamlConfiguration configuration = load(requiredConfigurationFile);
+            Object configured = configuration.get("settings.world-container");
+            if (configured == null) {
+                return DEFAULT_WORLD_CONTAINER;
+            }
+            if (!(configured instanceof String value)) {
+                throw new IOException("Bukkit settings.world-container must be a path string");
+            }
+            if (value.isBlank()) {
+                throw new IOException("Configured world container is empty");
+            }
+            return value;
+        }
+    }
+
     public static GeneratorReplacement replaceIfMatching(
             File configurationFile,
             String worldName,
@@ -82,7 +110,7 @@ public final class BukkitWorldConfiguration {
                 return new GeneratorReplacement(false, current, replacement);
             }
             apply(configuration, requiredWorldName, replacement);
-            saveAtomic(configurationFile.toPath(), configuration);
+            saveAtomic(configurationFile.toPath(), configuration, true);
             return new GeneratorReplacement(true, current, replacement);
         }
     }
@@ -104,7 +132,7 @@ public final class BukkitWorldConfiguration {
                 return false;
             }
             apply(configuration, requiredWorldName, requiredRestoration);
-            saveAtomic(configurationFile.toPath(), configuration);
+            saveAtomic(configurationFile.toPath(), configuration, true);
             return true;
         }
     }
@@ -194,7 +222,16 @@ public final class BukkitWorldConfiguration {
     }
 
     static void saveAtomic(Path target, YamlConfiguration configuration) throws IOException {
+        saveAtomic(target, configuration, false);
+    }
+
+    private static void saveAtomic(
+            Path target,
+            YamlConfiguration configuration,
+            boolean requireAtomicReplacement
+    ) throws IOException {
         Path absoluteTarget = target.toAbsolutePath().normalize();
+        requireRegularConfigurationFile(absoluteTarget);
         Path parent = absoluteTarget.getParent();
         if (parent == null) {
             throw new IOException("bukkit.yml target has no parent: " + absoluteTarget);
@@ -206,9 +243,16 @@ public final class BukkitWorldConfiguration {
             try (FileChannel channel = FileChannel.open(staged, StandardOpenOption.WRITE)) {
                 channel.force(true);
             }
+            requireRegularConfigurationFile(absoluteTarget);
             try {
                 Files.move(staged, absoluteTarget, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
             } catch (AtomicMoveNotSupportedException exception) {
+                if (requireAtomicReplacement) {
+                    throw new IOException(
+                            "Exact world replacement requires atomic bukkit.yml publication on this filesystem.",
+                            exception
+                    );
+                }
                 Files.move(staged, absoluteTarget, StandardCopyOption.REPLACE_EXISTING);
             }
         } finally {
@@ -217,6 +261,7 @@ public final class BukkitWorldConfiguration {
     }
 
     private static YamlConfiguration load(File configurationFile) throws IOException {
+        requireRegularConfigurationFile(configurationFile.toPath());
         YamlConfiguration configuration = new YamlConfiguration();
         try {
             configuration.load(configurationFile);
@@ -226,6 +271,18 @@ public final class BukkitWorldConfiguration {
         }
     }
 
+    private static void requireRegularConfigurationFile(Path configurationFile) throws IOException {
+        Path path = configurationFile.toAbsolutePath().normalize();
+        BasicFileAttributes attributes = Files.readAttributes(
+                path,
+                BasicFileAttributes.class,
+                LinkOption.NOFOLLOW_LINKS
+        );
+        if (attributes.isSymbolicLink() || !attributes.isRegularFile()) {
+            throw new IOException("bukkit.yml must be an existing regular file and was not changed: " + path);
+        }
+    }
+
     private static WorldGeneratorSnapshot snapshot(
             YamlConfiguration configuration,
             String worldName

core/src/main/java/art/arcane/iris/core/lifecycle/WorldReplacementBootstrap.java

@@ -102,21 +102,28 @@ public final class WorldReplacementBootstrap {
             feedback.accept("Restored the retained world for " + transaction.worldKey() + ".");
             return ReconcileAction.ROLLED_BACK;
         }
-        if (current.matchesGeneratorAndSeed(transaction.originalConfiguration())) {
-            rollback(dataDirectory, bukkitConfiguration, transaction, paths, current, replacement);
-            feedback.accept("Cancelled or rolled back Iris world replacement for " + transaction.worldKey() + ".");
-            return ReconcileAction.ROLLED_BACK;
-        }
-        if (!current.matchesGeneratorAndSeed(replacement)) {
-            throw conflict(transaction, "bukkit.yml matches neither the replacement nor its retained original state.");
-        }
-
         Transaction active = transaction;
         if (active.phase() == Phase.PREPARED) {
+            if (current.matchesGeneratorAndSeed(active.originalConfiguration())) {
+                rollback(dataDirectory, bukkitConfiguration, active, paths, current, replacement);
+                feedback.accept("Cancelled incomplete Iris world replacement for " + active.worldKey() + ".");
+                return ReconcileAction.ROLLED_BACK;
+            }
+            if (!current.matchesGeneratorAndSeed(replacement)) {
+                throw conflict(active, "bukkit.yml matches neither the replacement nor its retained original state.");
+            }
             active = active.withPhase(Phase.ARMED);
             WorldReplacementJournal.write(dataDirectory, active);
         }
         if (active.phase() == Phase.ARMED) {
+            if (!current.matchesGeneratorAndSeed(replacement)) {
+                if (current.matchesGeneratorAndSeed(active.originalConfiguration())) {
+                    rollback(dataDirectory, bukkitConfiguration, active, paths, current, replacement);
+                    feedback.accept("Cancelled Iris world replacement for " + active.worldKey() + ".");
+                    return ReconcileAction.ROLLED_BACK;
+                }
+                throw conflict(active, "bukkit.yml matches neither the replacement nor its retained original state.");
+            }
             WorldReplacementFilesystem.publish(
                     paths,
                     active.originalTargetPresent(),
@@ -129,6 +136,14 @@ public final class WorldReplacementBootstrap {
             return ReconcileAction.PUBLISHED;
         }
         if (active.phase() == Phase.PUBLISHED) {
+            if (!current.matchesGeneratorAndSeed(replacement)) {
+                if (current.matchesGeneratorAndSeed(active.originalConfiguration())) {
+                    rollback(dataDirectory, bukkitConfiguration, active, paths, current, replacement);
+                    feedback.accept("Rolled back Iris world replacement for " + active.worldKey() + ".");
+                    return ReconcileAction.ROLLED_BACK;
+                }
+                throw conflict(active, "bukkit.yml matches neither the replacement nor its retained original state.");
+            }
             WorldReplacementFilesystem.publish(
                     paths,
                     active.originalTargetPresent(),
@@ -154,7 +169,7 @@ public final class WorldReplacementBootstrap {
         Transaction rollback = transaction.phase() == Phase.ROLLBACK_PENDING
                 ? transaction
                 : transaction.withPhase(Phase.ROLLBACK_PENDING);
-        if (rollback != transaction) {
+        if (transaction.phase() != Phase.ROLLBACK_PENDING) {
             WorldReplacementJournal.write(dataDirectory, rollback);
         }
         WorldReplacementFilesystem.prepareRollback(paths, rollback.originalTargetPresent());

core/src/main/java/art/arcane/iris/core/lifecycle/WorldReplacementBootstrapMarker.java

@@ -0,0 +1,16 @@
+package art.arcane.iris.core.lifecycle;
+
+public final class WorldReplacementBootstrapMarker {
+    private static volatile boolean bootstrappedThisProcess;
+
+    private WorldReplacementBootstrapMarker() {
+    }
+
+    public static boolean wasBootstrappedThisProcess() {
+        return bootstrappedThisProcess;
+    }
+
+    public static void markBootstrapped() {
+        bootstrappedThisProcess = true;
+    }
+}

core/src/main/java/art/arcane/iris/core/lifecycle/WorldReplacementFilesystem.java

@@ -1,8 +1,10 @@
 package art.arcane.iris.core.lifecycle;
 
-import art.arcane.iris.core.SnapshotDirectoryTreeDeleter;
 import art.arcane.iris.core.ExactWorldSlotPathPolicy;
+import art.arcane.iris.core.SnapshotDirectoryTreeDeleter;
+import art.arcane.iris.spi.IrisLogging;
 
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.ByteBuffer;
@@ -26,6 +28,11 @@ import java.util.regex.Pattern;
 import java.util.stream.Stream;
 
 public final class WorldReplacementFilesystem {
+    private static final List<Path> PAPER_WORLD_METADATA = List.of(
+            Path.of("data/paper/metadata.dat"),
+            Path.of("data/paper/level_overrides.dat"),
+            Path.of("data/minecraft/world_gen_settings.dat")
+    );
     private static final Pattern STAGE_NAME = Pattern.compile(
             "^\\.iris-replace-[a-z0-9_-]+-[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\\.stage$");
     private static final Pattern BACKUP_NAME = Pattern.compile(
@@ -37,7 +44,7 @@ public final class WorldReplacementFilesystem {
     public static ReplacementPaths paths(ExactWorldSlotPathPolicy.Target target, UUID id) {
         ExactWorldSlotPathPolicy.Target requiredTarget = Objects.requireNonNull(target, "target");
         UUID requiredId = Objects.requireNonNull(id, "id");
-        String artifactBase = ".iris-replace-" + requiredTarget.worldKey().getKey() + "-" + requiredId;
+        String artifactBase = ".iris-replace-" + requiredTarget.worldKey().key() + "-" + requiredId;
         return new ReplacementPaths(
                 requiredTarget.worldDirectory(),
                 requiredTarget.namespaceRoot().resolve(artifactBase + ".stage"),
@@ -45,6 +52,17 @@ public final class WorldReplacementFilesystem {
         );
     }
 
+    public static void requireExistingTarget(ReplacementPaths paths) throws IOException {
+        ReplacementPaths requiredPaths = Objects.requireNonNull(paths, "paths");
+        State state = inspect(requiredPaths);
+        if (!state.targetPresent()) {
+            throw new IOException("overwrite=true requires an existing exact world slot; use ordinary create for a new world.");
+        }
+        if (state.stagePresent() || state.backupPresent()) {
+            throw new IOException("A replacement artifact already exists for this transaction.");
+        }
+    }
+
     public static void publish(
             ReplacementPaths paths,
             boolean originalTargetPresent,
@@ -56,6 +74,12 @@ public final class WorldReplacementFilesystem {
         if (state.stagePresent()) {
             requireSafeTree(requiredPaths.stage(), "replacement stage");
             requireFingerprint(requiredPaths.stage().resolve("iris/pack"), expectedFingerprint);
+            if (originalTargetPresent) {
+                Path retainedWorld = state.targetPresent()
+                        ? requiredPaths.target()
+                        : requiredPaths.backup();
+                preservePaperWorldMetadata(retainedWorld, requiredPaths.stage());
+            }
             if (state.targetPresent()) {
                 if (state.backupPresent()) {
                     throw new IOException("Replacement target, stage, and backup are all present.");
@@ -86,6 +110,11 @@ public final class WorldReplacementFilesystem {
         }
         requireSafeTree(requiredPaths.target(), "replacement target");
         requireFingerprint(requiredPaths.target().resolve("iris/pack"), expectedFingerprint);
+        if (originalTargetPresent) {
+            preservePaperWorldMetadata(requiredPaths.backup(), requiredPaths.target());
+            requireSafeTree(requiredPaths.target(), "replacement target");
+            requireFingerprint(requiredPaths.target().resolve("iris/pack"), expectedFingerprint);
+        }
     }
 
     public static void rollback(ReplacementPaths paths, boolean originalTargetPresent) throws IOException {
@@ -298,6 +327,71 @@ public final class WorldReplacementFilesystem {
         }
     }
 
+    private static void preservePaperWorldMetadata(Path retainedWorld, Path replacementWorld) throws IOException {
+        requireDirectory(retainedWorld, "retained world");
+        requireDirectory(replacementWorld, "replacement world");
+        requireDirectory(retainedWorld.resolve("data"), "retained world data");
+        requireDirectory(retainedWorld.resolve("data/paper"), "retained Paper data");
+        requireDirectory(retainedWorld.resolve("data/minecraft"), "retained Minecraft data");
+        for (Path relative : PAPER_WORLD_METADATA) {
+            BasicFileAttributes sourceAttributes = requireSafeEntry(retainedWorld.resolve(relative));
+            if (!sourceAttributes.isRegularFile()) {
+                throw new IOException("Retained Paper world metadata is not a regular file: " + relative);
+            }
+        }
+        ensureDirectory(replacementWorld.resolve("data"), "replacement world data");
+        ensureDirectory(replacementWorld.resolve("data/paper"), "replacement Paper data");
+        ensureDirectory(replacementWorld.resolve("data/minecraft"), "replacement Minecraft data");
+        for (Path relative : PAPER_WORLD_METADATA) {
+            Path destination = replacementWorld.resolve(relative);
+            if (Files.exists(destination, LinkOption.NOFOLLOW_LINKS)) {
+                BasicFileAttributes destinationAttributes = requireSafeEntry(destination);
+                if (!destinationAttributes.isRegularFile()) {
+                    throw new IOException("Replacement Paper world metadata is not a regular file: " + relative);
+                }
+                continue;
+            }
+            copyMetadataFile(retainedWorld.resolve(relative), destination);
+        }
+    }
+
+    private static void ensureDirectory(Path directory, String label) throws IOException {
+        if (Files.exists(directory, LinkOption.NOFOLLOW_LINKS)) {
+            requireDirectory(directory, label);
+            return;
+        }
+        Files.createDirectory(directory);
+        forceDirectoryRequired(directory.getParent());
+    }
+
+    private static void copyMetadataFile(Path source, Path destination) throws IOException {
+        Path temporary = destination.resolveSibling("." + destination.getFileName() + ".iris-replace.tmp");
+        if (Files.exists(temporary, LinkOption.NOFOLLOW_LINKS)) {
+            BasicFileAttributes attributes = requireSafeEntry(temporary);
+            if (!attributes.isRegularFile()) {
+                throw new IOException("Replacement metadata staging path is unsafe: " + temporary);
+            }
+            Files.delete(temporary);
+        }
+        try {
+            Files.copy(source, temporary, StandardCopyOption.COPY_ATTRIBUTES);
+            try (FileChannel channel = FileChannel.open(temporary, StandardOpenOption.WRITE)) {
+                channel.force(true);
+            }
+            try {
+                Files.move(temporary, destination, StandardCopyOption.ATOMIC_MOVE);
+            } catch (AtomicMoveNotSupportedException exception) {
+                throw new IOException(
+                        "Exact world replacement requires atomic Paper metadata publication on this filesystem.",
+                        exception
+                );
+            }
+            forceDirectoryRequired(destination.getParent());
+        } finally {
+            Files.deleteIfExists(temporary);
+        }
+    }
+
     private static void update(MessageDigest digest, String value) {
         byte[] bytes = value.getBytes(StandardCharsets.UTF_8);
         digest.update(ByteBuffer.allocate(Integer.BYTES).putInt(bytes.length).array());
@@ -313,13 +407,37 @@ public final class WorldReplacementFilesystem {
     }
 
     private static void move(Path source, Path target) throws IOException {
+        forceDirectoryRequired(source.getParent());
         try {
             Files.move(source, target, StandardCopyOption.ATOMIC_MOVE);
         } catch (AtomicMoveNotSupportedException exception) {
-            Files.move(source, target);
+            throw new IOException(
+                    "Exact world replacement requires atomic directory moves on this filesystem.",
+                    exception
+            );
         }
-        try (FileChannel channel = FileChannel.open(source.getParent(), StandardOpenOption.READ)) {
+        forceDirectoryAfterCommit(source.getParent());
+    }
+
+    private static void forceDirectoryRequired(Path directory) throws IOException {
+        if (File.separatorChar == '\\') {
+            return;
+        }
+        try (FileChannel channel = FileChannel.open(directory, StandardOpenOption.READ)) {
             channel.force(true);
+        } catch (UnsupportedOperationException failure) {
+            throw new IOException("Directory durability sync is unavailable for " + directory + ".", failure);
+        }
+    }
+
+    private static void forceDirectoryAfterCommit(Path directory) {
+        try {
+            forceDirectoryRequired(directory);
+        } catch (IOException failure) {
+            IrisLogging.reportError(
+                    "A world-replacement move completed, but its parent directory could not be durability-synced.",
+                    failure
+            );
         }
     }
 

core/src/main/java/art/arcane/iris/core/lifecycle/WorldReplacementJournal.java

@@ -1,10 +1,12 @@
 package art.arcane.iris.core.lifecycle;
 
 import art.arcane.iris.core.ExactWorldSlotPathPolicy;
+import art.arcane.iris.core.WorldSlotKey;
 import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.WorldGeneratorSnapshot;
-import org.bukkit.NamespacedKey;
+import art.arcane.iris.spi.IrisLogging;
 
 import java.io.ByteArrayOutputStream;
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.ByteBuffer;
@@ -18,9 +20,11 @@ import java.nio.file.StandardCopyOption;
 import java.nio.file.StandardOpenOption;
 import java.util.ArrayList;
 import java.util.Comparator;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
 import java.util.Properties;
+import java.util.Set;
 import java.util.UUID;
 
 public final class WorldReplacementJournal {
@@ -46,6 +50,12 @@ public final class WorldReplacementJournal {
             }
         }
         transactions.sort(Comparator.comparing(transaction -> transaction.id().toString()));
+        Set<WorldSlotKey> worldKeys = new HashSet<>();
+        for (Transaction transaction : transactions) {
+            if (!worldKeys.add(transaction.worldKey())) {
+                throw new IOException("Multiple replacement journals target " + transaction.worldKey() + ".");
+            }
+        }
         return List.copyOf(transactions);
     }
 
@@ -77,8 +87,9 @@ public final class WorldReplacementJournal {
         if (directory == null) {
             return;
         }
+        forceDirectoryRequired(directory);
         Files.deleteIfExists(directory.resolve(Objects.requireNonNull(id, "id") + JOURNAL_SUFFIX));
-        forceDirectory(directory);
+        forceDirectoryAfterCommit(directory);
     }
 
     public static ExactWorldSlotPathPolicy.Target resolveTarget(Transaction transaction, Path currentLevelRoot)
@@ -91,31 +102,44 @@ public final class WorldReplacementJournal {
         if (!target.levelRoot().equals(requiredTransaction.levelRoot())) {
             throw new IOException("The configured level root changed after the world replacement was staged.");
         }
-        String expectedWorldName = logicalWorldName(target.levelRoot(), requiredTransaction.worldKey());
+        String expectedWorldName;
+        try {
+            expectedWorldName = logicalWorldName(target.levelRoot(), requiredTransaction.worldKey());
+        } catch (IllegalArgumentException failure) {
+            throw new IOException("The replacement journal targets an ambiguous logical world name.", failure);
+        }
         if (!expectedWorldName.equals(requiredTransaction.worldName())) {
             throw new IOException("The logical world name changed after the world replacement was staged.");
         }
         return target;
     }
 
-    public static String logicalWorldName(Path levelRoot, NamespacedKey worldKey) {
+    public static String logicalWorldName(Path levelRoot, WorldSlotKey worldKey) {
         Path requiredLevelRoot = Objects.requireNonNull(levelRoot, "levelRoot").toAbsolutePath().normalize();
-        NamespacedKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
-        if ("iris".equals(requiredWorldKey.getNamespace())) {
-            return requiredWorldKey.getKey();
-        }
+        WorldSlotKey requiredWorldKey = Objects.requireNonNull(worldKey, "worldKey");
         Path fileName = requiredLevelRoot.getFileName();
         if (fileName == null || fileName.toString().isBlank()) {
             throw new IllegalArgumentException("Level root must have a logical world name.");
         }
         String levelName = fileName.toString();
-        if (NamespacedKey.minecraft("overworld").equals(requiredWorldKey)) {
+        if ("iris".equals(requiredWorldKey.namespace())) {
+            String logicalName = requiredWorldKey.key();
+            if (logicalName.equals(levelName)
+                    || logicalName.equals(levelName + "_nether")
+                    || logicalName.equals(levelName + "_the_end")) {
+                throw new IllegalArgumentException(
+                        "An Iris-managed world cannot use a configured vanilla world alias."
+                );
+            }
+            return logicalName;
+        }
+        if (WorldSlotKey.minecraft("overworld").equals(requiredWorldKey)) {
             return levelName;
         }
-        if (NamespacedKey.minecraft("the_nether").equals(requiredWorldKey)) {
+        if (WorldSlotKey.minecraft("the_nether").equals(requiredWorldKey)) {
             return levelName + "_nether";
         }
-        if (NamespacedKey.minecraft("the_end").equals(requiredWorldKey)) {
+        if (WorldSlotKey.minecraft("the_end").equals(requiredWorldKey)) {
             return levelName + "_the_end";
         }
         throw new IllegalArgumentException("World key is not an exact replaceable world slot: " + requiredWorldKey);
@@ -135,9 +159,11 @@ public final class WorldReplacementJournal {
         if (!file.getFileName().toString().equals(id + JOURNAL_SUFFIX)) {
             throw new IOException("Replacement journal filename does not match its transaction id.");
         }
-        NamespacedKey worldKey = NamespacedKey.fromString(required(properties, "worldKey"));
-        if (worldKey == null) {
-            throw new IOException("Replacement journal contains an invalid world key.");
+        WorldSlotKey worldKey;
+        try {
+            worldKey = WorldSlotKey.parse(required(properties, "worldKey"));
+        } catch (IllegalArgumentException failure) {
+            throw new IOException("Replacement journal contains an invalid world key.", failure);
         }
         String worldName = exact(properties, "worldName");
         Path recordedLevelRoot = recordedLevelRoot(properties);
@@ -300,26 +326,46 @@ public final class WorldReplacementJournal {
                 }
                 channel.force(true);
             }
+            forceDirectoryRequired(parent);
             try {
                 Files.move(temporary, target, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
             } catch (AtomicMoveNotSupportedException failure) {
-                Files.move(temporary, target, StandardCopyOption.REPLACE_EXISTING);
+                throw new IOException(
+                        "Exact world replacement requires atomic journal publication on this filesystem.",
+                        failure
+                );
             }
-            forceDirectory(parent);
+            forceDirectoryAfterCommit(parent);
         } finally {
             Files.deleteIfExists(temporary);
         }
     }
 
-    private static void forceDirectory(Path directory) throws IOException {
+    private static void forceDirectoryRequired(Path directory) throws IOException {
+        if (File.separatorChar == '\\') {
+            return;
+        }
         try (FileChannel channel = FileChannel.open(directory, StandardOpenOption.READ)) {
             channel.force(true);
+        } catch (UnsupportedOperationException failure) {
+            throw new IOException("Directory durability sync is unavailable for " + directory + ".", failure);
+        }
+    }
+
+    private static void forceDirectoryAfterCommit(Path directory) {
+        try {
+            forceDirectoryRequired(directory);
+        } catch (IOException failure) {
+            IrisLogging.reportError(
+                    "A world-replacement journal change completed, but its parent directory could not be durability-synced.",
+                    failure
+            );
         }
     }
 
     public record Transaction(
             UUID id,
-            NamespacedKey worldKey,
+            WorldSlotKey worldKey,
             String worldName,
             Path levelRoot,
             String dimension,

core/src/main/java/art/arcane/iris/core/pack/DefaultPackBootstrapProvisioner.java

@@ -1,6 +1,7 @@
 package art.arcane.iris.core.pack;
 
 import art.arcane.iris.core.IrisDatapackCompiler;
+import art.arcane.iris.core.lifecycle.BukkitStartupPaths;
 import art.arcane.iris.core.nms.datapack.DataVersion;
 import art.arcane.iris.core.nms.datapack.IDataFixer;
 import art.arcane.volmlib.util.collection.KList;
@@ -27,7 +28,9 @@ import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.HashSet;
 import java.util.HexFormat;
+import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
 import java.util.Set;
@@ -40,9 +43,20 @@ import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
 
 public final class DefaultPackBootstrapProvisioner {
-    private static final URI DEFAULT_SOURCE = URI.create("https://github.com/IrisDimensions/overworld/releases/download/beta/overworld.zip");
+    private static final List<PackSpec> DEFAULT_PACKS = List.of(
+            new PackSpec(
+                    "overworld",
+                    URI.create("https://github.com/IrisDimensions/overworld/releases/download/beta/overworld.zip"),
+                    "overworld"
+            ),
+            new PackSpec(
+                    "underworld",
+                    URI.create("https://github.com/IrisDimensions/underworld/releases/download/beta/underworld.zip"),
+                    "underworld"
+            )
+    );
     private static final String WORLD_DATAPACK_DIRECTORY = "iris";
-    private static final int MARKER_SCHEMA = 3;
+    private static final int MARKER_SCHEMA = 4;
     private static final int MAX_ARCHIVE_ENTRIES = 100_000;
     private static final long MAX_ARCHIVE_BYTES = 512L * 1024L * 1024L;
     private static final long MAX_EXPANDED_BYTES = 2L * 1024L * 1024L * 1024L;
@@ -53,17 +67,28 @@ public final class DefaultPackBootstrapProvisioner {
     private DefaultPackBootstrapProvisioner() {
     }
 
+    static List<PackSpec> defaultPacks() {
+        return DEFAULT_PACKS;
+    }
+
     public static ProvisionResult provision(Path dataDirectory, Consumer<String> feedback) throws IOException {
+        return provision(dataDirectory, feedback, BukkitStartupPaths.resolveCurrent());
+    }
+
+    public static ProvisionResult provision(
+            Path dataDirectory,
+            Consumer<String> feedback,
+            BukkitStartupPaths startupPaths
+    ) throws IOException {
         Objects.requireNonNull(dataDirectory, "dataDirectory");
         Objects.requireNonNull(feedback, "feedback");
-        Path serverRoot = Path.of("").toAbsolutePath().normalize();
-        Path levelRoot = resolveLevelRoot(serverRoot);
+        Path levelRoot = Objects.requireNonNull(startupPaths, "startupPaths").levelRoot();
         HttpClient client = HttpClient.newBuilder()
                 .connectTimeout(Duration.ofSeconds(15))
                 .followRedirects(HttpClient.Redirect.ALWAYS)
                 .build();
         ProvisionOptions options = new ProvisionOptions(
-                DEFAULT_SOURCE,
+                DEFAULT_PACKS,
                 client,
                 Clock.systemUTC(),
                 Duration.ofMinutes(30),
@@ -83,7 +108,7 @@ public final class DefaultPackBootstrapProvisioner {
         try {
             return isProvisioned(
                     dataDirectory,
-                    resolveLevelRoot(Path.of("").toAbsolutePath().normalize())
+                    BukkitStartupPaths.resolveCurrent().levelRoot()
             );
         } catch (IOException | RuntimeException exception) {
             return false;
@@ -91,14 +116,17 @@ public final class DefaultPackBootstrapProvisioner {
     }
 
     static boolean isProvisioned(Path dataDirectory, Path levelRoot) {
+        return isProvisioned(dataDirectory, levelRoot, DEFAULT_PACKS);
+    }
+
+    static boolean isProvisioned(Path dataDirectory, Path levelRoot, List<PackSpec> requiredPacks) {
         try {
             Path normalizedData = dataDirectory.toAbsolutePath().normalize();
             Path normalizedLevel = levelRoot.toAbsolutePath().normalize();
             Path bootstrapRoot = normalizedData.resolve("bootstrap");
             Path datapackRoot = worldDatapackRoot(normalizedLevel);
-            Path packRoot = normalizedData.resolve("packs/overworld");
             Path markerFile = bootstrapRoot.resolve("provisioned.properties");
-            if (!Files.isRegularFile(markerFile) || !isPackRoot(packRoot) || !isDatapackRoot(datapackRoot)) {
+            if (!Files.isRegularFile(markerFile) || !isDatapackRoot(datapackRoot)) {
                 return false;
             }
             Properties marker = loadProperties(markerFile);
@@ -111,8 +139,16 @@ public final class DefaultPackBootstrapProvisioner {
                     .equals(marker.getProperty("compilerIdentity"))) {
                 return false;
             }
-            return directoryFingerprint(packRoot).equals(marker.getProperty("defaultPackFingerprint"))
-                    && directoryFingerprint(datapackRoot).equals(marker.getProperty("datapackFingerprint"))
+            for (PackSpec spec : requiredPacks) {
+                Path packRoot = normalizedData.resolve("packs").resolve(spec.key());
+                if (!isPackRoot(packRoot, spec)
+                        || !spec.source().toString().equals(marker.getProperty(markerKey(spec, "source")))
+                        || !spec.requiredDimension().equals(marker.getProperty(markerKey(spec, "requiredDimension")))
+                        || !directoryFingerprint(packRoot).equals(marker.getProperty(markerKey(spec, "fingerprint")))) {
+                    return false;
+                }
+            }
+            return directoryFingerprint(datapackRoot).equals(marker.getProperty("datapackFingerprint"))
                     && datapackRoot.toString().equals(marker.getProperty("datapackPath"))
                     && packRootsFingerprint(IrisDatapackCompiler.collectPackRoots(
                             normalizedData,
@@ -136,7 +172,6 @@ public final class DefaultPackBootstrapProvisioner {
         PROVISIONED_COMPILER_INPUT_FINGERPRINT.set("");
         Path normalizedData = dataDirectory.toAbsolutePath().normalize();
         Path packsRoot = normalizedData.resolve("packs");
-        Path packRoot = packsRoot.resolve("overworld");
         Path bootstrapRoot = normalizedData.resolve("bootstrap");
         Path legacyDatapackRoot = bootstrapRoot.resolve("datapack");
         Path datapacksRoot = options.levelRoot().toAbsolutePath().normalize().resolve("datapacks");
@@ -149,40 +184,58 @@ public final class DefaultPackBootstrapProvisioner {
         Files.createDirectories(datapacksRoot);
 
         Properties previousMarker = Files.isRegularFile(markerFile) ? loadProperties(markerFile) : new Properties();
-        boolean existingPack = isPackRoot(packRoot);
         boolean existingDatapack = isDatapackRoot(datapackRoot);
-        String currentPackFingerprint = existingPack ? directoryFingerprint(packRoot) : "";
-        boolean markerOwned = "true".equals(previousMarker.getProperty("managedDefault"));
-        boolean unchangedManagedPack = markerOwned
-                && currentPackFingerprint.equals(previousMarker.getProperty("defaultPackFingerprint"));
-        boolean managedDefault = !existingPack || unchangedManagedPack;
-        if (Files.isSymbolicLink(packRoot)) {
-            managedDefault = false;
+        List<PackPlan> plans = new ArrayList<>(options.packs().size());
+        for (PackSpec spec : options.packs()) {
+            Path packRoot = packsRoot.resolve(spec.key()).normalize();
+            if (!Objects.equals(packRoot.getParent(), packsRoot)) {
+                throw new IOException("Bootstrap pack target escapes the packs directory: " + packRoot);
+            }
+            boolean existingPack = isPackRoot(packRoot, spec);
+            String currentFingerprint = existingPack ? directoryFingerprint(packRoot) : "";
+            boolean markerOwned = "true".equals(markerProperty(previousMarker, spec, "managed"));
+            boolean unchangedManagedPack = markerOwned
+                    && currentFingerprint.equals(markerProperty(previousMarker, spec, "fingerprint"));
+            boolean managed = !existingPack || unchangedManagedPack;
+            if (Files.isSymbolicLink(packRoot)) {
+                managed = false;
+            }
+            String retainedSourceSha = markerProperty(previousMarker, spec, "sourceSha256");
+            Archive archive = managed
+                    ? acquireArchive(cacheRoot, previousMarker, spec, feedback, options)
+                    : new Archive(null, retainedSourceSha == null || retainedSourceSha.isBlank()
+                    ? currentFingerprint
+                    : retainedSourceSha);
+            boolean replacePack = !existingPack || managed
+                    && (!archive.sha256().equals(markerProperty(previousMarker, spec, "sourceSha256"))
+                    || !currentFingerprint.equals(markerProperty(previousMarker, spec, "fingerprint")));
+            plans.add(new PackPlan(spec, packRoot, existingPack, managed, archive, replacePack));
         }
-        Archive archive = managedDefault
-                ? acquireArchive(cacheRoot, previousMarker, feedback, options)
-                : new Archive(null, currentPackFingerprint);
-        boolean replacePack = !existingPack || managedDefault
-                && (!archive.sha256().equals(previousMarker.getProperty("sourceSha256"))
-                || !currentPackFingerprint.equals(previousMarker.getProperty("defaultPackFingerprint")));
 
-        Path stagedPack = null;
-        Path extractionRoot = null;
+        Map<PackPlan, Path> stagedPacks = new LinkedHashMap<>();
+        List<Path> extractionRoots = new ArrayList<>();
         Path compileContainer = null;
-        Path packBackup = null;
         Path datapackBackup = null;
-        boolean packReplaced = false;
         boolean datapackReplaced = false;
+        List<PackPublication> packPublications = new ArrayList<>();
+        boolean committed = false;
         try {
-            if (replacePack) {
-                extractionRoot = cacheRoot.resolve(".extract-" + UUID.randomUUID());
+            for (PackPlan plan : plans) {
+                if (!plan.replace()) {
+                    continue;
+                }
+                Path extractionRoot = cacheRoot.resolve(".extract-" + plan.spec().key() + "-" + UUID.randomUUID());
+                extractionRoots.add(extractionRoot);
                 Files.createDirectories(extractionRoot);
-                Path extractedPack = extractArchive(archive.path(), extractionRoot);
-                stagedPack = packsRoot.resolve(".overworld-stage-" + UUID.randomUUID());
+                Path extractedPack = extractArchive(plan.archive().path(), extractionRoot, plan.spec());
+                Path stagedPack = packsRoot.resolve("." + plan.spec().key() + "-stage-" + UUID.randomUUID());
                 copyDirectory(extractedPack, stagedPack);
-                validatePackRoot(stagedPack);
-                packBackup = replaceWithBackup(stagedPack, packRoot);
-                packReplaced = true;
+                validatePackRoot(stagedPack, plan.spec());
+                stagedPacks.put(plan, stagedPack);
+            }
+            for (Map.Entry<PackPlan, Path> entry : stagedPacks.entrySet()) {
+                Path backup = replaceWithBackup(entry.getValue(), entry.getKey().root());
+                packPublications.add(new PackPublication(entry.getKey().root(), backup));
             }
 
             List<File> packRoots = IrisDatapackCompiler.collectPackRoots(normalizedData, options.levelRoot());
@@ -195,7 +248,8 @@ public final class DefaultPackBootstrapProvisioner {
             }
             String compilerIdentity = IrisDatapackCompiler.compilerIdentity(fixer);
             String aggregateFingerprint = packRootsFingerprint(packRoots);
-            boolean rebuildDatapack = replacePack
+            boolean anyPackReplaced = !packPublications.isEmpty();
+            boolean rebuildDatapack = anyPackReplaced
                     || !existingDatapack
                     || !aggregateFingerprint.equals(previousMarker.getProperty("aggregateFingerprint"))
                     || !compilerIdentity.equals(previousMarker.getProperty("compilerIdentity"))
@@ -214,11 +268,12 @@ public final class DefaultPackBootstrapProvisioner {
                 datapackReplaced = true;
             }
 
-            validatePackRoot(packRoot);
+            for (PackPlan plan : plans) {
+                validatePackRoot(plan.root(), plan.spec());
+            }
             if (!isDatapackRoot(datapackRoot)) {
                 throw new IOException("Bootstrap datapack output is incomplete at " + datapackRoot);
             }
-            String finalPackFingerprint = directoryFingerprint(packRoot);
             List<File> finalPackRoots = IrisDatapackCompiler.collectPackRoots(
                     normalizedData,
                     options.levelRoot());
@@ -230,47 +285,76 @@ public final class DefaultPackBootstrapProvisioner {
             String finalDatapackFingerprint = directoryFingerprint(datapackRoot);
             Properties marker = new Properties();
             marker.setProperty("schema", Integer.toString(MARKER_SCHEMA));
-            marker.setProperty("source", options.source().toString());
-            marker.setProperty("sourceSha256", archive.sha256());
-            marker.setProperty("managedDefault", Boolean.toString(managedDefault));
-            marker.setProperty("defaultPackFingerprint", finalPackFingerprint);
+            for (PackPlan plan : plans) {
+                marker.setProperty(markerKey(plan.spec(), "source"), plan.spec().source().toString());
+                marker.setProperty(markerKey(plan.spec(), "sourceSha256"), plan.archive().sha256());
+                marker.setProperty(markerKey(plan.spec(), "managed"), Boolean.toString(plan.managed()));
+                marker.setProperty(markerKey(plan.spec(), "fingerprint"), directoryFingerprint(plan.root()));
+                marker.setProperty(markerKey(plan.spec(), "requiredDimension"), plan.spec().requiredDimension());
+            }
             marker.setProperty("aggregateFingerprint", finalAggregateFingerprint);
             marker.setProperty("compilerIdentity", compilerIdentity);
             marker.setProperty("datapackFingerprint", finalDatapackFingerprint);
             marker.setProperty("datapackPath", datapackRoot.toString());
             marker.setProperty("completedAt", Long.toString(options.clock().millis()));
             storePropertiesAtomic(markerFile, marker);
+            committed = true;
             PROVISIONED_COMPILER_INPUT_FINGERPRINT.set(finalCompilerInputFingerprint);
             PROVISIONED_THIS_STARTUP.set(true);
-            deleteQuietly(packBackup, feedback);
+            for (PackPublication publication : packPublications) {
+                deleteQuietly(publication.backup(), feedback);
+            }
             deleteQuietly(datapackBackup, feedback);
             deleteQuietly(legacyDatapackRoot, feedback);
 
+            boolean everyPackMissing = plans.stream().noneMatch(PackPlan::existed);
             ProvisionStatus status;
-            if (!existingPack && !existingDatapack) {
+            if (everyPackMissing && !existingDatapack) {
                 status = ProvisionStatus.INSTALLED;
-            } else if (replacePack || rebuildDatapack) {
+            } else if (anyPackReplaced || rebuildDatapack) {
                 status = ProvisionStatus.UPDATED;
             } else {
                 status = ProvisionStatus.UNCHANGED;
             }
-            feedback.accept("Iris bootstrap pack is " + status.name().toLowerCase() + ".");
-            return new ProvisionResult(packRoot, datapackRoot, status);
+            feedback.accept("Iris bootstrap packs are " + status.name().toLowerCase() + ".");
+            Map<String, Path> provisionedPacks = new LinkedHashMap<>();
+            for (PackPlan plan : plans) {
+                provisionedPacks.put(plan.spec().key(), plan.root());
+            }
+            return new ProvisionResult(provisionedPacks, datapackRoot, status);
         } catch (IOException failure) {
-            IOException rollbackFailure = rollback(packRoot, packBackup, packReplaced, datapackRoot, datapackBackup, datapackReplaced);
-            if (rollbackFailure != null) {
-                failure.addSuppressed(rollbackFailure);
+            if (!committed) {
+                IOException rollbackFailure = rollback(
+                        packPublications,
+                        datapackRoot,
+                        datapackBackup,
+                        datapackReplaced
+                );
+                if (rollbackFailure != null) {
+                    failure.addSuppressed(rollbackFailure);
+                }
             }
             throw failure;
         } catch (RuntimeException | LinkageError failure) {
-            IOException rollbackFailure = rollback(packRoot, packBackup, packReplaced, datapackRoot, datapackBackup, datapackReplaced);
-            if (rollbackFailure != null) {
-                failure.addSuppressed(rollbackFailure);
+            if (!committed) {
+                IOException rollbackFailure = rollback(
+                        packPublications,
+                        datapackRoot,
+                        datapackBackup,
+                        datapackReplaced
+                );
+                if (rollbackFailure != null) {
+                    failure.addSuppressed(rollbackFailure);
+                }
             }
             throw new IOException("Iris bootstrap provisioning failed", failure);
         } finally {
-            deleteQuietly(stagedPack, feedback);
-            deleteQuietly(extractionRoot, feedback);
+            for (Path stagedPack : stagedPacks.values()) {
+                deleteQuietly(stagedPack, feedback);
+            }
+            for (Path extractionRoot : extractionRoots) {
+                deleteQuietly(extractionRoot, feedback);
+            }
             deleteQuietly(compileContainer, feedback);
         }
     }
@@ -278,35 +362,44 @@ public final class DefaultPackBootstrapProvisioner {
     private static Archive acquireArchive(
             Path cacheRoot,
             Properties marker,
+            PackSpec spec,
             Consumer<String> feedback,
             ProvisionOptions options
     ) throws IOException {
-        Path archivePath = cacheRoot.resolve("default-overworld.zip");
-        Path metadataPath = cacheRoot.resolve("default-overworld.properties");
+        Path archivePath = cacheRoot.resolve("default-" + spec.key() + ".zip");
+        Path metadataPath = cacheRoot.resolve("default-" + spec.key() + ".properties");
         Properties metadata = Files.isRegularFile(metadataPath) ? loadProperties(metadataPath) : new Properties();
         boolean validCache = false;
         if (Files.isRegularFile(archivePath)) {
             try {
-                validCache = validateArchive(archivePath);
+                validCache = validateArchive(archivePath, spec);
             } catch (IOException exception) {
-                feedback.accept("Cached default overworld archive is invalid; downloading a replacement.");
+                feedback.accept("Cached Iris " + spec.key() + " beta archive is invalid; downloading a replacement.");
             }
         }
+        String cachedSource = metadata.getProperty("source", "");
+        boolean cacheMatchesSource = validCache && (spec.source().toString().equals(cachedSource)
+                || cachedSource.isBlank() && "overworld".equals(spec.key()));
         long fetchedAt = parseLong(metadata.getProperty("fetchedAt"), 0L);
-        boolean fresh = validCache && options.clock().millis() - fetchedAt < options.refreshInterval().toMillis();
+        boolean fresh = cacheMatchesSource
+                && options.clock().millis() - fetchedAt < options.refreshInterval().toMillis();
         if (fresh) {
+            if (cachedSource.isBlank()) {
+                metadata.setProperty("source", spec.source().toString());
+                storePropertiesAtomic(metadataPath, metadata);
+            }
             return new Archive(archivePath, sha256(archivePath));
         }
 
         IOException networkFailure = null;
         for (int attempt = 1; attempt <= options.attempts(); attempt++) {
             try {
-                HttpRequest.Builder request = HttpRequest.newBuilder(options.source())
+                HttpRequest.Builder request = HttpRequest.newBuilder(spec.source())
                         .timeout(options.requestTimeout())
                         .header("Accept", "application/octet-stream")
                         .header("User-Agent", "Iris-Bootstrap")
                         .GET();
-                if (validCache) {
+                if (cacheMatchesSource) {
                     String etag = metadata.getProperty("etag");
                     String lastModified = metadata.getProperty("lastModified");
                     if (etag != null && !etag.isBlank()) {
@@ -318,8 +411,9 @@ public final class DefaultPackBootstrapProvisioner {
                 }
                 HttpResponse<InputStream> response = options.client().send(request.build(), HttpResponse.BodyHandlers.ofInputStream());
                 int status = response.statusCode();
-                if (status == 304 && validCache) {
+                if (status == 304 && cacheMatchesSource) {
                     close(response.body());
+                    metadata.setProperty("source", spec.source().toString());
                     metadata.setProperty("fetchedAt", Long.toString(options.clock().millis()));
                     storePropertiesAtomic(metadataPath, metadata);
                     return new Archive(archivePath, sha256(archivePath));
@@ -328,10 +422,10 @@ public final class DefaultPackBootstrapProvisioner {
                     Path temporary = cacheRoot.resolve(".download-" + UUID.randomUUID() + ".zip");
                     try {
                         try (InputStream input = response.body(); OutputStream output = Files.newOutputStream(temporary)) {
-                            copyLimited(input, output, options.maxArchiveBytes());
+                            copyLimited(input, output, options.maxArchiveBytes(), spec);
                         }
-                        if (!validateArchive(temporary)) {
-                            throw new IOException("Downloaded default overworld archive is invalid");
+                        if (!validateArchive(temporary, spec)) {
+                            throw new IOException("Downloaded Iris " + spec.key() + " beta archive is invalid");
                         }
                         move(temporary, archivePath, true);
                     } finally {
@@ -340,14 +434,15 @@ public final class DefaultPackBootstrapProvisioner {
                     Properties updated = new Properties();
                     response.headers().firstValue("etag").ifPresent(value -> updated.setProperty("etag", value));
                     response.headers().firstValue("last-modified").ifPresent(value -> updated.setProperty("lastModified", value));
+                    updated.setProperty("source", spec.source().toString());
                     updated.setProperty("fetchedAt", Long.toString(options.clock().millis()));
                     updated.setProperty("sha256", sha256(archivePath));
                     storePropertiesAtomic(metadataPath, updated);
-                    feedback.accept("Downloaded the Iris default overworld beta archive.");
+                    feedback.accept("Downloaded the Iris " + spec.key() + " beta archive.");
                     return new Archive(archivePath, updated.getProperty("sha256"));
                 }
                 close(response.body());
-                IOException statusFailure = new IOException("Default overworld download returned HTTP " + status);
+                IOException statusFailure = new IOException("Iris " + spec.key() + " beta download returned HTTP " + status);
                 if (!retryableStatus(status)) {
                     networkFailure = statusFailure;
                     break;
@@ -355,7 +450,7 @@ public final class DefaultPackBootstrapProvisioner {
                 networkFailure = statusFailure;
             } catch (InterruptedException exception) {
                 Thread.currentThread().interrupt();
-                throw new IOException("Default overworld download was interrupted", exception);
+                throw new IOException("Iris " + spec.key() + " beta download was interrupted", exception);
             } catch (IOException exception) {
                 networkFailure = exception;
             }
@@ -364,79 +459,38 @@ public final class DefaultPackBootstrapProvisioner {
             }
         }
 
-        if (validCache) {
-            feedback.accept("Default overworld download failed; using the validated cached archive.");
+        if (cacheMatchesSource) {
+            feedback.accept("Iris " + spec.key() + " beta download failed; using the validated cached archive.");
             return new Archive(archivePath, sha256(archivePath));
         }
-        if (isManagedPackOutputUsable(marker, cacheRoot.getParent().getParent())) {
-            String sourceSha = marker.getProperty("sourceSha256");
+        if (isManagedPackOutputUsable(marker, cacheRoot.getParent().getParent(), spec)) {
+            String sourceSha = markerProperty(marker, spec, "sourceSha256");
             return new Archive(null, sourceSha);
         }
         throw networkFailure == null
-                ? new IOException("Default overworld archive is unavailable and no valid cache exists")
-                : new IOException("Default overworld archive is unavailable and no valid cache exists", networkFailure);
+                ? new IOException("Iris " + spec.key() + " beta archive is unavailable and no valid cache exists")
+                : new IOException("Iris " + spec.key() + " beta archive is unavailable and no valid cache exists", networkFailure);
     }
 
-    private static boolean isManagedPackOutputUsable(Properties marker, Path dataDirectory) {
-        String sourceSha = marker.getProperty("sourceSha256");
-        String expectedFingerprint = marker.getProperty("defaultPackFingerprint");
+    private static boolean isManagedPackOutputUsable(Properties marker, Path dataDirectory, PackSpec spec) {
+        String sourceSha = markerProperty(marker, spec, "sourceSha256");
+        String expectedFingerprint = markerProperty(marker, spec, "fingerprint");
         if (sourceSha == null || sourceSha.isBlank() || expectedFingerprint == null || expectedFingerprint.isBlank()) {
             return false;
         }
-        Path packRoot = dataDirectory.toAbsolutePath().normalize().resolve("packs/overworld");
+        Path packRoot = dataDirectory.toAbsolutePath().normalize().resolve("packs").resolve(spec.key());
         try {
-            return isPackRoot(packRoot) && directoryFingerprint(packRoot).equals(expectedFingerprint);
+            return isPackRoot(packRoot, spec) && directoryFingerprint(packRoot).equals(expectedFingerprint);
         } catch (IOException | RuntimeException exception) {
             return false;
         }
     }
 
-    static Path resolveLevelRoot(Path serverRoot) throws IOException {
-        Path normalizedServerRoot = serverRoot.toAbsolutePath().normalize();
-        String levelName = readConfiguredLevelName(normalizedServerRoot);
-        Path configured = Path.of(levelName);
-        return configured.isAbsolute()
-                ? configured.normalize()
-                : normalizedServerRoot.resolve(configured).normalize();
+    public static Path resolveLevelRoot(Path serverRoot) throws IOException {
+        return BukkitStartupPaths.resolve(serverRoot).levelRoot();
     }
 
-    private static String readConfiguredLevelName(Path serverRoot) throws IOException {
-        String levelName = "world";
-        Path propertiesFile = serverRoot.resolve("server.properties");
-        if (Files.isRegularFile(propertiesFile)) {
-            Properties properties = loadProperties(propertiesFile);
-            levelName = properties.getProperty("level-name", levelName);
-        }
-
-        String[] arguments = ProcessHandle.current().info().arguments().orElse(new String[0]);
-        for (int index = 0; index < arguments.length; index++) {
-            String argument = arguments[index];
-            String following = index + 1 < arguments.length ? arguments[index + 1] : null;
-            String parsed = parseLevelArgument(argument, following);
-            if (parsed != null) {
-                levelName = parsed;
-            }
-        }
-        if (levelName.isBlank()) {
-            throw new IOException("Configured level name is empty");
-        }
-        return levelName;
-    }
-
-    private static String parseLevelArgument(String argument, String following) {
-        for (String key : List.of("-w", "--level-name", "--world")) {
-            if (argument.equals(key) && following != null && !following.isBlank()) {
-                return following;
-            }
-            String prefix = key + "=";
-            if (argument.startsWith(prefix) && argument.length() > prefix.length()) {
-                return argument.substring(prefix.length());
-            }
-        }
-        return null;
-    }
-
-    private static boolean validateArchive(Path archive) throws IOException {
+    private static boolean validateArchive(Path archive, PackSpec spec) throws IOException {
         int entries = 0;
         long expanded = 0L;
         boolean dimensionFound = false;
@@ -446,13 +500,14 @@ public final class DefaultPackBootstrapProvisioner {
             while ((entry = zip.getNextEntry()) != null) {
                 entries++;
                 if (entries > MAX_ARCHIVE_ENTRIES) {
-                    throw new IOException("Default overworld archive contains too many entries");
+                    throw new IOException("Iris " + spec.key() + " beta archive contains too many entries");
                 }
                 String name = normalizedZipEntry(entry.getName());
                 if (!paths.add(name)) {
-                    throw new IOException("Default overworld archive contains duplicate path " + name);
+                    throw new IOException("Iris " + spec.key() + " beta archive contains duplicate path " + name);
                 }
-                if (name.equals("dimensions/overworld.json") || name.endsWith("/dimensions/overworld.json")) {
+                String requiredDimension = "dimensions/" + spec.requiredDimension() + ".json";
+                if (name.equals(requiredDimension) || name.endsWith("/" + requiredDimension)) {
                     dimensionFound = true;
                 }
                 if (!entry.isDirectory()) {
@@ -461,7 +516,7 @@ public final class DefaultPackBootstrapProvisioner {
                     while ((read = zip.read(buffer)) >= 0) {
                         expanded += read;
                         if (expanded > MAX_EXPANDED_BYTES) {
-                            throw new IOException("Default overworld archive expands beyond the safety limit");
+                            throw new IOException("Iris " + spec.key() + " beta archive expands beyond the safety limit");
                         }
                     }
                 }
@@ -471,9 +526,9 @@ public final class DefaultPackBootstrapProvisioner {
         return entries > 0 && dimensionFound;
     }
 
-    private static Path extractArchive(Path archive, Path extractionRoot) throws IOException {
+    private static Path extractArchive(Path archive, Path extractionRoot, PackSpec spec) throws IOException {
         if (archive == null) {
-            throw new IOException("Cached default pack archive is unavailable for required pack rebuild");
+            throw new IOException("Cached Iris " + spec.key() + " beta archive is unavailable for required pack rebuild");
         }
         long expanded = 0L;
         int entries = 0;
@@ -482,12 +537,12 @@ public final class DefaultPackBootstrapProvisioner {
             while ((entry = zip.getNextEntry()) != null) {
                 entries++;
                 if (entries > MAX_ARCHIVE_ENTRIES) {
-                    throw new IOException("Default overworld archive contains too many entries");
+                    throw new IOException("Iris " + spec.key() + " beta archive contains too many entries");
                 }
                 String name = normalizedZipEntry(entry.getName());
                 Path output = extractionRoot.resolve(name).normalize();
                 if (!output.startsWith(extractionRoot)) {
-                    throw new IOException("Unsafe path in default overworld archive: " + entry.getName());
+                    throw new IOException("Unsafe path in Iris " + spec.key() + " beta archive: " + entry.getName());
                 }
                 if (entry.isDirectory()) {
                     Files.createDirectories(output);
@@ -499,7 +554,7 @@ public final class DefaultPackBootstrapProvisioner {
                         while ((read = zip.read(buffer)) >= 0) {
                             expanded += read;
                             if (expanded > MAX_EXPANDED_BYTES) {
-                                throw new IOException("Default overworld archive expands beyond the safety limit");
+                                throw new IOException("Iris " + spec.key() + " beta archive expands beyond the safety limit");
                             }
                             file.write(buffer, 0, read);
                         }
@@ -508,42 +563,44 @@ public final class DefaultPackBootstrapProvisioner {
                 zip.closeEntry();
             }
         }
-        if (isPackRoot(extractionRoot)) {
+        if (isPackRoot(extractionRoot, spec)) {
             return extractionRoot;
         }
         List<Path> candidates = new ArrayList<>();
         try (DirectoryStream<Path> children = Files.newDirectoryStream(extractionRoot)) {
             for (Path child : children) {
-                if (Files.isDirectory(child) && isPackRoot(child)) {
+                if (Files.isDirectory(child) && isPackRoot(child, spec)) {
                     candidates.add(child);
                 }
             }
         }
         if (candidates.size() != 1) {
-            throw new IOException("Default overworld archive has an invalid root layout");
+            throw new IOException("Iris " + spec.key() + " beta archive has an invalid root layout");
         }
         return candidates.getFirst();
     }
 
     private static String normalizedZipEntry(String raw) throws IOException {
         if (raw == null || raw.isBlank() || raw.indexOf('\0') >= 0 || raw.startsWith("/") || raw.startsWith("\\")) {
-            throw new IOException("Invalid path in default overworld archive");
+            throw new IOException("Invalid path in Iris bootstrap pack archive");
         }
         String normalized = raw.replace('\\', '/');
         Path path = Path.of(normalized).normalize();
         if (path.isAbsolute() || path.startsWith("..") || normalized.matches("^[A-Za-z]:.*")) {
-            throw new IOException("Unsafe path in default overworld archive: " + raw);
+            throw new IOException("Unsafe path in Iris bootstrap pack archive: " + raw);
         }
         return path.toString().replace('\\', '/');
     }
 
-    private static boolean isPackRoot(Path path) {
-        return path != null && Files.isRegularFile(path.resolve("dimensions/overworld.json"));
+    private static boolean isPackRoot(Path path, PackSpec spec) {
+        return path != null
+                && Files.isRegularFile(path.resolve("dimensions").resolve(spec.requiredDimension() + ".json"));
     }
 
-    private static void validatePackRoot(Path path) throws IOException {
-        if (!isPackRoot(path)) {
-            throw new IOException("Default overworld pack is missing dimensions/overworld.json at " + path);
+    private static void validatePackRoot(Path path, PackSpec spec) throws IOException {
+        if (!isPackRoot(path, spec)) {
+            throw new IOException("Iris " + spec.key() + " beta pack is missing dimensions/"
+                    + spec.requiredDimension() + ".json at " + path);
         }
     }
 
@@ -636,26 +693,27 @@ public final class DefaultPackBootstrapProvisioner {
     }
 
     private static IOException rollback(
-            Path packRoot,
-            Path packBackup,
-            boolean packReplaced,
+            List<PackPublication> packPublications,
             Path datapackRoot,
             Path datapackBackup,
             boolean datapackReplaced
     ) {
         IOException failure = null;
         try {
-            restore(packRoot, packBackup, packReplaced);
+            restore(datapackRoot, datapackBackup, datapackReplaced);
         } catch (IOException exception) {
             failure = exception;
         }
-        try {
-            restore(datapackRoot, datapackBackup, datapackReplaced);
-        } catch (IOException exception) {
-            if (failure == null) {
-                failure = exception;
-            } else {
-                failure.addSuppressed(exception);
+        for (int index = packPublications.size() - 1; index >= 0; index--) {
+            PackPublication publication = packPublications.get(index);
+            try {
+                restore(publication.root(), publication.backup(), true);
+            } catch (IOException exception) {
+                if (failure == null) {
+                    failure = exception;
+                } else {
+                    failure.addSuppressed(exception);
+                }
             }
         }
         return failure;
@@ -726,14 +784,19 @@ public final class DefaultPackBootstrapProvisioner {
         }
     }
 
-    private static void copyLimited(InputStream input, OutputStream output, long limit) throws IOException {
+    private static void copyLimited(
+            InputStream input,
+            OutputStream output,
+            long limit,
+            PackSpec spec
+    ) throws IOException {
         byte[] buffer = new byte[8192];
         long total = 0L;
         int read;
         while ((read = input.read(buffer)) >= 0) {
             total += read;
             if (total > limit) {
-                throw new IOException("Default overworld archive exceeds the download size limit");
+                throw new IOException("Iris " + spec.key() + " beta archive exceeds the download size limit");
             }
             output.write(buffer, 0, read);
         }
@@ -772,7 +835,7 @@ public final class DefaultPackBootstrapProvisioner {
             Thread.sleep(duration.toMillis());
         } catch (InterruptedException exception) {
             Thread.currentThread().interrupt();
-            throw new IOException("Default overworld retry wait was interrupted", exception);
+            throw new IOException("Iris bootstrap pack retry wait was interrupted", exception);
         }
     }
 
@@ -787,6 +850,24 @@ public final class DefaultPackBootstrapProvisioner {
         }
     }
 
+    private static String markerKey(PackSpec spec, String property) {
+        return "pack." + spec.key() + "." + property;
+    }
+
+    private static String markerProperty(Properties marker, PackSpec spec, String property) {
+        String current = marker.getProperty(markerKey(spec, property));
+        if (current != null || !"overworld".equals(spec.key())) {
+            return current;
+        }
+        return switch (property) {
+            case "source" -> marker.getProperty("source");
+            case "sourceSha256" -> marker.getProperty("sourceSha256");
+            case "managed" -> marker.getProperty("managedDefault");
+            case "fingerprint" -> marker.getProperty("defaultPackFingerprint");
+            default -> null;
+        };
+    }
+
     private static void close(InputStream input) {
         try {
             input.close();
@@ -800,16 +881,16 @@ public final class DefaultPackBootstrapProvisioner {
         UNCHANGED
     }
 
-    public record ProvisionResult(Path packRoot, Path datapackRoot, ProvisionStatus status) {
+    public record ProvisionResult(Map<String, Path> packRoots, Path datapackRoot, ProvisionStatus status) {
         public ProvisionResult {
-            Objects.requireNonNull(packRoot, "packRoot");
+            packRoots = Map.copyOf(Objects.requireNonNull(packRoots, "packRoots"));
             Objects.requireNonNull(datapackRoot, "datapackRoot");
             Objects.requireNonNull(status, "status");
         }
     }
 
     record ProvisionOptions(
-            URI source,
+            List<PackSpec> packs,
             HttpClient client,
             Clock clock,
             Duration refreshInterval,
@@ -820,19 +901,49 @@ public final class DefaultPackBootstrapProvisioner {
             Path levelRoot
     ) {
         ProvisionOptions {
-            Objects.requireNonNull(source, "source");
+            packs = List.copyOf(Objects.requireNonNull(packs, "packs"));
             Objects.requireNonNull(client, "client");
             Objects.requireNonNull(clock, "clock");
             Objects.requireNonNull(refreshInterval, "refreshInterval");
             Objects.requireNonNull(requestTimeout, "requestTimeout");
             Objects.requireNonNull(retryDelay, "retryDelay");
             Objects.requireNonNull(levelRoot, "levelRoot");
-            if (attempts < 1 || maxArchiveBytes < 1L) {
+            if (packs.isEmpty() || attempts < 1 || maxArchiveBytes < 1L) {
                 throw new IllegalArgumentException("Invalid bootstrap provisioning options");
             }
+            Set<String> keys = new HashSet<>();
+            for (PackSpec pack : packs) {
+                if (!keys.add(pack.key())) {
+                    throw new IllegalArgumentException("Duplicate bootstrap pack key '" + pack.key() + "'");
+                }
+            }
         }
     }
 
+    record PackSpec(String key, URI source, String requiredDimension) {
+        PackSpec {
+            Objects.requireNonNull(key, "key");
+            Objects.requireNonNull(source, "source");
+            Objects.requireNonNull(requiredDimension, "requiredDimension");
+            if (!key.matches("[a-z0-9_-]+") || !requiredDimension.matches("[a-z0-9_/-]+")) {
+                throw new IllegalArgumentException("Invalid bootstrap pack specification for '" + key + "'");
+            }
+        }
+    }
+
+    private record PackPlan(
+            PackSpec spec,
+            Path root,
+            boolean existed,
+            boolean managed,
+            Archive archive,
+            boolean replace
+    ) {
+    }
+
+    private record PackPublication(Path root, Path backup) {
+    }
+
     private record Archive(Path path, String sha256) {
         private Archive {
             Objects.requireNonNull(sha256, "sha256");

core/src/main/java/art/arcane/iris/core/pack/PackDownloader.java

@@ -36,10 +36,12 @@ import java.nio.file.Files;
 import java.nio.file.LinkOption;
 import java.nio.file.Path;
 import java.nio.file.StandardOpenOption;
+import java.util.Arrays;
 import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
@@ -55,6 +57,14 @@ public final class PackDownloader {
     private static final String DEFAULT_OVERWORLD_PACK = "overworld";
     private static final String DEFAULT_OVERWORLD_REPOSITORY = "IrisDimensions/overworld";
     private static final String DEFAULT_OVERWORLD_RELEASE_URL = "https://github.com/IrisDimensions/overworld/releases/download/beta/overworld.zip";
+    private static final String UNDERWORLD_PACK = "underworld";
+    private static final String UNDERWORLD_REPOSITORY = "IrisDimensions/underworld";
+    private static final String UNDERWORLD_RELEASE_URL = "https://github.com/IrisDimensions/underworld/releases/download/beta/underworld.zip";
+    private static final List<String> MANAGED_BETA_PACK_KEYS = List.of(DEFAULT_OVERWORLD_PACK, UNDERWORLD_PACK);
+    private static final Map<String, ManagedBetaPack> MANAGED_BETA_PACKS = Map.of(
+            DEFAULT_OVERWORLD_PACK, new ManagedBetaPack(DEFAULT_OVERWORLD_REPOSITORY, DEFAULT_OVERWORLD_RELEASE_URL),
+            UNDERWORLD_PACK, new ManagedBetaPack(UNDERWORLD_REPOSITORY, UNDERWORLD_RELEASE_URL)
+    );
     private static final Pattern GITHUB_REPOSITORY = Pattern.compile("[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+");
     private static final Pattern GITHUB_REF = Pattern.compile("[A-Za-z0-9][A-Za-z0-9._/-]*");
     private static final Pattern COMMIT_SHA = Pattern.compile("[0-9a-fA-F]{40}");
@@ -74,6 +84,14 @@ public final class PackDownloader {
         return DEFAULT_OVERWORLD_PACK.equals(pack);
     }
 
+    public static boolean isManagedBetaPack(String pack) {
+        return pack != null && MANAGED_BETA_PACKS.containsKey(pack);
+    }
+
+    public static List<String> managedBetaPacks() {
+        return MANAGED_BETA_PACK_KEYS;
+    }
+
     public static String defaultOverworldPack() {
         return DEFAULT_OVERWORLD_PACK;
     }
@@ -115,8 +133,40 @@ public final class PackDownloader {
         }
     }
 
+    public static boolean isManagedBetaPackPresent(File packsFolder, String key) {
+        if (!isManagedBetaPack(key) || !isPackPresent(packsFolder, key)) {
+            return false;
+        }
+        File resolvedPack = PackDirectoryResolver.resolveExisting(packsFolder, key);
+        if (resolvedPack == null) {
+            return false;
+        }
+        Path primaryDimension = resolvedPack.toPath().toAbsolutePath().normalize()
+                .resolve("dimensions")
+                .resolve(key + ".json");
+        return !Files.isSymbolicLink(primaryDimension)
+                && Files.isRegularFile(primaryDimension, LinkOption.NOFOLLOW_LINKS);
+    }
+
     public static PackInstallResult downloadDefaultOverworld(File packsFolder, boolean forceOverwrite, Consumer<String> feedback) throws IOException {
-        return download(packsFolder, DEFAULT_OVERWORLD_REPOSITORY, defaultOverworldReleaseUrl(), forceOverwrite, true, DEFAULT_OVERWORLD_PACK, feedback);
+        return downloadManagedBeta(packsFolder, DEFAULT_OVERWORLD_PACK, forceOverwrite, feedback);
+    }
+
+    public static PackInstallResult downloadManagedBeta(File packsFolder, String pack, boolean forceOverwrite,
+                                                        Consumer<String> feedback) throws IOException {
+        ManagedBetaPack managed = pack == null ? null : MANAGED_BETA_PACKS.get(pack);
+        if (managed == null) {
+            throw new IllegalArgumentException("Pack '" + pack + "' has no managed beta release");
+        }
+        return download(
+                packsFolder,
+                managed.repository(),
+                managed.releaseUrl(),
+                forceOverwrite,
+                true,
+                pack,
+                feedback
+        );
     }
 
     /**
@@ -135,7 +185,10 @@ public final class PackDownloader {
         }
         String lockKey = expectedKey != null && !expectedKey.isBlank() ? "key:" + expectedKey : "ref:" + repo + "|" + ref;
         return withDownloadLock(lockKey, () -> {
-            if (!forceOverwrite && isPackPresent(packsFolder, expectedKey)) {
+            boolean present = isManagedBetaPack(expectedKey)
+                    ? isManagedBetaPackPresent(packsFolder, expectedKey)
+                    : isPackPresent(packsFolder, expectedKey);
+            if (!forceOverwrite && present) {
                 sendFeedback(output, IrisLanguage.plain(PackDownloadMessages.ALREADY_INSTALLED, MessageArgument.untrusted("key", expectedKey)));
                 return new PackInstallResult(expectedKey, false, false);
             }
@@ -235,11 +288,11 @@ public final class PackDownloader {
                 sendFeedback(feedback, IrisLanguage.plain(PackDownloadMessages.CHECK_GITHUB));
                 return null;
             }
-            if (dimensions.length != 1) {
-                sendFeedback(feedback, IrisLanguage.plain(PackDownloadMessages.ONE_DIMENSION_REQUIRED));
+            String selectedDimension = selectDimensionKey(dimensions, expectedKey, feedback);
+            if (selectedDimension == null) {
                 return null;
             }
-            IrisDimension dimension = data.getDimensionLoader().load(dimensions[0]);
+            IrisDimension dimension = data.getDimensionLoader().load(selectedDimension);
             if (dimension == null) {
                 sendFeedback(feedback, IrisLanguage.plain(PackDownloadMessages.INVALID_DIMENSION));
                 return null;
@@ -280,6 +333,29 @@ public final class PackDownloader {
         return new PreparedPack(key, name, validation);
     }
 
+    private static String selectDimensionKey(String[] dimensions, String expectedKey,
+                                             Consumer<String> feedback) throws IOException {
+        if (expectedKey == null || expectedKey.isBlank()) {
+            if (dimensions.length != 1) {
+                sendFeedback(feedback, IrisLanguage.plain(PackDownloadMessages.ONE_DIMENSION_REQUIRED));
+                return null;
+            }
+            return dimensions[0];
+        }
+
+        int matches = 0;
+        for (String dimension : dimensions) {
+            if (expectedKey.equals(dimension)) {
+                matches++;
+            }
+        }
+        if (matches != 1) {
+            throw new IOException("Downloaded pack dimensions " + Arrays.toString(dimensions)
+                    + " do not contain exactly one requested key '" + expectedKey + "'");
+        }
+        return expectedKey;
+    }
+
     private static PackInstallResult publishPreparedPack(File packsFolder, Path packsRoot, Path staging, PreparedPack prepared,
                                                          boolean forceOverwrite, Consumer<String> feedback) throws IOException {
         Path target = packsRoot.resolve(prepared.key()).normalize();
@@ -299,15 +375,19 @@ public final class PackDownloader {
             ));
             return null;
         }
-        if (!forceOverwrite && isPackPresent(packsRoot.toFile(), prepared.key())) {
+        boolean present = isManagedBetaPack(prepared.key())
+                ? isManagedBetaPackPresent(packsRoot.toFile(), prepared.key())
+                : isPackPresent(packsRoot.toFile(), prepared.key());
+        if (!forceOverwrite && present) {
             sendFeedback(feedback, IrisLanguage.plain(
                     PackDownloadMessages.PACK_KEY_CONFLICT,
                     MessageArgument.untrusted("key", prepared.key())
             ));
             return null;
         }
-        if (!forceOverwrite && Files.exists(target) && !isPackPresent(packsRoot.toFile(), prepared.key())) {
-            IrisLogging.warn("Replacing partial pack folder " + target + " (no dimension files found).");
+        if (!forceOverwrite && Files.exists(target) && !present) {
+            IrisLogging.warn("Replacing partial pack folder " + target
+                    + " (required primary dimension is missing).");
         }
 
         Optional<IrisData> loadedData = IrisData.getLoaded(new File(packsFolder, prepared.key()));
@@ -572,6 +652,10 @@ public final class PackDownloader {
         return DEFAULT_OVERWORLD_RELEASE_URL;
     }
 
+    static String underworldReleaseUrl() {
+        return UNDERWORLD_RELEASE_URL;
+    }
+
     private static void validateGithubRef(String qualifiedRef) {
         String refPath = qualifiedRef.startsWith("refs/heads/")
                 ? qualifiedRef.substring("refs/heads/".length())
@@ -597,6 +681,9 @@ public final class PackDownloader {
     private record PreparedPack(String key, String name, PackValidationResult validation) {
     }
 
+    private record ManagedBetaPack(String repository, String releaseUrl) {
+    }
+
     public record PackInstallResult(String key, boolean changed, boolean restartRequired) {
     }
 

core/src/main/java/art/arcane/iris/core/service/StudioSVC.java

@@ -68,12 +68,13 @@ import java.nio.file.Files;
 import java.nio.file.LinkOption;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
+import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.LinkedHashSet;
 import java.util.List;
-import java.util.Optional;
 import java.util.Locale;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.function.Consumer;
@@ -97,20 +98,17 @@ public class StudioSVC implements IrisService {
 
     @Override
     public void onEnable() {
-        J.a(() -> {
-            String pack = IrisSettings.get().getGenerator().getDefaultWorldType();
+        VolmitSender console = BukkitPlatform.console();
+        runPackMutation(console, LifecycleOperationCoordinator.OperationKind.PACK_DOWNLOAD,
+                "managed-beta-packs", () -> installMissingManagedBetaPacks(console),
+                "Failed to install Iris managed beta packs at startup.");
 
-            // Presence means a non-empty pack folder: an empty leftover folder must still
-            // trigger the install instead of shadowing it forever.
-            if (!PackDownloader.isPackPresent(getWorkspaceFolder(), pack)) {
-                if (PackDownloader.isDefaultOverworld(pack)) {
-                    IrisLogging.info("Downloading Default Pack " + pack + " (beta release)");
-                    IrisServices.get(StudioSVC.class).downloadDefaultOverworld(BukkitPlatform.console(), false);
-                } else {
-                    IrisLogging.warn("Default pack '" + pack + "' is not installed. Please download it manually with /iris download " + pack);
-                }
-            }
-        });
+        String configuredPack = IrisSettings.get().getGenerator().getDefaultWorldType();
+        if (!PackDownloader.isManagedBetaPack(configuredPack)
+                && !PackDownloader.isPackPresent(getWorkspaceFolder(), configuredPack)) {
+            IrisLogging.warn("Default pack '" + configuredPack
+                    + "' is not installed. Please download it manually with /iris download " + configuredPack);
+        }
     }
 
     @Override
@@ -348,12 +346,15 @@ public class StudioSVC implements IrisService {
         }, IrisLanguage.text(BukkitRuntimeMessages.STUDIO_S_V_C_FAILED_DOWNLOAD, MessageArgument.untrusted("key", String.valueOf(key))));
     }
 
-    public void downloadDefaultOverworld(VolmitSender sender, boolean forceOverwrite) {
-        String key = PackDownloader.defaultOverworldPack();
+    public void downloadManagedBeta(VolmitSender sender, String key, boolean forceOverwrite) {
+        if (!PackDownloader.isManagedBetaPack(key)) {
+            sender.sendMessage("Iris pack '" + key + "' does not have a managed beta release.");
+            return;
+        }
         runPackMutation(sender, LifecycleOperationCoordinator.OperationKind.PACK_DOWNLOAD, key, () -> {
-            DownloadOutcome outcome = downloadDefaultOverworldLocked(sender, forceOverwrite);
+            DownloadOutcome outcome = downloadManagedBetaLocked(sender, key, forceOverwrite);
             return finishStandalonePackMutation(sender, outcome);
-        }, IrisLanguage.text(BukkitRuntimeMessages.STUDIO_S_V_C_FAILED_DOWNLOAD_IRISDIMENSIONS_OVERWORLD_BETA_RELEASE));
+        }, "Failed to download IrisDimensions/" + key + " beta release.");
     }
 
     public void downloadBranch(VolmitSender sender, String repo, String branch, boolean forceOverwrite) {
@@ -380,8 +381,8 @@ public class StudioSVC implements IrisService {
     }
 
     private DownloadOutcome downloadSearchLocked(VolmitSender sender, String key, boolean forceOverwrite) throws IOException {
-        if (PackDownloader.isDefaultOverworld(key)) {
-            return downloadDefaultOverworldLocked(sender, forceOverwrite);
+        if (PackDownloader.isManagedBetaPack(key)) {
+            return downloadManagedBetaLocked(sender, key, forceOverwrite);
         }
 
         String descriptor = key.contains("/") ? key : getListing(false).get(key);
@@ -411,21 +412,52 @@ public class StudioSVC implements IrisService {
         return new PackListingReference(repository, ref, expectedKey);
     }
 
-    private DownloadOutcome downloadDefaultOverworldLocked(VolmitSender sender, boolean forceOverwrite) throws IOException {
-        String expectedKey = PackDownloader.defaultOverworldPack();
-        if (!forceOverwrite && PackDownloader.isPackPresent(getWorkspaceFolder(), expectedKey)) {
+    private DownloadOutcome downloadManagedBetaLocked(
+            VolmitSender sender,
+            String expectedKey,
+            boolean forceOverwrite
+    ) throws IOException {
+        if (!forceOverwrite && PackDownloader.isManagedBetaPackPresent(getWorkspaceFolder(), expectedKey)) {
             sender.sendMessage(IrisLanguage.text(PackDownloadMessages.ALREADY_INSTALLED, MessageArgument.untrusted("key", expectedKey)));
             return DownloadOutcome.notChanged();
         }
 
-        PackDownloader.PackInstallResult result = PackDownloader.downloadDefaultOverworld(
+        PackDownloader.PackInstallResult result = PackDownloader.downloadManagedBeta(
                 getWorkspaceFolder(),
+                expectedKey,
                 forceOverwrite,
                 sender::sendMessage
         );
         return DownloadOutcome.from(result);
     }
 
+    private boolean installMissingManagedBetaPacks(VolmitSender sender) {
+        boolean changed = false;
+        boolean restartRequired = false;
+        for (String key : missingManagedBetaPacks(getWorkspaceFolder())) {
+            IrisLogging.info("Downloading managed Iris pack " + key + " (beta release)");
+            try {
+                DownloadOutcome outcome = downloadManagedBetaLocked(sender, key, false);
+                changed |= outcome.changed();
+                restartRequired |= outcome.restartRequired();
+            } catch (Throwable failure) {
+                IrisLogging.reportError("Failed to download IrisDimensions/" + key + " beta release.", failure);
+                sender.sendMessage("Failed to download IrisDimensions/" + key + " beta release. " + errorDetail(failure));
+            }
+        }
+        return finishStandalonePackMutation(sender, new DownloadOutcome(changed, restartRequired));
+    }
+
+    static List<String> missingManagedBetaPacks(File workspaceFolder) {
+        List<String> missing = new ArrayList<>();
+        for (String key : PackDownloader.managedBetaPacks()) {
+            if (!PackDownloader.isManagedBetaPackPresent(workspaceFolder, key)) {
+                missing.add(key);
+            }
+        }
+        return List.copyOf(missing);
+    }
+
     private DownloadOutcome downloadLocked(
             VolmitSender sender,
             String repo,

core/src/main/java/art/arcane/iris/util/common/misc/ServerProperties.java

@@ -1,7 +1,6 @@
 package art.arcane.iris.util.common.misc;
 
-import org.jetbrains.annotations.NotNull;
-import org.jetbrains.annotations.Nullable;
+import art.arcane.iris.core.lifecycle.BukkitStartupPaths;
 
 import java.io.File;
 import java.io.FileInputStream;
@@ -16,48 +15,19 @@ public class ServerProperties {
     public static final String LEVEL_NAME;
 
     static {
-        String[] args = ProcessHandle.current()
-                .info()
-                .arguments()
-                .orElse(new String[0]);
-
-        String propertiesPath = "server.properties";
-        String bukkitYml = "bukkit.yml";
-        String levelName = null;
-
-        for (int i = 0; i < args.length; i++) {
-            String arg = args[i];
-            String next = i < args.length - 1 ? args[i + 1] : null;
-
-            propertiesPath = parse(arg, next, propertiesPath, "-c", "--config");
-            bukkitYml = parse(arg, next, bukkitYml, "-b", "--bukkit-settings");
-            levelName = parse(arg, next, levelName, "-w", "--level-name", "--world");
+        BukkitStartupPaths startupPaths;
+        try {
+            startupPaths = BukkitStartupPaths.resolveCurrent();
+        } catch (IOException exception) {
+            throw new RuntimeException(exception);
         }
-
-        SERVER_PROPERTIES = new File(propertiesPath);
-        BUKKIT_YML = new File(bukkitYml);
-        try (FileInputStream in = new FileInputStream(SERVER_PROPERTIES)){
+        SERVER_PROPERTIES = startupPaths.serverProperties().toFile();
+        BUKKIT_YML = startupPaths.bukkitConfiguration().toFile();
+        try (FileInputStream in = new FileInputStream(SERVER_PROPERTIES)) {
             DATA.load(in);
-        } catch (IOException e) {
-            throw new RuntimeException(e);
+        } catch (IOException exception) {
+            throw new RuntimeException(exception);
         }
-
-        if (levelName != null) LEVEL_NAME = levelName;
-        else LEVEL_NAME = DATA.getProperty("level-name", "world");
-    }
-
-    private static String parse(
-            @NotNull String current,
-            @Nullable String next,
-            String fallback,
-            @NotNull String @NotNull ... keys
-    ) {
-        for (String k : keys) {
-            if (current.equals(k) && next != null)
-                return next;
-            if (current.startsWith(k + "=") && current.length() > k.length() + 1)
-                return current.substring(k.length() + 1);
-        }
-        return fallback;
+        LEVEL_NAME = startupPaths.levelName();
     }
 }

core/src/main/resources/languages/de_DE.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Dimension oder Pack, mit der bzw. dem die Welt erstellt wird",
     "iris.director.commandiris.param.seed_generate_world_with": "Seed für die Generierung der Welt",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Ob man diese Welt automatisch als Hauptwelt benutzt oder nicht",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Den exakten vorhandenen Welt-Slot beim nächsten Neustart ersetzen",
     "iris.director.commandiris.director.teleport_another_world": "Teleportieren in eine andere Welt",
     "iris.director.commandiris.param.world_teleport": "Zielwelt der Teleportation",
     "iris.director.commandiris.param.player_teleport": "Zu teleportierender Spieler",

core/src/main/resources/languages/es_ES.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "La dimensión o el pack con el que se creará el mundo",
     "iris.director.commandiris.param.seed_generate_world_with": "La semilla con la que se generará el mundo",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Indica si este mundo debe usarse automáticamente como mundo principal",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Reemplazar el espacio exacto del mundo existente en el próximo reinicio",
     "iris.director.commandiris.director.teleport_another_world": "Teletransportarse a otro mundo",
     "iris.director.commandiris.param.world_teleport": "El mundo al que se teletransportará",
     "iris.director.commandiris.param.player_teleport": "El jugador que se teletransportará",

core/src/main/resources/languages/fi_FI.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Ulottuvuus / paketti luoda maailma",
     "iris.director.commandiris.param.seed_generate_world_with": "Siemenet tuottaa maailman kanssa",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Käytetäänkö tätä maailmaa automaattisesti päämaailmana",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Korvaa täsmällinen olemassa oleva maailmapaikka seuraavalla uudelleenkäynnistyksellä",
     "iris.director.commandiris.director.teleport_another_world": "Teleporttautuminen toiseen maailmaan",
     "iris.director.commandiris.param.world_teleport": "Maailman teleportata",
     "iris.director.commandiris.param.player_teleport": "Pelaaja teleportata",

core/src/main/resources/languages/fr_FR.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "La dimension ou le pack avec lequel créer le monde",
     "iris.director.commandiris.param.seed_generate_world_with": "La graine avec laquelle générer le monde",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Indique si ce monde doit être utilisé automatiquement comme monde principal",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Remplacer l’emplacement exact du monde existant au prochain redémarrage",
     "iris.director.commandiris.director.teleport_another_world": "Se téléporter vers un autre monde",
     "iris.director.commandiris.param.world_teleport": "Le monde vers lequel se téléporter",
     "iris.director.commandiris.param.player_teleport": "Le joueur à téléporter",

core/src/main/resources/languages/he_IL.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "המימד / החבילה ליצירת העולם עם",
     "iris.director.commandiris.param.seed_generate_world_with": "הזרע ליצור את העולם עם",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "בין אם להשתמש בעולם באופן אוטומטי כעולם הראשי",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "החלפת משבצת העולם הקיימת המדויקת בהפעלה מחדש הבאה",
     "iris.director.commandiris.director.teleport_another_world": "טלפורט לעולם אחר",
     "iris.director.commandiris.param.world_teleport": "העולם לטלפורט",
     "iris.director.commandiris.param.player_teleport": "שחקן לטלפורט",

core/src/main/resources/languages/it_IT.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "La dimensione o il Pack con cui creare il mondo",
     "iris.director.commandiris.param.seed_generate_world_with": "Il seed con cui generare il mondo",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Che sia o meno utilizzare automaticamente questo mondo come il mondo principale",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Sostituisci lo slot esatto del mondo esistente al prossimo riavvio",
     "iris.director.commandiris.director.teleport_another_world": "Teletrasporto in un altro mondo",
     "iris.director.commandiris.param.world_teleport": "Il mondo verso cui teletrasportarsi",
     "iris.director.commandiris.param.player_teleport": "Il giocatore da teletrasportare",

core/src/main/resources/languages/ja-JP.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "ワールドの作成に使用するディメンションまたはパック",
     "iris.director.commandiris.param.seed_generate_world_with": "ワールドの生成に使用するシード",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "このワールドをメインワールドとして自動設定するかどうか",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "次回の再起動時に既存の正確なワールドスロットを置き換える",
     "iris.director.commandiris.director.teleport_another_world": "別のワールドへテレポートします",
     "iris.director.commandiris.param.world_teleport": "テレポート先のワールド",
     "iris.director.commandiris.param.player_teleport": "テレポートさせるプレイヤー",

core/src/main/resources/languages/ko_KR.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "월드 생성에 사용할 차원 또는 팩",
     "iris.director.commandiris.param.seed_generate_world_with": "월드 생성에 사용할 시드",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "이 월드를 메인 월드로 자동 사용할지 여부",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "다음 재시작 시 기존의 정확한 월드 슬롯 교체",
     "iris.director.commandiris.director.teleport_another_world": "다른 월드로 순간이동합니다",
     "iris.director.commandiris.param.world_teleport": "순간이동할 월드",
     "iris.director.commandiris.param.player_teleport": "순간이동시킬 플레이어",

core/src/main/resources/languages/lt_LT.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "dimensija / paketas sukurti pasaulį su",
     "iris.director.commandiris.param.seed_generate_world_with": "Sėkla generuoti pasaulį su",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Ar automatiškai naudoti šį pasaulį kaip pagrindinį pasaulį",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Per kitą paleidimą iš naujo pakeisti tikslų esamą pasaulio lizdą",
     "iris.director.commandiris.director.teleport_another_world": "Teleportas į kitą pasaulį",
     "iris.director.commandiris.param.world_teleport": "Pasaulis teleportui į",
     "iris.director.commandiris.param.player_teleport": "Žaidėjas į teleportą",

core/src/main/resources/languages/nl_NL.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "De dimensie/pack om de wereld te creëren met",
     "iris.director.commandiris.param.seed_generate_world_with": "Het zaad om de wereld te genereren met",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Of deze wereld automatisch gebruikt moet worden als de belangrijkste wereld",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "De exacte bestaande wereldsleuf bij de volgende herstart vervangen",
     "iris.director.commandiris.director.teleport_another_world": "Teleporteren naar een andere wereld",
     "iris.director.commandiris.param.world_teleport": "Wereld te teleporteren naar",
     "iris.director.commandiris.param.player_teleport": "Speler naar teleporteren",

core/src/main/resources/languages/pl_PL.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Wymiar / pakiet do tworzenia świata z",
     "iris.director.commandiris.param.seed_generate_world_with": "Nasienie do generowania świata z",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Czy automatycznie używać tego świata jako głównego świata",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Zastąp dokładny istniejący slot świata przy następnym restarcie",
     "iris.director.commandiris.director.teleport_another_world": "Teleport do innego świata",
     "iris.director.commandiris.param.world_teleport": "Świat teleportować do",
     "iris.director.commandiris.param.player_teleport": "Gracz do teleportowania",

core/src/main/resources/languages/pt_PT.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "A dimensão/pack para criar o mundo com",
     "iris.director.commandiris.param.seed_generate_world_with": "A semente para gerar o mundo com",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Se deve ou não usar automaticamente este mundo como o mundo principal",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Substituir o espaço exato do mundo existente no próximo reinício",
     "iris.director.commandiris.director.teleport_another_world": "Teletransporte para outro mundo",
     "iris.director.commandiris.param.world_teleport": "Mundo para teletransportar",
     "iris.director.commandiris.param.player_teleport": "Jogador para teletransportar",

core/src/main/resources/languages/ru_RU.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Размер/пакет для создания мира",
     "iris.director.commandiris.param.seed_generate_world_with": "Семя, чтобы создать мир с",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Использовать или не использовать этот мир как основной",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Заменить точный существующий слот мира при следующем перезапуске",
     "iris.director.commandiris.director.teleport_another_world": "Телепорт в другой мир",
     "iris.director.commandiris.param.world_teleport": "Телепортироваться в мир",
     "iris.director.commandiris.param.player_teleport": "Игрок телепортируется",

core/src/main/resources/languages/tr_TR.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Dünyayı yaratmak için boyut / paket",
     "iris.director.commandiris.param.seed_generate_world_with": "Dünyayı üretmek için tohum",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Bu dünyayı ana dünya olarak otomatik olarak kullanıp kullanmayalım",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Bir sonraki yeniden başlatmada mevcut tam dünya yuvasını değiştir",
     "iris.director.commandiris.director.teleport_another_world": "Teleport başka bir dünyaya",
     "iris.director.commandiris.param.world_teleport": "Dünya teleport'a",
     "iris.director.commandiris.param.player_teleport": "Oyuncuya teleport",

core/src/main/resources/languages/vi_VI.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "Kích thước/ lốc tạo ra thế giới với",
     "iris.director.commandiris.param.seed_generate_world_with": "Hạt giống để tạo ra thế giới với",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "Có nên tự động sử dụng thế giới này làm thế giới chính hay không",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "Thay thế đúng vị trí thế giới hiện có vào lần khởi động lại tiếp theo",
     "iris.director.commandiris.director.teleport_another_world": "Name",
     "iris.director.commandiris.param.world_teleport": "Thế giới có thể dịch chuyển",
     "iris.director.commandiris.param.player_teleport": "Người chơi cần dịch chuyển",

core/src/main/resources/languages/zh_CN.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "用于创建世界的维度包",
     "iris.director.commandiris.param.seed_generate_world_with": "用于生成世界的种子",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "是否自动将此世界设为主世界",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "在下次重启时替换指定的现有世界槽位",
     "iris.director.commandiris.director.teleport_another_world": "传送到另一个世界",
     "iris.director.commandiris.param.world_teleport": "要传送到的世界",
     "iris.director.commandiris.param.player_teleport": "要传送的玩家",

core/src/main/resources/languages/zh_TW.json

@@ -697,6 +697,7 @@
     "iris.director.commandiris.param.dimension_pack_create_world_with": "用於建立世界的維度包",
     "iris.director.commandiris.param.seed_generate_world_with": "用於生成世界的種子",
     "iris.director.commandiris.param.whether_not_automatically_use_this_world_as_main_world": "是否自動將此世界設為主世界",
+    "iris.director.commandiris.param.replace_exact_existing_world_slot_next_restart": "在下次重新啟動時取代指定的現有世界槽位",
     "iris.director.commandiris.director.teleport_another_world": "傳送到另一個世界",
     "iris.director.commandiris.param.world_teleport": "要傳送到的世界",
     "iris.director.commandiris.param.player_teleport": "要傳送的玩家",

core/src/test/java/art/arcane/iris/core/ExactWorldSlotPathPolicyTest.java

@@ -1,6 +1,5 @@
 package art.arcane.iris.core;
 
-import org.bukkit.NamespacedKey;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
@@ -22,22 +21,22 @@ public class ExactWorldSlotPathPolicyTest {
         Path canonicalRoot = levelRoot.toRealPath();
         List<SlotExpectation> expectations = List.of(
                 new SlotExpectation(
-                        new NamespacedKey("iris", "underworld"),
+                        new WorldSlotKey("iris", "underworld"),
                         ExactWorldSlotPathPolicy.SlotKind.IRIS_MANAGED,
                         "dimensions/iris/underworld"
                 ),
                 new SlotExpectation(
-                        NamespacedKey.minecraft("overworld"),
+                        WorldSlotKey.minecraft("overworld"),
                         ExactWorldSlotPathPolicy.SlotKind.VANILLA_OVERWORLD,
                         "dimensions/minecraft/overworld"
                 ),
                 new SlotExpectation(
-                        NamespacedKey.minecraft("the_nether"),
+                        WorldSlotKey.minecraft("the_nether"),
                         ExactWorldSlotPathPolicy.SlotKind.VANILLA_NETHER,
                         "dimensions/minecraft/the_nether"
                 ),
                 new SlotExpectation(
-                        NamespacedKey.minecraft("the_end"),
+                        WorldSlotKey.minecraft("the_end"),
                         ExactWorldSlotPathPolicy.SlotKind.VANILLA_END,
                         "dimensions/minecraft/the_end"
                 )
@@ -63,7 +62,7 @@ public class ExactWorldSlotPathPolicyTest {
 
         ExactWorldSlotPathPolicy.Target target = ExactWorldSlotPathPolicy.resolve(
                 levelRoot,
-                NamespacedKey.minecraft("the_nether")
+                WorldSlotKey.minecraft("the_nether")
         );
 
         assertEquals(worldDirectory.toRealPath(), target.worldDirectory());
@@ -75,15 +74,15 @@ public class ExactWorldSlotPathPolicyTest {
 
         ExactWorldSlotPathPolicy.Rejection foreign = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
-                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, new NamespacedKey("foreign", "world"))
+                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, new WorldSlotKey("foreign", "world"))
         );
         ExactWorldSlotPathPolicy.Rejection nestedIris = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
-                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, new NamespacedKey("iris", "nested/world"))
+                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, new WorldSlotKey("iris", "nested/world"))
         );
         ExactWorldSlotPathPolicy.Rejection unsupportedMinecraft = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
-                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, NamespacedKey.minecraft("custom"))
+                () -> ExactWorldSlotPathPolicy.resolve(levelRoot, WorldSlotKey.minecraft("custom"))
         );
 
         assertEquals(ExactWorldSlotPathPolicy.RejectionReason.FOREIGN_NAMESPACE, foreign.reason());
@@ -97,7 +96,7 @@ public class ExactWorldSlotPathPolicyTest {
     @Test
     public void validatesOnlyTheExactExpectedCandidate() throws Exception {
         Path levelRoot = temporaryFolder.newFolder("candidate-policy").toPath();
-        NamespacedKey worldKey = NamespacedKey.minecraft("the_nether");
+        WorldSlotKey worldKey = WorldSlotKey.minecraft("the_nether");
         Path expected = levelRoot.toRealPath().resolve("dimensions/minecraft/the_nether");
 
         ExactWorldSlotPathPolicy.Target target = ExactWorldSlotPathPolicy.validate(
@@ -136,7 +135,7 @@ public class ExactWorldSlotPathPolicyTest {
                 ExactWorldSlotPathPolicy.Rejection.class,
                 () -> ExactWorldSlotPathPolicy.resolve(
                         levelRoot.resolve("child/.."),
-                        new NamespacedKey("iris", "underworld")
+                        new WorldSlotKey("iris", "underworld")
                 )
         );
 
@@ -148,41 +147,41 @@ public class ExactWorldSlotPathPolicyTest {
         Path linkedLevelTarget = temporaryFolder.newFolder("linked-level-target").toPath();
         Path levelLink = temporaryFolder.getRoot().toPath().resolve("linked-level");
         Files.createSymbolicLink(levelLink, linkedLevelTarget);
-        assertSymbolicLinkRejected(levelLink, new NamespacedKey("iris", "underworld"));
+        assertSymbolicLinkRejected(levelLink, new WorldSlotKey("iris", "underworld"));
 
         Path dimensionsLevel = temporaryFolder.newFolder("linked-dimensions").toPath();
         Path externalDimensions = temporaryFolder.newFolder("external-dimensions").toPath();
         Files.createSymbolicLink(dimensionsLevel.resolve("dimensions"), externalDimensions);
-        assertSymbolicLinkRejected(dimensionsLevel, new NamespacedKey("iris", "underworld"));
+        assertSymbolicLinkRejected(dimensionsLevel, new WorldSlotKey("iris", "underworld"));
 
         Path namespaceLevel = temporaryFolder.newFolder("linked-namespace").toPath();
         Path dimensions = Files.createDirectories(namespaceLevel.resolve("dimensions"));
         Path externalNamespace = temporaryFolder.newFolder("external-namespace").toPath();
         Files.createSymbolicLink(dimensions.resolve("minecraft"), externalNamespace);
-        assertSymbolicLinkRejected(namespaceLevel, NamespacedKey.minecraft("the_nether"));
+        assertSymbolicLinkRejected(namespaceLevel, WorldSlotKey.minecraft("the_nether"));
 
         Path targetLevel = temporaryFolder.newFolder("linked-target").toPath();
         Path namespace = Files.createDirectories(targetLevel.resolve("dimensions/minecraft"));
         Path externalTarget = temporaryFolder.newFolder("external-target").toPath();
         Files.createSymbolicLink(namespace.resolve("the_nether"), externalTarget);
-        assertSymbolicLinkRejected(targetLevel, NamespacedKey.minecraft("the_nether"));
+        assertSymbolicLinkRejected(targetLevel, WorldSlotKey.minecraft("the_nether"));
     }
 
     @Test
     public void rejectsNonDirectoryStorageEntries() throws Exception {
         Path dimensionsLevel = temporaryFolder.newFolder("file-dimensions").toPath();
         Files.writeString(dimensionsLevel.resolve("dimensions"), "not a directory");
-        assertUnsafeEntryRejected(dimensionsLevel, new NamespacedKey("iris", "underworld"));
+        assertUnsafeEntryRejected(dimensionsLevel, new WorldSlotKey("iris", "underworld"));
 
         Path namespaceLevel = temporaryFolder.newFolder("file-namespace").toPath();
         Path dimensions = Files.createDirectories(namespaceLevel.resolve("dimensions"));
         Files.writeString(dimensions.resolve("iris"), "not a directory");
-        assertUnsafeEntryRejected(namespaceLevel, new NamespacedKey("iris", "underworld"));
+        assertUnsafeEntryRejected(namespaceLevel, new WorldSlotKey("iris", "underworld"));
 
         Path targetLevel = temporaryFolder.newFolder("file-target").toPath();
         Path namespace = Files.createDirectories(targetLevel.resolve("dimensions/iris"));
         Files.writeString(namespace.resolve("underworld"), "not a directory");
-        assertUnsafeEntryRejected(targetLevel, new NamespacedKey("iris", "underworld"));
+        assertUnsafeEntryRejected(targetLevel, new WorldSlotKey("iris", "underworld"));
     }
 
     @Test
@@ -191,13 +190,13 @@ public class ExactWorldSlotPathPolicyTest {
 
         ExactWorldSlotPathPolicy.Rejection missingFailure = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
-                () -> ExactWorldSlotPathPolicy.resolve(missing, new NamespacedKey("iris", "underworld"))
+                () -> ExactWorldSlotPathPolicy.resolve(missing, new WorldSlotKey("iris", "underworld"))
         );
         ExactWorldSlotPathPolicy.Rejection filesystemFailure = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
                 () -> ExactWorldSlotPathPolicy.resolve(
                         missing.toAbsolutePath().getRoot(),
-                        new NamespacedKey("iris", "underworld")
+                        new WorldSlotKey("iris", "underworld")
                 )
         );
 
@@ -205,7 +204,7 @@ public class ExactWorldSlotPathPolicyTest {
         assertEquals(ExactWorldSlotPathPolicy.RejectionReason.UNSAFE_ENTRY, filesystemFailure.reason());
     }
 
-    private void assertSymbolicLinkRejected(Path levelRoot, NamespacedKey worldKey) {
+    private void assertSymbolicLinkRejected(Path levelRoot, WorldSlotKey worldKey) {
         ExactWorldSlotPathPolicy.Rejection failure = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
                 () -> ExactWorldSlotPathPolicy.resolve(levelRoot, worldKey)
@@ -214,7 +213,7 @@ public class ExactWorldSlotPathPolicyTest {
         assertEquals(ExactWorldSlotPathPolicy.RejectionReason.SYMBOLIC_LINK, failure.reason());
     }
 
-    private void assertUnsafeEntryRejected(Path levelRoot, NamespacedKey worldKey) {
+    private void assertUnsafeEntryRejected(Path levelRoot, WorldSlotKey worldKey) {
         ExactWorldSlotPathPolicy.Rejection failure = assertThrows(
                 ExactWorldSlotPathPolicy.Rejection.class,
                 () -> ExactWorldSlotPathPolicy.resolve(levelRoot, worldKey)
@@ -224,7 +223,7 @@ public class ExactWorldSlotPathPolicyTest {
     }
 
     private record SlotExpectation(
-            NamespacedKey worldKey,
+            WorldSlotKey worldKey,
             ExactWorldSlotPathPolicy.SlotKind slotKind,
             String relativePath
     ) {

core/src/test/java/art/arcane/iris/core/lifecycle/BukkitStartupPathsTest.java

@@ -0,0 +1,260 @@
+package art.arcane.iris.core.lifecycle;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+public class BukkitStartupPathsTest {
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    @Test
+    public void defaultsResolveAgainstServerWorkingDirectory() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(serverRoot, new String[0]);
+
+        assertEquals(serverRoot, paths.serverRoot());
+        assertEquals(serverRoot.resolve("server.properties"), paths.serverProperties());
+        assertEquals(serverRoot.resolve("bukkit.yml"), paths.bukkitConfiguration());
+        assertEquals(serverRoot, paths.worldContainer());
+        assertEquals("world", paths.levelName());
+        assertEquals(serverRoot.resolve("world"), paths.levelRoot());
+    }
+
+    @Test
+    public void shortSeparatedArgumentsOverrideRelativeDefaults() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path configurationRoot = Files.createDirectories(serverRoot.resolve("short-config"));
+        Path properties = configurationRoot.resolve("custom.properties");
+        Path bukkit = configurationRoot.resolve("custom-bukkit.yml");
+        Files.writeString(properties, "level-name=property-world\n", StandardCharsets.UTF_8);
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: relative-worlds\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "-c", "short-config/custom.properties",
+                        "-b", "short-config/custom-bukkit.yml",
+                        "-w", "argument-world"
+                }
+        );
+
+        assertEquals(properties, paths.serverProperties());
+        assertEquals(bukkit, paths.bukkitConfiguration());
+        assertEquals(serverRoot.resolve("relative-worlds"), paths.worldContainer());
+        assertEquals("argument-world", paths.levelName());
+        assertEquals(serverRoot.resolve("relative-worlds/argument-world"), paths.levelRoot());
+    }
+
+    @Test
+    public void longSeparatedArgumentsUseCustomPropertiesAndWorldContainer() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path configurationRoot = Files.createDirectories(serverRoot.resolve("long-config"));
+        Path properties = configurationRoot.resolve("server.properties");
+        Path bukkit = configurationRoot.resolve("bukkit.yml");
+        Files.writeString(properties, "level-name=ignored-property-world\n", StandardCharsets.UTF_8);
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: long-worlds\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "--config", "long-config/server.properties",
+                        "--bukkit-settings", "long-config/bukkit.yml",
+                        "--world", "long-world"
+                }
+        );
+
+        assertEquals(properties, paths.serverProperties());
+        assertEquals(bukkit, paths.bukkitConfiguration());
+        assertEquals(serverRoot.resolve("long-worlds"), paths.worldContainer());
+        assertEquals("long-world", paths.levelName());
+        assertEquals(serverRoot.resolve("long-worlds/long-world"), paths.levelRoot());
+    }
+
+    @Test
+    public void equalsArgumentsResolveRelativePathsAndLastWorldOverride() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path configurationRoot = Files.createDirectories(serverRoot.resolve("equals-config"));
+        Path properties = configurationRoot.resolve("server.properties");
+        Path bukkit = configurationRoot.resolve("bukkit.yml");
+        Files.writeString(properties, "level-name=property-world\n", StandardCharsets.UTF_8);
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: equals-worlds\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "--config=equals-config/server.properties",
+                        "--bukkit-settings=equals-config/bukkit.yml",
+                        "--world=first-world",
+                        "-w=last-world"
+                }
+        );
+
+        assertEquals(properties, paths.serverProperties());
+        assertEquals(bukkit, paths.bukkitConfiguration());
+        assertEquals(serverRoot.resolve("equals-worlds"), paths.worldContainer());
+        assertEquals("last-world", paths.levelName());
+        assertEquals(serverRoot.resolve("equals-worlds/last-world"), paths.levelRoot());
+    }
+
+    @Test
+    public void absoluteConfigurationContainerAndLevelPathsRemainAbsolute() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path configurationRoot = Files.createDirectories(serverRoot.resolve("absolute-config"));
+        Path worldContainer = Files.createDirectories(serverRoot.resolve("absolute-worlds"));
+        Path absoluteLevel = serverRoot.resolve("absolute-level");
+        Path properties = configurationRoot.resolve("server.properties");
+        Path bukkit = configurationRoot.resolve("bukkit.yml");
+        Files.writeString(properties, "level-name=property-world\n", StandardCharsets.UTF_8);
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: '" + worldContainer + "'\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths propertiesPaths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "--config=" + properties,
+                        "--bukkit-settings=" + bukkit
+                }
+        );
+        BukkitStartupPaths argumentPaths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "-c", properties.toString(),
+                        "-b", bukkit.toString(),
+                        "-w", absoluteLevel.toString()
+                }
+        );
+
+        assertEquals(properties, propertiesPaths.serverProperties());
+        assertEquals(bukkit, propertiesPaths.bukkitConfiguration());
+        assertEquals(worldContainer, propertiesPaths.worldContainer());
+        assertEquals(worldContainer.resolve("property-world"), propertiesPaths.levelRoot());
+        assertEquals(absoluteLevel.toString(), argumentPaths.levelName());
+        assertEquals(absoluteLevel, argumentPaths.levelRoot());
+    }
+
+    @Test
+    public void worldContainerArgumentsOverrideBukkitConfiguration() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path bukkit = serverRoot.resolve("bukkit.yml");
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: ignored-worlds\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths shortPaths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{"-W", "short-worlds", "--world=short-level"}
+        );
+        BukkitStartupPaths longPaths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{"--world-dir=first-worlds", "--universe", "second-worlds"}
+        );
+        BukkitStartupPaths explicitPaths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{"--world-container=explicit-worlds"}
+        );
+
+        assertEquals(serverRoot.resolve("short-worlds"), shortPaths.worldContainer());
+        assertEquals(serverRoot.resolve("short-worlds/short-level"), shortPaths.levelRoot());
+        assertEquals(serverRoot.resolve("second-worlds"), longPaths.worldContainer());
+        assertEquals(serverRoot.resolve("second-worlds/world"), longPaths.levelRoot());
+        assertEquals(serverRoot.resolve("explicit-worlds"), explicitPaths.worldContainer());
+        assertEquals(serverRoot.resolve("explicit-worlds/world"), explicitPaths.levelRoot());
+    }
+
+    @Test
+    public void compactShortArgumentsMatchServerOptionParsing() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+        Path configurationRoot = Files.createDirectories(serverRoot.resolve("compact-config"));
+        Path properties = configurationRoot.resolve("server.properties");
+        Path bukkit = configurationRoot.resolve("bukkit.yml");
+        Files.writeString(properties, "level-name=ignored-world\n", StandardCharsets.UTF_8);
+        Files.writeString(
+                bukkit,
+                "settings:\n  world-container: ignored-worlds\n",
+                StandardCharsets.UTF_8
+        );
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{
+                        "-ccompact-config/server.properties",
+                        "-bcompact-config/bukkit.yml",
+                        "-Wcompact-worlds",
+                        "-wcompact-level"
+                }
+        );
+
+        assertEquals(properties, paths.serverProperties());
+        assertEquals(bukkit, paths.bukkitConfiguration());
+        assertEquals(serverRoot.resolve("compact-worlds"), paths.worldContainer());
+        assertEquals("compact-level", paths.levelName());
+        assertEquals(serverRoot.resolve("compact-worlds/compact-level"), paths.levelRoot());
+    }
+
+    @Test
+    public void applicationArgumentsExcludeJvmAndLauncherOptions() {
+        assertArrayEquals(
+                new String[]{"-bconfig/bukkit.yml", "-Wworlds", "-wlevel", "-cserver.properties"},
+                BukkitStartupPaths.applicationArguments(new String[]{
+                        "-Xmx4G",
+                        "-jar",
+                        "paper.jar",
+                        "-bconfig/bukkit.yml",
+                        "-Wworlds",
+                        "-wlevel",
+                        "-cserver.properties"
+                })
+        );
+        assertArrayEquals(
+                new String[]{"-bconfig/bukkit.yml"},
+                BukkitStartupPaths.applicationArguments(new String[]{
+                        "-Xmx4G",
+                        "-cp",
+                        "paper.jar",
+                        "org.bukkit.craftbukkit.Main",
+                        "-bconfig/bukkit.yml"
+                })
+        );
+    }
+
+    @Test
+    public void endOfOptionsStopsStartupOptionParsing() throws Exception {
+        Path serverRoot = temporaryFolder.getRoot().toPath().toRealPath();
+
+        BukkitStartupPaths paths = BukkitStartupPaths.resolve(
+                serverRoot,
+                new String[]{"--", "-bcustom-bukkit.yml", "-Wcustom-worlds", "-wcustom-level"}
+        );
+
+        assertEquals(serverRoot.resolve("bukkit.yml"), paths.bukkitConfiguration());
+        assertEquals(serverRoot, paths.worldContainer());
+        assertEquals("world", paths.levelName());
+        assertEquals(serverRoot.resolve("world"), paths.levelRoot());
+    }
+}

core/src/test/java/art/arcane/iris/core/lifecycle/BukkitWorldConfigurationTest.java

@@ -7,13 +7,20 @@ import org.junit.rules.TemporaryFolder;
 
 import java.io.File;
 import java.io.IOException;
+import java.net.StandardProtocolFamily;
+import java.net.UnixDomainSocketAddress;
+import java.nio.channels.ServerSocketChannel;
+import java.nio.file.FileSystems;
 import java.nio.file.Files;
+import java.nio.file.Path;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assume.assumeNoException;
+import static org.junit.Assume.assumeTrue;
 
 public class BukkitWorldConfigurationTest {
     @Rule
@@ -317,4 +324,48 @@ public class BukkitWorldConfigurationTest {
         assertTrue(failure.getMessage().contains("generator"));
         assertEquals(malformed, Files.readString(configuration.toPath()));
     }
+
+    @Test
+    public void replacementRejectsSymbolicConfigurationWithoutChangingLinkOrTarget() throws Exception {
+        Path target = temporaryFolder.newFile("shared-bukkit.yml").toPath();
+        String original = "settings:\n  allow-end: true\n";
+        Files.writeString(target, original);
+        Path link = temporaryFolder.getRoot().toPath().resolve("bukkit.yml");
+        try {
+            Files.createSymbolicLink(link, target.getFileName());
+        } catch (IOException | UnsupportedOperationException failure) {
+            assumeNoException(failure);
+        }
+        BukkitWorldConfiguration.WorldGeneratorSnapshot expected =
+                new BukkitWorldConfiguration.WorldGeneratorSnapshot(false, false, false, null, false, null);
+
+        assertThrows(IOException.class, () -> BukkitWorldConfiguration.replaceIfMatching(
+                link.toFile(),
+                "world_nether",
+                expected,
+                "underworld",
+                1337L
+        ));
+
+        assertTrue(Files.isSymbolicLink(link));
+        assertEquals(original, Files.readString(target));
+    }
+
+    @Test
+    public void snapshotRejectsSpecialConfigurationWithoutOpeningIt() throws Exception {
+        assumeTrue(FileSystems.getDefault().supportedFileAttributeViews().contains("unix"));
+        Path socket = temporaryFolder.getRoot().toPath().resolve("bukkit.yml");
+        try (ServerSocketChannel channel = ServerSocketChannel.open(StandardProtocolFamily.UNIX)) {
+            try {
+                channel.bind(UnixDomainSocketAddress.of(socket));
+            } catch (IOException | UnsupportedOperationException failure) {
+                assumeNoException(failure);
+            }
+
+            assertThrows(IOException.class, () -> BukkitWorldConfiguration.snapshot(
+                    socket.toFile(),
+                    "world_nether"
+            ));
+        }
+    }
 }

core/src/test/java/art/arcane/iris/core/lifecycle/WorldReplacementBootstrapTest.java

@@ -0,0 +1,386 @@
+package art.arcane.iris.core.lifecycle;
+
+import art.arcane.iris.core.ExactWorldSlotPathPolicy;
+import art.arcane.iris.core.WorldSlotKey;
+import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.WorldGeneratorSnapshot;
+import art.arcane.iris.core.lifecycle.WorldReplacementFilesystem.ReplacementPaths;
+import art.arcane.iris.core.lifecycle.WorldReplacementJournal.Phase;
+import art.arcane.iris.core.lifecycle.WorldReplacementJournal.Transaction;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import java.util.UUID;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+
+public class WorldReplacementBootstrapTest {
+    private static final WorldSlotKey WORLD_KEY = WorldSlotKey.minecraft("the_nether");
+    private static final long SEED = 4242424242L;
+
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    private Path serverRoot;
+    private Path dataDirectory;
+    private Path levelRoot;
+    private Path bukkitConfiguration;
+    private ExactWorldSlotPathPolicy.Target target;
+
+    @Before
+    public void setUp() throws Exception {
+        serverRoot = temporaryFolder.newFolder("server").toPath();
+        dataDirectory = Files.createDirectories(serverRoot.resolve("plugins/Iris"));
+        levelRoot = Files.createDirectories(serverRoot.resolve("world"));
+        bukkitConfiguration = Files.createFile(serverRoot.resolve("bukkit.yml"));
+        target = ExactWorldSlotPathPolicy.resolve(levelRoot, WORLD_KEY);
+        Files.createDirectories(target.namespaceRoot());
+    }
+
+    @Test
+    public void publishesArmedReplacementBeforeRegistryCompilation() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.ARMED, true, "original");
+        configureReplacement(transaction);
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.published());
+        assertEquals("replacement", replacementContent(target.worldDirectory()));
+        assertEquals("original", Files.readString(backup(transaction).resolve("original.txt")));
+        assertEquals(Phase.PUBLISHED, loadSingle().phase());
+    }
+
+    @Test
+    public void publishesArmedReplacementWhenOriginalConfigurationAlreadyMatchesReplacement() throws Exception {
+        configureExistingReplacement();
+        Transaction transaction = stagedTransaction(Phase.ARMED, true, "original");
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.published());
+        assertEquals("replacement", replacementContent(target.worldDirectory()));
+        assertEquals("original", Files.readString(backup(transaction).resolve("original.txt")));
+        assertEquals(Phase.PUBLISHED, loadSingle().phase());
+    }
+
+    @Test
+    public void retainsPublishedReplacementWhenOriginalConfigurationAlreadyMatchesReplacement() throws Exception {
+        configureExistingReplacement();
+        Transaction transaction = stagedTransaction(Phase.PUBLISHED, true, "original");
+        WorldReplacementFilesystem.publish(paths(transaction), true, transaction.packFingerprint());
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.retained());
+        assertEquals("replacement", replacementContent(target.worldDirectory()));
+        assertEquals("original", Files.readString(backup(transaction).resolve("original.txt")));
+        assertEquals(Phase.PUBLISHED, loadSingle().phase());
+    }
+
+    @Test
+    public void cancelsPreparedReplacementWhenOriginalConfigurationAlreadyMatchesReplacement() throws Exception {
+        configureExistingReplacement();
+        Transaction transaction = stagedTransaction(Phase.PREPARED, true, "original");
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.rolledBack());
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertFalse(Files.exists(paths(transaction).stage()));
+        assertTrue(WorldReplacementJournal.load(dataDirectory, levelRoot).isEmpty());
+    }
+
+    @Test
+    public void resumesPublicationAfterOriginalMoveCrash() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.ARMED, true, "original");
+        configureReplacement(transaction);
+        ReplacementPaths paths = paths(transaction);
+        Files.move(paths.target(), paths.backup());
+
+        reconcile();
+
+        assertEquals("replacement", replacementContent(paths.target()));
+        assertEquals("original", Files.readString(paths.backup().resolve("original.txt")));
+        assertEquals(Phase.PUBLISHED, loadSingle().phase());
+    }
+
+    @Test
+    public void cancelsPreparedTransactionWhenConfigurationWasNotApplied() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.PREPARED, true, "original");
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.rolledBack());
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertFalse(Files.exists(paths(transaction).stage()));
+        assertTrue(WorldReplacementJournal.load(dataDirectory, levelRoot).isEmpty());
+    }
+
+    @Test
+    public void restoresPublishedWorldWhenConfigurationWasReverted() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.PUBLISHED, true, "original");
+        configureReplacement(transaction);
+        WorldReplacementFilesystem.publish(paths(transaction), true, transaction.packFingerprint());
+        restoreOriginalConfiguration(transaction);
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.rolledBack());
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertFalse(Files.exists(paths(transaction).stage()));
+        assertFalse(Files.exists(paths(transaction).backup()));
+        assertTrue(WorldReplacementJournal.load(dataDirectory, levelRoot).isEmpty());
+    }
+
+    @Test
+    public void rejectsThirdPartyConfigurationAfterPublicationWithoutMovingStorage() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.PUBLISHED, true, "original");
+        configureReplacement(transaction);
+        WorldReplacementFilesystem.publish(paths(transaction), true, transaction.packFingerprint());
+        WorldGeneratorSnapshot replacement = WorldReplacementBootstrap.replacementSnapshot(transaction);
+        BukkitWorldConfiguration.replaceIfMatching(
+                bukkitConfiguration.toFile(),
+                transaction.worldName(),
+                replacement,
+                "other",
+                SEED
+        );
+
+        assertThrows(IOException.class, this::reconcile);
+
+        assertEquals("replacement", replacementContent(target.worldDirectory()));
+        assertEquals("original", Files.readString(backup(transaction).resolve("original.txt")));
+        assertEquals(Phase.PUBLISHED, loadSingle().phase());
+    }
+
+    @Test
+    public void completesRollbackAcrossPreparedStorageCrashBoundary() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.ROLLBACK_PENDING, true, "original");
+        configureReplacement(transaction);
+        WorldReplacementFilesystem.publish(paths(transaction), true, transaction.packFingerprint());
+        WorldReplacementFilesystem.prepareRollback(paths(transaction), true);
+        restoreOriginalConfiguration(transaction);
+        WorldReplacementJournal.write(dataDirectory, transaction.withPhase(Phase.ROLLBACK_CLEANUP));
+
+        reconcile();
+
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertFalse(Files.exists(paths(transaction).stage()));
+        assertTrue(WorldReplacementJournal.load(dataDirectory, levelRoot).isEmpty());
+    }
+
+    @Test
+    public void retainsVerifiedTargetWhenBackupWasAlreadyCleaned() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.CLEANUP_PENDING, true, "original");
+        configureReplacement(transaction);
+        WorldReplacementFilesystem.publish(paths(transaction), true, transaction.packFingerprint());
+        WorldReplacementFilesystem.cleanupBackup(paths(transaction));
+
+        WorldReplacementBootstrap.ReconcileResult result = reconcile();
+
+        assertEquals(1, result.retained());
+        assertEquals("replacement", replacementContent(target.worldDirectory()));
+        assertEquals(Phase.CLEANUP_PENDING, loadSingle().phase());
+    }
+
+    @Test
+    public void rejectsChangedLevelRootBeforeTouchingStagedStorage() throws Exception {
+        Transaction transaction = stagedTransaction(Phase.ARMED, true, "original");
+        configureReplacement(transaction);
+        Path otherLevelRoot = Files.createDirectories(serverRoot.resolve("renamed-world"));
+
+        assertThrows(
+                IOException.class,
+                () -> WorldReplacementBootstrap.reconcile(
+                        dataDirectory,
+                        otherLevelRoot,
+                        bukkitConfiguration,
+                        ignored -> {
+                        }
+                )
+        );
+
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertTrue(Files.isDirectory(paths(transaction).stage()));
+        assertFalse(Files.exists(paths(transaction).backup()));
+    }
+
+    @Test
+    public void rejectsDuplicateWorldJournalsBeforePublishingEither() throws Exception {
+        Transaction first = stagedTransaction(Phase.ARMED, true, "original");
+        configureReplacement(first);
+        Transaction second = new Transaction(
+                UUID.randomUUID(),
+                first.worldKey(),
+                first.worldName(),
+                first.levelRoot(),
+                first.dimension(),
+                first.seed(),
+                first.packFingerprint(),
+                first.originalConfiguration(),
+                first.originalTargetPresent(),
+                first.phase()
+        );
+        WorldReplacementJournal.write(dataDirectory, second);
+
+        assertThrows(IOException.class, this::reconcile);
+
+        assertTrue(Files.isDirectory(paths(first).stage()));
+        assertEquals("original", Files.readString(target.worldDirectory().resolve("original.txt")));
+        assertFalse(Files.exists(paths(first).backup()));
+    }
+
+    @Test
+    public void roundTripsBlankAndWhitespaceOriginalGenerators() throws Exception {
+        for (String generator : List.of("", "   ")) {
+            WorldGeneratorSnapshot original = new WorldGeneratorSnapshot(
+                    true,
+                    true,
+                    true,
+                    generator,
+                    false,
+                    null
+            );
+            Transaction transaction = transaction(UUID.randomUUID(), original, Phase.PREPARED, false, "replacement");
+            WorldReplacementJournal.write(dataDirectory, transaction);
+
+            Transaction loaded = loadSingle();
+
+            assertEquals(generator, loaded.originalConfiguration().generator());
+            WorldReplacementJournal.delete(dataDirectory, transaction.id());
+        }
+    }
+
+    @Test
+    public void rejectsIrisWorldKeysThatCollideWithConfiguredVanillaAliases() {
+        WorldGeneratorSnapshot original = new WorldGeneratorSnapshot(false, false, false, null, false, null);
+        for (String alias : List.of("world", "world_nether", "world_the_end")) {
+            Transaction transaction = new Transaction(
+                    UUID.randomUUID(),
+                    new WorldSlotKey("iris", alias),
+                    alias,
+                    levelRoot,
+                    "underworld",
+                    SEED,
+                    "0".repeat(64),
+                    original,
+                    false,
+                    Phase.ARMED
+            );
+
+            assertThrows(IOException.class, () -> WorldReplacementJournal.resolveTarget(transaction, levelRoot));
+        }
+    }
+
+    private WorldReplacementBootstrap.ReconcileResult reconcile() throws Exception {
+        return WorldReplacementBootstrap.reconcile(
+                dataDirectory,
+                levelRoot,
+                bukkitConfiguration,
+                ignored -> {
+                }
+        );
+    }
+
+    private Transaction stagedTransaction(Phase phase, boolean originalPresent, String originalContent)
+            throws Exception {
+        WorldGeneratorSnapshot original = BukkitWorldConfiguration.snapshot(
+                bukkitConfiguration.toFile(),
+                "world_nether"
+        );
+        return transaction(UUID.randomUUID(), original, phase, originalPresent, originalContent);
+    }
+
+    private Transaction transaction(
+            UUID id,
+            WorldGeneratorSnapshot original,
+            Phase phase,
+            boolean originalPresent,
+            String originalContent
+    ) throws Exception {
+        ReplacementPaths paths = WorldReplacementFilesystem.paths(target, id);
+        if (originalPresent) {
+            writeOriginalTarget(paths, originalContent);
+        }
+        Path dimension = paths.stage().resolve("iris/pack/dimensions/underworld.json");
+        Files.createDirectories(dimension.getParent());
+        Files.writeString(dimension, "replacement");
+        String fingerprint = WorldReplacementFilesystem.fingerprintPack(paths.stage().resolve("iris/pack"));
+        Transaction transaction = new Transaction(
+                id,
+                WORLD_KEY,
+                "world_nether",
+                target.levelRoot(),
+                "underworld",
+                SEED,
+                fingerprint,
+                original,
+                originalPresent,
+                phase
+        );
+        WorldReplacementJournal.write(dataDirectory, transaction);
+        return transaction;
+    }
+
+    private void configureReplacement(Transaction transaction) throws Exception {
+        BukkitWorldConfiguration.GeneratorReplacement result = BukkitWorldConfiguration.replaceIfMatching(
+                bukkitConfiguration.toFile(),
+                transaction.worldName(),
+                transaction.originalConfiguration(),
+                transaction.dimension(),
+                transaction.seed()
+        );
+        assertTrue(result.applied());
+    }
+
+    private void configureExistingReplacement() throws Exception {
+        BukkitWorldConfiguration.register(
+                bukkitConfiguration.toFile(),
+                "world_nether",
+                "underworld",
+                SEED
+        );
+    }
+
+    private void restoreOriginalConfiguration(Transaction transaction) throws Exception {
+        assertTrue(BukkitWorldConfiguration.restoreIfMatching(
+                bukkitConfiguration.toFile(),
+                transaction.worldName(),
+                WorldReplacementBootstrap.replacementSnapshot(transaction),
+                transaction.originalConfiguration()
+        ));
+    }
+
+    private Transaction loadSingle() throws Exception {
+        return WorldReplacementJournal.load(dataDirectory, levelRoot).getFirst();
+    }
+
+    private ReplacementPaths paths(Transaction transaction) {
+        return WorldReplacementFilesystem.paths(target, transaction.id());
+    }
+
+    private Path backup(Transaction transaction) {
+        return paths(transaction).backup();
+    }
+
+    private void writeOriginalTarget(ReplacementPaths paths, String originalContent) throws Exception {
+        Files.createDirectories(paths.target().resolve("data/paper"));
+        Files.createDirectories(paths.target().resolve("data/minecraft"));
+        Files.writeString(paths.target().resolve("original.txt"), originalContent);
+        Files.writeString(paths.target().resolve("data/paper/metadata.dat"), "metadata");
+        Files.writeString(paths.target().resolve("data/paper/level_overrides.dat"), "overrides");
+        Files.writeString(paths.target().resolve("data/minecraft/world_gen_settings.dat"), "generation");
+    }
+
+    private String replacementContent(Path worldDirectory) throws Exception {
+        return Files.readString(worldDirectory.resolve("iris/pack/dimensions/underworld.json"));
+    }
+}

core/src/test/java/art/arcane/iris/core/lifecycle/WorldReplacementFilesystemTest.java

@@ -31,15 +31,83 @@ public class WorldReplacementFilesystemTest {
     public void publishesReplacementAndRetainsOriginalBackup() throws Exception {
         WorldReplacementFilesystem.ReplacementPaths paths = paths("publish-existing", TRANSACTION_ID);
         writeOriginalTarget(paths, "original");
+        Files.createDirectories(paths.target().resolve("region"));
+        Files.createDirectories(paths.target().resolve("entities"));
+        Files.createDirectories(paths.target().resolve("poi"));
+        Files.writeString(paths.target().resolve("region/r.0.0.mca"), "old-region");
+        Files.writeString(paths.target().resolve("entities/r.0.0.mca"), "old-entities");
+        Files.writeString(paths.target().resolve("poi/r.0.0.mca"), "old-poi");
         String fingerprint = writeStage(paths, "replacement");
 
         WorldReplacementFilesystem.publish(paths, true, fingerprint);
 
         assertEquals("replacement", readPackContent(paths.target()));
         assertEquals("original", Files.readString(paths.backup().resolve("original.txt")));
+        assertEquals("metadata", Files.readString(paths.target().resolve("data/paper/metadata.dat")));
+        assertEquals("overrides", Files.readString(paths.target().resolve("data/paper/level_overrides.dat")));
+        assertEquals(
+                "generation",
+                Files.readString(paths.target().resolve("data/minecraft/world_gen_settings.dat"))
+        );
+        assertFalse(Files.exists(paths.target().resolve("region/r.0.0.mca")));
+        assertFalse(Files.exists(paths.target().resolve("entities/r.0.0.mca")));
+        assertFalse(Files.exists(paths.target().resolve("poi/r.0.0.mca")));
+        assertTrue(Files.exists(paths.backup().resolve("region/r.0.0.mca")));
+        assertTrue(Files.exists(paths.backup().resolve("entities/r.0.0.mca")));
+        assertTrue(Files.exists(paths.backup().resolve("poi/r.0.0.mca")));
         assertFalse(Files.exists(paths.stage()));
     }
 
+    @Test
+    public void rejectsAbsentTargetForReplacementAdmission() throws Exception {
+        WorldReplacementFilesystem.ReplacementPaths paths = paths("admit-absent", TRANSACTION_ID);
+
+        IOException failure = assertThrows(
+                IOException.class,
+                () -> WorldReplacementFilesystem.requireExistingTarget(paths)
+        );
+
+        assertTrue(failure.getMessage().contains("requires an existing exact world slot"));
+        assertFalse(Files.exists(paths.target()));
+        assertFalse(Files.exists(paths.stage()));
+        assertFalse(Files.exists(paths.backup()));
+    }
+
+    @Test
+    public void completesMetadataForAlreadyPublishedReplacement() throws Exception {
+        WorldReplacementFilesystem.ReplacementPaths paths = paths("published-metadata", TRANSACTION_ID);
+        writeOriginalTarget(paths, "original");
+        String fingerprint = writeStage(paths, "replacement");
+        Files.move(paths.target(), paths.backup());
+        Files.move(paths.stage(), paths.target());
+
+        WorldReplacementFilesystem.publish(paths, true, fingerprint);
+
+        assertEquals("metadata", Files.readString(paths.target().resolve("data/paper/metadata.dat")));
+        assertEquals("overrides", Files.readString(paths.target().resolve("data/paper/level_overrides.dat")));
+        assertEquals(
+                "generation",
+                Files.readString(paths.target().resolve("data/minecraft/world_gen_settings.dat"))
+        );
+    }
+
+    @Test
+    public void rejectsUnmigratedRetainedWorldBeforePublication() throws Exception {
+        WorldReplacementFilesystem.ReplacementPaths paths = paths("missing-paper-metadata", TRANSACTION_ID);
+        Files.createDirectories(paths.target());
+        Files.writeString(paths.target().resolve("original.txt"), "original");
+        String fingerprint = writeStage(paths, "replacement");
+
+        assertThrows(
+                IOException.class,
+                () -> WorldReplacementFilesystem.publish(paths, true, fingerprint)
+        );
+
+        assertTrue(Files.isDirectory(paths.target()));
+        assertTrue(Files.isDirectory(paths.stage()));
+        assertFalse(Files.exists(paths.backup()));
+    }
+
     @Test
     public void publishesReplacementWithoutCreatingBackupForAbsentTarget() throws Exception {
         WorldReplacementFilesystem.ReplacementPaths paths = paths("publish-absent", TRANSACTION_ID);
@@ -136,6 +204,37 @@ public class WorldReplacementFilesystemTest {
         assertFalse(Files.exists(paths.backup()));
     }
 
+    @Test
+    public void preparedRollbackCanRepublishWhenConfigurationRestoreFails() throws Exception {
+        WorldReplacementFilesystem.ReplacementPaths paths = paths("rollback-republish", TRANSACTION_ID);
+        writeOriginalTarget(paths, "original");
+        String fingerprint = writeStage(paths, "replacement");
+        WorldReplacementFilesystem.publish(paths, true, fingerprint);
+
+        WorldReplacementFilesystem.prepareRollback(paths, true);
+        WorldReplacementFilesystem.publish(paths, true, fingerprint);
+
+        assertEquals("replacement", readPackContent(paths.target()));
+        assertEquals("original", Files.readString(paths.backup().resolve("original.txt")));
+        assertFalse(Files.exists(paths.stage()));
+    }
+
+    @Test
+    public void preparedRollbackCleanupIsRetryableAfterStageDeletion() throws Exception {
+        WorldReplacementFilesystem.ReplacementPaths paths = paths("rollback-cleanup-retry", TRANSACTION_ID);
+        writeOriginalTarget(paths, "original");
+        String fingerprint = writeStage(paths, "replacement");
+        WorldReplacementFilesystem.publish(paths, true, fingerprint);
+        WorldReplacementFilesystem.prepareRollback(paths, true);
+
+        WorldReplacementFilesystem.finishPreparedRollback(paths, true);
+        WorldReplacementFilesystem.finishPreparedRollback(paths, true);
+
+        assertEquals("original", Files.readString(paths.target().resolve("original.txt")));
+        assertFalse(Files.exists(paths.stage()));
+        assertFalse(Files.exists(paths.backup()));
+    }
+
     @Test
     public void rejectsPackMutationBeforePublication() throws Exception {
         WorldReplacementFilesystem.ReplacementPaths paths = paths("fingerprint-mutation", TRANSACTION_ID);
@@ -289,6 +388,11 @@ public class WorldReplacementFilesystemTest {
     private void writeOriginalTarget(WorldReplacementFilesystem.ReplacementPaths paths, String content) throws Exception {
         Files.createDirectories(paths.target());
         Files.writeString(paths.target().resolve("original.txt"), content);
+        Files.createDirectories(paths.target().resolve("data/paper"));
+        Files.createDirectories(paths.target().resolve("data/minecraft"));
+        Files.writeString(paths.target().resolve("data/paper/metadata.dat"), "metadata");
+        Files.writeString(paths.target().resolve("data/paper/level_overrides.dat"), "overrides");
+        Files.writeString(paths.target().resolve("data/minecraft/world_gen_settings.dat"), "generation");
     }
 
     private String readPackContent(Path worldDirectory) throws Exception {

core/src/test/java/art/arcane/iris/core/pack/DefaultPackBootstrapProvisionerTest.java

@@ -7,6 +7,7 @@ import org.junit.Test;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.net.InetSocketAddress;
 import java.net.URI;
 import java.net.http.HttpClient;
@@ -17,10 +18,14 @@ import java.time.Clock;
 import java.time.Duration;
 import java.time.Instant;
 import java.time.ZoneOffset;
+import java.util.Arrays;
+import java.util.Comparator;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Properties;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Stream;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
 import java.util.zip.ZipOutputStream;
@@ -31,6 +36,26 @@ import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 public class DefaultPackBootstrapProvisionerTest {
+    @Test
+    public void defaultBetaSourcesArePinnedPerRequiredPack() {
+        Map<String, DefaultPackBootstrapProvisioner.PackSpec> packs = new LinkedHashMap<>();
+        for (DefaultPackBootstrapProvisioner.PackSpec pack : DefaultPackBootstrapProvisioner.defaultPacks()) {
+            packs.put(pack.key(), pack);
+        }
+
+        assertEquals(2, packs.size());
+        assertEquals(
+                URI.create("https://github.com/IrisDimensions/overworld/releases/download/beta/overworld.zip"),
+                packs.get("overworld").source()
+        );
+        assertEquals("overworld", packs.get("overworld").requiredDimension());
+        assertEquals(
+                URI.create("https://github.com/IrisDimensions/underworld/releases/download/beta/underworld.zip"),
+                packs.get("underworld").source()
+        );
+        assertEquals("underworld", packs.get("underworld").requiredDimension());
+    }
+
     @Test
     public void coldInstallUsesFreshCacheWithoutSecondRequest() throws Exception {
         byte[] archive = packArchive("overworld", "bootstrap_biome");
@@ -58,14 +83,23 @@ public class DefaultPackBootstrapProvisionerTest {
 
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.INSTALLED, installed.status());
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.UNCHANGED, unchanged.status());
-            assertEquals(1, requests.get());
-            assertTrue(Files.isRegularFile(installed.packRoot().resolve("dimensions/overworld.json")));
+            assertEquals(2, requests.get());
+            assertTrue(Files.isRegularFile(installed.packRoots().get("overworld").resolve("dimensions/overworld.json")));
+            assertTrue(Files.isRegularFile(installed.packRoots().get("underworld").resolve("dimensions/underworld.json")));
+            assertTrue(Files.isRegularFile(installed.packRoots().get("underworld").resolve("dimensions/underworld_roof.json")));
             assertEquals(root.resolve("datapacks/iris"), installed.datapackRoot());
             assertTrue(Files.isRegularFile(installed.datapackRoot().resolve("pack.mcmeta")));
             assertTrue(Files.isRegularFile(installed.datapackRoot().resolve("data/overworld/worldgen/biome/bootstrap_biome.json")));
+            assertTrue(Files.isRegularFile(installed.datapackRoot().resolve("data/underworld/worldgen/biome/underworld_biome.json")));
             assertFalse(Files.exists(dataDirectory.resolve("bootstrap/datapack")));
-            assertTrue(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root));
+            assertTrue(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root, options.packs()));
             assertTrue(DefaultPackBootstrapProvisioner.wasProvisionedThisStartup());
+            Properties marker = loadProperties(dataDirectory.resolve("bootstrap/provisioned.properties"));
+            assertEquals("true", marker.getProperty("pack.overworld.managed"));
+            assertEquals("true", marker.getProperty("pack.underworld.managed"));
+            assertEquals("underworld", marker.getProperty("pack.underworld.requiredDimension"));
+            delete(installed.packRoots().get("underworld"));
+            assertFalse(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root, options.packs()));
         } finally {
             server.stop(0);
             delete(root);
@@ -101,9 +135,10 @@ public class DefaultPackBootstrapProvisionerTest {
             );
 
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.UPDATED, rebuilt.status());
-            assertEquals(1, requests.get());
+            assertEquals(2, requests.get());
             assertTrue(Files.isRegularFile(rebuilt.datapackRoot().resolve("pack.mcmeta")));
             assertTrue(Files.isRegularFile(rebuilt.datapackRoot().resolve("data/overworld/worldgen/biome/bootstrap_biome.json")));
+            assertTrue(Files.isRegularFile(rebuilt.datapackRoot().resolve("data/underworld/worldgen/biome/underworld_biome.json")));
         } finally {
             if (!serverStopped) {
                 server.stop(0);
@@ -138,9 +173,10 @@ public class DefaultPackBootstrapProvisionerTest {
             );
 
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.INSTALLED, installed.status());
-            assertEquals(1, requests.get());
+            assertEquals(2, requests.get());
             assertTrue(Files.isSymbolicLink(dataDirectory.resolve("packs")));
             assertTrue(Files.isRegularFile(sharedPacks.resolve("overworld/dimensions/overworld.json")));
+            assertTrue(Files.isRegularFile(sharedPacks.resolve("underworld/dimensions/underworld.json")));
             assertTrue(Files.isRegularFile(installed.datapackRoot().resolve("pack.mcmeta")));
         } finally {
             server.stop(0);
@@ -165,7 +201,7 @@ public class DefaultPackBootstrapProvisionerTest {
             DefaultPackBootstrapProvisioner.provision(dataDirectory, ignored -> {
             }, options);
 
-            assertEquals(2, requests.get());
+            assertEquals(3, requests.get());
             try (ZipInputStream zip = new ZipInputStream(Files.newInputStream(cache))) {
                 assertTrue(zip.getNextEntry() != null);
             }
@@ -245,13 +281,13 @@ public class DefaultPackBootstrapProvisionerTest {
                     options
             );
 
-            assertEquals(0, requests.get());
+            assertEquals(1, requests.get());
             assertTrue(Files.isSymbolicLink(link));
             assertEquals(target.toRealPath(), link.toRealPath());
             assertTrue(Files.isRegularFile(result.datapackRoot().resolve("data/overworld/worldgen/biome/local_biome.json")));
 
             Files.writeString(target.resolve("biomes/local.json"), biomeJson("changed_biome"), StandardCharsets.UTF_8);
-            assertFalse(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root));
+            assertFalse(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root, options.packs()));
             DefaultPackBootstrapProvisioner.ProvisionResult updated = DefaultPackBootstrapProvisioner.provision(
                     dataDirectory,
                     ignored -> {
@@ -259,7 +295,7 @@ public class DefaultPackBootstrapProvisionerTest {
                     options
             );
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.UPDATED, updated.status());
-            assertEquals(0, requests.get());
+            assertEquals(2, requests.get());
             assertTrue(Files.isSymbolicLink(link));
         } finally {
             server.stop(0);
@@ -280,7 +316,7 @@ public class DefaultPackBootstrapProvisionerTest {
             writePack(dataDirectory.resolve("packs/second"), "second", "second_biome");
             writePack(root.resolve("dimensions/example/world/iris/pack"), "world_local", "world_local_biome");
 
-            assertFalse(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root));
+            assertFalse(DefaultPackBootstrapProvisioner.isProvisioned(dataDirectory, root, options.packs()));
             DefaultPackBootstrapProvisioner.ProvisionResult updated = DefaultPackBootstrapProvisioner.provision(
                     dataDirectory,
                     ignored -> {
@@ -289,7 +325,7 @@ public class DefaultPackBootstrapProvisionerTest {
             );
 
             assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.UPDATED, updated.status());
-            assertEquals(1, requests.get());
+            assertEquals(2, requests.get());
             assertTrue(Files.isRegularFile(updated.datapackRoot().resolve("data/second/worldgen/biome/second_biome.json")));
             assertTrue(Files.isRegularFile(updated.datapackRoot().resolve("data/world_local/worldgen/biome/world_local_biome.json")));
         } finally {
@@ -319,14 +355,15 @@ public class DefaultPackBootstrapProvisionerTest {
                     refreshOptions
             );
 
-            assertEquals(1, requests.get());
+            assertEquals(3, requests.get());
             assertTrue(Files.readString(editedBiome).contains("locally_edited_biome"));
             assertTrue(Files.isRegularFile(updated.datapackRoot().resolve("data/overworld/worldgen/biome/locally_edited_biome.json")));
             Properties marker = new Properties();
-            try (java.io.InputStream input = Files.newInputStream(dataDirectory.resolve("bootstrap/provisioned.properties"))) {
+            try (InputStream input = Files.newInputStream(dataDirectory.resolve("bootstrap/provisioned.properties"))) {
                 marker.load(input);
             }
-            assertEquals("false", marker.getProperty("managedDefault"));
+            assertEquals("false", marker.getProperty("pack.overworld.managed"));
+            assertEquals("true", marker.getProperty("pack.underworld.managed"));
         } finally {
             server.stop(0);
             delete(root);
@@ -334,39 +371,185 @@ public class DefaultPackBootstrapProvisionerTest {
     }
 
     @Test
-    public void failedAggregateCompilationPreservesPreviousOutputAndMarker() throws Exception {
+    public void underworldLocalEditRelinquishesOnlyUnderworldOwnership() throws Exception {
         AtomicInteger requests = new AtomicInteger();
-        HttpServer server = server(packArchive("overworld", "first_biome"), requests);
-        Path root = Files.createTempDirectory("iris-bootstrap-rollback");
+        HttpServer server = server(packArchive("overworld", "overworld_managed"), requests);
+        Path root = Files.createTempDirectory("iris-bootstrap-underworld-edit");
         try {
             Path dataDirectory = root.resolve("plugins/Iris");
-            DefaultPackBootstrapProvisioner.ProvisionOptions options = options(server, root, Duration.ofHours(1));
-            DefaultPackBootstrapProvisioner.ProvisionResult first = DefaultPackBootstrapProvisioner.provision(
+            DefaultPackBootstrapProvisioner.ProvisionOptions initialOptions = options(
+                    server,
+                    root,
+                    Duration.ofHours(1)
+            );
+            DefaultPackBootstrapProvisioner.provision(dataDirectory, ignored -> {
+            }, initialOptions);
+            Path editedBiome = dataDirectory.resolve("packs/underworld/biomes/local.json");
+            Files.writeString(editedBiome, biomeJson("underworld_local_edit"), StandardCharsets.UTF_8);
+            DefaultPackBootstrapProvisioner.ProvisionOptions refreshOptions = options(server, root, Duration.ZERO);
+
+            DefaultPackBootstrapProvisioner.ProvisionResult updated = DefaultPackBootstrapProvisioner.provision(
                     dataDirectory,
                     ignored -> {
                     },
-                    options
+                    refreshOptions
             );
-            byte[] marker = Files.readAllBytes(dataDirectory.resolve("bootstrap/provisioned.properties"));
-            byte[] metadata = Files.readAllBytes(first.datapackRoot().resolve("pack.mcmeta"));
-            Path invalidPack = dataDirectory.resolve("packs/invalid");
-            Files.createDirectories(invalidPack.resolve("dimensions"));
-            Files.writeString(invalidPack.resolve("dimensions/broken.json"), "{", StandardCharsets.UTF_8);
+
+            assertEquals(3, requests.get());
+            assertTrue(Files.readString(editedBiome).contains("underworld_local_edit"));
+            assertTrue(Files.isRegularFile(updated.datapackRoot()
+                    .resolve("data/underworld/worldgen/biome/underworld_local_edit.json")));
+            Properties marker = loadProperties(dataDirectory.resolve("bootstrap/provisioned.properties"));
+            assertEquals("true", marker.getProperty("pack.overworld.managed"));
+            assertEquals("false", marker.getProperty("pack.underworld.managed"));
+        } finally {
+            server.stop(0);
+            delete(root);
+        }
+    }
+
+    @Test
+    public void betaPacksUpdateIndependentlyAndRecompileOneAggregateDatapack() throws Exception {
+        byte[] overworld = packArchive("overworld", "overworld_first");
+        byte[] underworldFirst = underworldArchive("underworld_first");
+        AtomicInteger requests = new AtomicInteger();
+        HttpServer initialServer = server(overworld, underworldFirst, requests);
+        HttpServer updateServer = null;
+        Path root = Files.createTempDirectory("iris-bootstrap-independent-update");
+        try {
+            Path dataDirectory = root.resolve("plugins/Iris");
+            DefaultPackBootstrapProvisioner.provision(
+                    dataDirectory,
+                    ignored -> {
+                    },
+                    options(initialServer, root, Duration.ofHours(1))
+            );
+            initialServer.stop(0);
+            initialServer = null;
+
+            byte[] underworldSecond = underworldArchive("underworld_second");
+            updateServer = server(overworld, underworldSecond, requests);
+            DefaultPackBootstrapProvisioner.ProvisionResult updated = DefaultPackBootstrapProvisioner.provision(
+                    dataDirectory,
+                    ignored -> {
+                    },
+                    options(updateServer, root, Duration.ZERO)
+            );
+
+            assertEquals(DefaultPackBootstrapProvisioner.ProvisionStatus.UPDATED, updated.status());
+            assertEquals(4, requests.get());
+            assertTrue(Files.readString(dataDirectory.resolve("packs/overworld/biomes/local.json"))
+                    .contains("overworld_first"));
+            assertTrue(Files.readString(dataDirectory.resolve("packs/underworld/biomes/local.json"))
+                    .contains("underworld_second"));
+            assertTrue(Files.isRegularFile(updated.datapackRoot()
+                    .resolve("data/overworld/worldgen/biome/overworld_first.json")));
+            assertTrue(Files.isRegularFile(updated.datapackRoot()
+                    .resolve("data/underworld/worldgen/biome/underworld_second.json")));
+            assertFalse(Files.exists(updated.datapackRoot()
+                    .resolve("data/underworld/worldgen/biome/underworld_first.json")));
+        } finally {
+            if (initialServer != null) {
+                initialServer.stop(0);
+            }
+            if (updateServer != null) {
+                updateServer.stop(0);
+            }
+            delete(root);
+        }
+    }
+
+    @Test
+    public void invalidUnderworldArchivePublishesNeitherRequiredPack() throws Exception {
+        byte[] overworld = packArchive("overworld", "overworld_valid");
+        byte[] invalidUnderworld = packArchive("underworld_roof", "roof_only");
+        AtomicInteger requests = new AtomicInteger();
+        HttpServer server = server(overworld, invalidUnderworld, requests);
+        Path root = Files.createTempDirectory("iris-bootstrap-underworld-invalid");
+        try {
+            Path dataDirectory = root.resolve("plugins/Iris");
 
             assertThrows(IOException.class, () -> DefaultPackBootstrapProvisioner.provision(
                     dataDirectory,
                     ignored -> {
                     },
-                    options
+                    options(server, root, Duration.ZERO)
             ));
-            assertTrue(java.util.Arrays.equals(marker, Files.readAllBytes(dataDirectory.resolve("bootstrap/provisioned.properties"))));
-            assertTrue(java.util.Arrays.equals(metadata, Files.readAllBytes(first.datapackRoot().resolve("pack.mcmeta"))));
+
+            assertEquals(2, requests.get());
+            assertFalse(Files.exists(dataDirectory.resolve("packs/overworld")));
+            assertFalse(Files.exists(dataDirectory.resolve("packs/underworld")));
+            assertFalse(Files.exists(dataDirectory.resolve("bootstrap/provisioned.properties")));
+            assertFalse(Files.exists(root.resolve("datapacks/iris")));
         } finally {
             server.stop(0);
             delete(root);
         }
     }
 
+    @Test
+    public void failedAggregateCompilationRollsBackBothPackUpdatesAndDatapack() throws Exception {
+        AtomicInteger requests = new AtomicInteger();
+        HttpServer initialServer = server(
+                packArchive("overworld", "overworld_first"),
+                underworldArchive("underworld_first"),
+                requests
+        );
+        HttpServer updateServer = null;
+        Path root = Files.createTempDirectory("iris-bootstrap-rollback");
+        try {
+            Path dataDirectory = root.resolve("plugins/Iris");
+            DefaultPackBootstrapProvisioner.ProvisionResult first = DefaultPackBootstrapProvisioner.provision(
+                    dataDirectory,
+                    ignored -> {
+                    },
+                    options(initialServer, root, Duration.ofHours(1))
+            );
+            initialServer.stop(0);
+            initialServer = null;
+            byte[] marker = Files.readAllBytes(dataDirectory.resolve("bootstrap/provisioned.properties"));
+            byte[] metadata = Files.readAllBytes(first.datapackRoot().resolve("pack.mcmeta"));
+            byte[] originalOverworld = Files.readAllBytes(dataDirectory.resolve("packs/overworld/biomes/local.json"));
+            byte[] originalUnderworld = Files.readAllBytes(dataDirectory.resolve("packs/underworld/biomes/local.json"));
+            Path invalidPack = dataDirectory.resolve("packs/invalid");
+            Files.createDirectories(invalidPack.resolve("dimensions"));
+            Files.writeString(invalidPack.resolve("dimensions/broken.json"), "{", StandardCharsets.UTF_8);
+            updateServer = server(
+                    packArchive("overworld", "overworld_second"),
+                    underworldArchive("underworld_second"),
+                    requests
+            );
+            DefaultPackBootstrapProvisioner.ProvisionOptions updateOptions = options(
+                    updateServer,
+                    root,
+                    Duration.ZERO
+            );
+
+            assertThrows(IOException.class, () -> DefaultPackBootstrapProvisioner.provision(
+                    dataDirectory,
+                    ignored -> {
+                    },
+                    updateOptions
+            ));
+            assertTrue(Arrays.equals(marker, Files.readAllBytes(dataDirectory.resolve("bootstrap/provisioned.properties"))));
+            assertTrue(Arrays.equals(metadata, Files.readAllBytes(first.datapackRoot().resolve("pack.mcmeta"))));
+            assertTrue(Arrays.equals(originalOverworld,
+                    Files.readAllBytes(dataDirectory.resolve("packs/overworld/biomes/local.json"))));
+            assertTrue(Arrays.equals(originalUnderworld,
+                    Files.readAllBytes(dataDirectory.resolve("packs/underworld/biomes/local.json"))));
+            assertNoBootstrapTransactionPaths(dataDirectory.resolve("packs"));
+            assertNoBootstrapTransactionPaths(root.resolve("datapacks"));
+        } finally {
+            if (initialServer != null) {
+                initialServer.stop(0);
+            }
+            if (updateServer != null) {
+                updateServer.stop(0);
+            }
+            delete(root);
+        }
+    }
+
     @Test
     public void resolvesConfiguredLevelRootFromServerProperties() throws Exception {
         Path serverRoot = Files.createTempDirectory("iris-bootstrap-level-root");
@@ -378,7 +561,7 @@ public class DefaultPackBootstrapProvisionerTest {
             );
 
             assertEquals(
-                    serverRoot.resolve("levels/primary").normalize(),
+                    serverRoot.toRealPath().resolve("levels/primary").normalize(),
                     DefaultPackBootstrapProvisioner.resolveLevelRoot(serverRoot)
             );
         } finally {
@@ -391,9 +574,20 @@ public class DefaultPackBootstrapProvisionerTest {
             Path serverRoot,
             Duration refreshInterval
     ) {
-        URI source = URI.create("http://127.0.0.1:" + server.getAddress().getPort() + "/overworld.zip");
+        String sourceRoot = "http://127.0.0.1:" + server.getAddress().getPort();
         return new DefaultPackBootstrapProvisioner.ProvisionOptions(
-                source,
+                List.of(
+                        new DefaultPackBootstrapProvisioner.PackSpec(
+                                "overworld",
+                                URI.create(sourceRoot + "/overworld.zip"),
+                                "overworld"
+                        ),
+                        new DefaultPackBootstrapProvisioner.PackSpec(
+                                "underworld",
+                                URI.create(sourceRoot + "/underworld.zip"),
+                                "underworld"
+                        )
+                ),
                 HttpClient.newBuilder().connectTimeout(Duration.ofSeconds(2)).build(),
                 Clock.fixed(Instant.parse("2026-07-12T12:00:00Z"), ZoneOffset.UTC),
                 refreshInterval,
@@ -406,8 +600,17 @@ public class DefaultPackBootstrapProvisionerTest {
     }
 
     private static HttpServer server(byte[] response, AtomicInteger requests) throws IOException {
+        return server(response, underworldArchive("underworld_biome"), requests);
+    }
+
+    private static HttpServer server(
+            byte[] overworldResponse,
+            byte[] underworldResponse,
+            AtomicInteger requests
+    ) throws IOException {
         HttpServer server = HttpServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
-        server.createContext("/overworld.zip", exchange -> respond(exchange, response, requests));
+        server.createContext("/overworld.zip", exchange -> respond(exchange, overworldResponse, requests));
+        server.createContext("/underworld.zip", exchange -> respond(exchange, underworldResponse, requests));
         server.start();
         return server;
     }
@@ -428,6 +631,33 @@ public class DefaultPackBootstrapProvisionerTest {
         return zip(files);
     }
 
+    private static byte[] underworldArchive(String biomeId) throws IOException {
+        LinkedHashMap<String, String> files = new LinkedHashMap<>();
+        files.put("dimensions/underworld.json", dimensionJson("underworld"));
+        files.put("dimensions/underworld_roof.json", dimensionJson("underworld_roof"));
+        files.put("regions/local.json", "{\"name\":\"Local\",\"landBiomes\":[\"local\"]}");
+        files.put("biomes/local.json", biomeJson(biomeId));
+        return zip(files);
+    }
+
+    private static Properties loadProperties(Path path) throws IOException {
+        Properties properties = new Properties();
+        try (InputStream input = Files.newInputStream(path)) {
+            properties.load(input);
+        }
+        return properties;
+    }
+
+    private static void assertNoBootstrapTransactionPaths(Path root) throws IOException {
+        if (!Files.isDirectory(root)) {
+            return;
+        }
+        try (Stream<Path> stream = Files.list(root)) {
+            assertFalse(stream.anyMatch(path -> path.getFileName().toString().contains("-stage-")
+                    || path.getFileName().toString().contains("-backup-")));
+        }
+    }
+
     private static void writePack(Path root, String dimensionKey, String biomeId) throws IOException {
         Files.createDirectories(root.resolve("dimensions"));
         Files.createDirectories(root.resolve("regions"));
@@ -462,8 +692,8 @@ public class DefaultPackBootstrapProvisionerTest {
         if (!Files.exists(root)) {
             return;
         }
-        try (java.util.stream.Stream<Path> stream = Files.walk(root)) {
-            for (Path path : stream.sorted(java.util.Comparator.reverseOrder()).toList()) {
+        try (Stream<Path> stream = Files.walk(root)) {
+            for (Path path : stream.sorted(Comparator.reverseOrder()).toList()) {
                 Files.deleteIfExists(path);
             }
         }

core/src/test/java/art/arcane/iris/core/pack/PackDownloaderTest.java

@@ -49,6 +49,7 @@ import java.util.zip.ZipOutputStream;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
@@ -95,6 +96,60 @@ public class PackDownloaderTest {
                 PackDownloader.defaultOverworldReleaseUrl()
         );
         assertTrue(PackDownloader.isDefaultOverworld("overworld"));
+        assertTrue(PackDownloader.isManagedBetaPack("overworld"));
+        assertEquals(List.of("overworld", "underworld"), PackDownloader.managedBetaPacks());
+    }
+
+    @Test
+    public void resolvesUnderworldBetaRelease() {
+        assertEquals(
+                "https://github.com/IrisDimensions/underworld/releases/download/beta/underworld.zip",
+                PackDownloader.underworldReleaseUrl()
+        );
+        assertTrue(PackDownloader.isManagedBetaPack("underworld"));
+    }
+
+    @Test
+    public void managedPackPresenceRequiresItsPrimaryDimension() throws Exception {
+        File packsFolder = temp.newFolder("managed-presence");
+        Path dimensions = Files.createDirectories(
+                packsFolder.toPath().resolve("underworld/dimensions")
+        );
+        writeDimension(packsFolder.toPath().resolve("underworld"), "underworld_roof");
+
+        assertTrue(PackDownloader.isPackPresent(packsFolder, "underworld"));
+        assertFalse(PackDownloader.isManagedBetaPackPresent(packsFolder, "underworld"));
+
+        writeDimension(packsFolder.toPath().resolve("underworld"), "underworld");
+        assertTrue(Files.isDirectory(dimensions));
+        assertTrue(PackDownloader.isManagedBetaPackPresent(packsFolder, "underworld"));
+    }
+
+    @Test
+    public void repairsManagedFolderMissingItsPrimaryDimension() throws Exception {
+        File packsFolder = temp.newFolder("managed-repair-packs");
+        Path target = packsFolder.toPath().resolve("underworld");
+        Files.createDirectories(target.resolve("dimensions"));
+        writeDimension(target, "underworld_roof");
+        Files.writeString(target.resolve("partial.txt"), "partial", StandardCharsets.UTF_8);
+        File extracted = writePack(temp.newFolder("managed-repair-source").toPath(), "underworld", "new");
+        writeDimension(extracted.toPath(), "underworld_roof");
+
+        PackDownloader.PackInstallResult result = PackDownloader.installExtractedPack(
+                packsFolder,
+                extracted,
+                false,
+                "underworld",
+                ignored -> {
+                }
+        );
+
+        assertNotNull(result);
+        assertTrue(result.changed());
+        assertTrue(Files.isRegularFile(target.resolve("dimensions/underworld.json")));
+        assertTrue(Files.isRegularFile(target.resolve("dimensions/underworld_roof.json")));
+        assertFalse(Files.exists(target.resolve("partial.txt")));
+        assertTransactionStateClean(packsFolder);
     }
 
     @Test
@@ -102,6 +157,9 @@ public class PackDownloaderTest {
         assertFalse(PackDownloader.isDefaultOverworld("theend"));
         assertFalse(PackDownloader.isDefaultOverworld(""));
         assertFalse(PackDownloader.isDefaultOverworld(null));
+        assertFalse(PackDownloader.isManagedBetaPack("theend"));
+        assertFalse(PackDownloader.isManagedBetaPack(""));
+        assertFalse(PackDownloader.isManagedBetaPack(null));
     }
 
     @Test
@@ -345,6 +403,71 @@ public class PackDownloaderTest {
         assertEquals(0, PackDownloader.downloadLockCount());
     }
 
+    @Test
+    public void importsExpectedDimensionFromMultiDimensionPack() throws Exception {
+        File packsFolder = temp.newFolder("multi-dimension-packs");
+        File extracted = writePack(temp.newFolder("multi-dimension-source").toPath(), "underworld", "new");
+        writeDimension(extracted.toPath(), "underworld_roof");
+
+        PackDownloader.PackInstallResult result = PackDownloader.installExtractedPack(
+                packsFolder,
+                extracted,
+                false,
+                "underworld",
+                ignored -> {
+                }
+        );
+
+        assertEquals("underworld", result.key());
+        assertTrue(result.changed());
+        assertTrue(Files.isRegularFile(
+                packsFolder.toPath().resolve("underworld/dimensions/underworld_roof.json")
+        ));
+        assertTransactionStateClean(packsFolder);
+    }
+
+    @Test
+    public void rejectsMultiDimensionPackWithoutExpectedDimension() throws Exception {
+        File packsFolder = temp.newFolder("missing-dimension-packs");
+        File extracted = writePack(temp.newFolder("missing-dimension-source").toPath(), "underworld", "new");
+        writeDimension(extracted.toPath(), "underworld_roof");
+
+        IOException failure = assertThrows(IOException.class, () -> PackDownloader.installExtractedPack(
+                packsFolder,
+                extracted,
+                false,
+                "missing",
+                ignored -> {
+                }
+        ));
+
+        assertTrue(failure.getMessage().contains("missing"));
+        assertTrue(failure.getMessage().contains("underworld"));
+        assertFalse(new File(packsFolder, "missing").exists());
+        assertTransactionStateClean(packsFolder);
+    }
+
+    @Test
+    public void rejectsAmbiguousMultiDimensionPackWithoutExpectedKey() throws Exception {
+        File packsFolder = temp.newFolder("ambiguous-dimension-packs");
+        File extracted = writePack(temp.newFolder("ambiguous-dimension-source").toPath(), "underworld", "new");
+        writeDimension(extracted.toPath(), "underworld_roof");
+        List<String> feedback = new ArrayList<>();
+
+        PackDownloader.PackInstallResult result = PackDownloader.installExtractedPack(
+                packsFolder,
+                extracted,
+                false,
+                null,
+                feedback::add
+        );
+
+        assertNull(result);
+        assertFalse(feedback.isEmpty());
+        assertFalse(new File(packsFolder, "underworld").exists());
+        assertTransactionStateClean(packsFolder);
+    }
+
     @Test
     public void concurrentImportsForSameKeyPublishOnlyOnePack() throws Exception {
         File packsFolder = temp.newFolder("concurrent-packs");
@@ -465,12 +588,7 @@ public class PackDownloaderTest {
         Files.createDirectories(root.resolve("dimensions"));
         Files.createDirectories(root.resolve("regions"));
         Files.createDirectories(root.resolve("biomes"));
-        Files.writeString(
-                root.resolve("dimensions/" + key + ".json"),
-                "{\"name\":\"" + key + "\",\"regions\":[\"local\"],\"logicalHeight\":256,"
-                        + "\"dimensionHeight\":{\"min\":-64,\"max\":320}}",
-                StandardCharsets.UTF_8
-        );
+        writeDimension(root, key);
         Files.writeString(
                 root.resolve("regions/local.json"),
                 "{\"name\":\"Local\",\"landBiomes\":[\"local\"]}",
@@ -485,6 +603,15 @@ public class PackDownloaderTest {
         return root.toFile();
     }
 
+    private static void writeDimension(Path root, String key) throws IOException {
+        Files.writeString(
+                root.resolve("dimensions/" + key + ".json"),
+                "{\"name\":\"" + key + "\",\"regions\":[\"local\"],\"logicalHeight\":256,"
+                        + "\"dimensionHeight\":{\"min\":-64,\"max\":320}}",
+                StandardCharsets.UTF_8
+        );
+    }
+
     private static void writeArchive(Path archive, Map<String, String> entries) throws IOException {
         try (ZipOutputStream zip = new ZipOutputStream(Files.newOutputStream(archive))) {
             for (Map.Entry<String, String> entry : entries.entrySet()) {

core/src/test/java/art/arcane/iris/core/service/StudioSVCManagedBetaPackTest.java

@@ -0,0 +1,78 @@
+/*
+ * Iris is a World Generator for Minecraft Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.core.service;
+
+import org.junit.Test;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+
+public class StudioSVCManagedBetaPackTest {
+    @Test
+    public void startupSelectsOnlyMissingManagedBetaPacks() throws IOException {
+        Path workspace = Files.createTempDirectory("iris-managed-beta-startup");
+        try {
+            assertEquals(
+                    List.of("overworld", "underworld"),
+                    StudioSVC.missingManagedBetaPacks(workspace.toFile())
+            );
+
+            createPack(workspace, "overworld");
+            assertEquals(
+                    List.of("underworld"),
+                    StudioSVC.missingManagedBetaPacks(workspace.toFile())
+            );
+
+            createDimension(workspace, "underworld", "underworld_roof");
+            assertEquals(
+                    List.of("underworld"),
+                    StudioSVC.missingManagedBetaPacks(workspace.toFile())
+            );
+
+            createPack(workspace, "underworld");
+            assertEquals(List.of(), StudioSVC.missingManagedBetaPacks(workspace.toFile()));
+        } finally {
+            deleteTree(workspace);
+        }
+    }
+
+    private static void createPack(Path workspace, String key) throws IOException {
+        createDimension(workspace, key, key);
+    }
+
+    private static void createDimension(Path workspace, String folder, String key) throws IOException {
+        Path dimensions = Files.createDirectories(workspace.resolve(folder).resolve("dimensions"));
+        Files.writeString(dimensions.resolve(key + ".json"), "{}", StandardCharsets.UTF_8);
+    }
+
+    private static void deleteTree(Path root) throws IOException {
+        try (Stream<Path> paths = Files.walk(root)) {
+            for (Path path : paths.sorted(Comparator.comparingInt(Path::getNameCount).reversed()).toList()) {
+                Files.deleteIfExists(path);
+            }
+        }
+    }
+}

docs/01 - Installation & Platforms.md

@@ -1,13 +1,13 @@
 # 01 - Installation & Platforms
 
-Iris installs as either a Bukkit-family plugin jar or a self-contained Fabric, Forge, or NeoForge mod jar. Java 25 is required on every platform. On first boot the default `overworld` pack is downloaded when missing; packs live under each platform’s data directory.
+Iris installs as either a Bukkit-family plugin jar or a self-contained Fabric, Forge, or NeoForge mod jar. Java 25 is required on every platform. On first boot the managed `overworld` and `underworld` beta packs are downloaded when missing; packs live under each platform’s data directory.
 
 ## Installation outcome
 
 Complete one platform path below. A successful install has all three results:
 
 1. Iris reaches its enabled/ready state without an exception.
-2. The platform data directory contains `settings.json` and a loadable `packs/overworld/` directory.
+2. The platform data directory contains `settings.json` and loadable `packs/overworld/` and `packs/underworld/` directories.
 3. `/iris` prints help from the server console. On a modded client, the Iris keybind category is an additional client-side check, not a substitute for the server check.
 
 Keep the previous jar/mod and the entire Iris data directory until the new build passes these checks. Replacing the binary does not update pack snapshots already stored inside worlds.
@@ -22,7 +22,7 @@ Keep the previous jar/mod and the entire Iris data directory until the new build
 | Fabric Loader | 0.19.3+ |
 | Forge | 65.0.4+ |
 | NeoForge | 26.2.0.12-beta+ |
-| Network | Outbound HTTPS on first boot for default pack download (GitHub IrisDimensions overworld release / pack install) |
+| Network | Outbound HTTPS on first boot for the GitHub IrisDimensions Overworld and Underworld beta release assets |
 
 Before replacing an existing installation:
 
@@ -37,7 +37,7 @@ Do not copy multiple Iris platform jars into the same `plugins/` or `mods/` dire
 
 1. Place the CraftBukkit-labelled plugin jar into `plugins/`.
 2. Start the server. Iris loads at `STARTUP` (`plugin.yml` / `paper-plugin.yml`).
-3. On first boot Iris provisions the default `overworld` pack into `plugins/Iris/packs/overworld` when missing (source: IrisDimensions overworld `beta` release zip).
+3. On first boot Iris provisions `overworld` and `underworld` into `plugins/Iris/packs/` when missing from their IrisDimensions `beta` release ZIPs.
 4. Settings are written at `plugins/Iris/settings.json` if absent (`IrisSettings.read()`).
 
 Validate the plugin install from the server console:
@@ -45,6 +45,7 @@ Validate the plugin install from the server console:
 ```text
 /iris version
 /iris pack validate pack=overworld
+/iris pack validate pack=underworld
 ```
 
 The first command must report the running Iris, platform, and Minecraft versions. The second must resolve the downloaded pack and finish without blocking validation errors. Then complete the disposable-world workflow in `02 - Getting Started.md`; a command response alone does not prove that the generator can create chunks.
@@ -66,16 +67,17 @@ Before creating a real world, run the Bukkit fresh-install smoke in `31 - Operat
 1. Place the matching mod jar into `mods/`.
 2. Start the dedicated server (or a client for singleplayer; see below).
 3. The jar is self-contained: core, SPI, and required Fabric API modules are bundled where applicable. Mod id: `irisworldgen`.
-4. On first boot, if `config/irisworldgen/modded.json` has `autoDownloadDefaultPack` true (default) and `defaultPack` (default `overworld`) is missing, Iris downloads `IrisDimensions/<pack>` (branch `master` for the auto-prefetch path) into the packs folder before the forced worldgen datapack is written.
+4. On first boot, if `config/irisworldgen/modded.json` has `autoDownloadDefaultPack` true (default), Iris installs the managed `overworld` and `underworld` beta releases when missing, followed by a configured non-managed `defaultPack` when applicable, before the forced worldgen datapack is written.
 
 Validate the server-side mod install:
 
 ```text
 /iris version
 /iris pack validate overworld
+/iris pack validate underworld
 ```
 
-The install passes when Iris reports the expected loader/version, `config/irisworldgen/packs/overworld/dimensions/` contains a dimension JSON file, and validation has no blocking errors. Restart once before creating a world if the pack or its generated dimension-type datapack was installed during this boot.
+The install passes when Iris reports the expected loader/version, both managed pack directories contain their primary dimension JSON, and validation has no blocking errors. Restart once before creating a world if a pack or its generated dimension-type datapack was installed during this boot.
 
 Packs installed later register custom dimension types (height ranges) and custom biomes through the forced datapack at server start. **Restart once after adding a pack** so worlds get full heights and biomes. Worlds created before that restart run with fallback heights.
 
@@ -118,7 +120,7 @@ Pack resolution for engines, commands, and the forced datapack uses `config/iris
 | Key | Default | Effect |
 |---|---|---|
 | `defaultPack` | `overworld` | Pack auto-download and default create pack name |
-| `autoDownloadDefaultPack` | `true` | Async prefetch at boot when pack missing |
+| `autoDownloadDefaultPack` | `true` | Async prefetch of both managed beta packs and any configured non-managed default when missing |
 | `primaryWorld` | `""` | Primary-world router target dimension id |
 | `routePlayersToPrimaryWorld` | `true` | Route players from vanilla overworld when primary is set |
 | `mainWorldPack` | `""` | Main-world generator override pack ref |
@@ -142,17 +144,17 @@ Full key list: `03 - Configuration.md`.
 
 | Platform | Behavior |
 |---|---|
-| Plugin | `DefaultPackBootstrapProvisioner` downloads `https://github.com/IrisDimensions/overworld/releases/download/beta/overworld.zip` into `packs/overworld` when not already provisioned |
-| Mod | If `autoDownloadDefaultPack` and pack missing, async install of configured `defaultPack` into `config/irisworldgen/packs` |
+| Plugin | `DefaultPackBootstrapProvisioner` independently manages the Overworld and Underworld beta assets under `packs/overworld` and `packs/underworld`, then compiles the aggregate datapack once |
+| Mod | If `autoDownloadDefaultPack` is enabled, async install of both managed beta packs plus any distinct configured default into `config/irisworldgen/packs` |
 
-Manual install: `/iris download <pack>` (alias `dl`). Default overworld uses the beta-release path; other packs use `IrisDimensions/<pack>/<branch>` (plugin default branch `stable` for non-default; mod download defaults branch `stable` unless auto-prefetch uses `master` — see `25 - Pack Management.md`).
+Manual install: `/iris download <pack>` (alias `dl`). `overworld` and `underworld` use their beta-release assets and ignore the branch argument; other packs use `IrisDimensions/<pack>/<branch>` (default branch `stable` — see `25 - Pack Management.md`).
 
 ## Installation recovery
 
 | Symptom | Check | Recovery |
 |---|---|---|
 | Iris does not appear in `/iris version` | Wrong directory, wrong platform jar, duplicate jar, Java mismatch, or an enable exception | Stop the server, keep only the matching artifact, confirm Java 25, and fix the first Iris exception in the startup log |
-| `settings.json` exists but `packs/overworld` does not | Default-pack download failed or is still incomplete | Restore outbound HTTPS or install a complete pack, then restart; do not create an empty `overworld` folder |
+| `settings.json` exists but a managed pack is absent | Managed beta download failed or is still incomplete | Restore outbound HTTPS or install the complete release pack, then restart; do not create an empty pack folder |
 | Pack validates but modded height/biomes use fallbacks | Forced datapack was generated after registries loaded | Restart once with the pack already installed, then create a new disposable world |
 | Bukkit command is denied for a non-op | `iris.all` is missing | Grant `iris.all`; `iris.treefeller` controls only survival tree felling |
 | Client HUD is absent but server commands work | Client mod missing, disabled keybind, or server capability not negotiated | Install the matching client mod, reconnect, and verify the Iris keybind category; server generation does not require the client HUD |

docs/02 - Getting Started.md

@@ -15,7 +15,7 @@ Treat each numbered section as a gate. Confirm the world is loaded before telepo
 - Iris installed per `01 - Installation & Platforms.md`
 - Java 25 server or mod instance running
 - Operator / gamemaster access (`iris` commands; modded mutating commands require permission level 2 / gamemasters)
-- Default pack present (auto-downloaded on first boot) or an installed pack under the platform packs directory
+- Managed Overworld and Underworld packs present (auto-downloaded on first boot) or the required project pack installed under the platform packs directory
 
 ## Argument style
 
@@ -263,7 +263,7 @@ The session passes when the production world loads again after a clean restart a
 | Bukkit optional args without `key=` | Parse error | Use `seed=1337`, not a bare second number for optional params |
 | Mod pregen while another job runs | Start fails | `/iris pregen stop` then start again |
 | Studio closed mid-edit | World discarded | Edits on disk in `packs/` remain; reopen studio |
-| Default pack download blocked | Create/open fails missing pack | Allow HTTPS or `/iris download overworld` offline install of a pack tree |
+| Managed pack download blocked | Startup or create/open fails with a missing pack | Allow HTTPS or install with `/iris download overworld` and `/iris download underworld`; an offline install must contain each complete pack tree |
 | `type=default` vs pack key | Resolves via `generator.defaultWorldType` | Prefer explicit `type=overworld` or your pack key |
 
 ## Quick reference

docs/03 - Configuration.md

@@ -244,8 +244,8 @@ Path: `<configDir>/irisworldgen/modded.json`. Written with defaults on first loa
 
 | Key | Type | Default | Notes |
 |-----|------|---------|-------|
-| `defaultPack` | string | `"overworld"` | Default pack for bootstrap download/install |
-| `autoDownloadDefaultPack` | boolean | `true` | Download default pack when missing |
+| `defaultPack` | string | `"overworld"` | Default create pack; a distinct non-managed value is also prefetched when enabled |
+| `autoDownloadDefaultPack` | boolean | `true` | Download missing managed Overworld/Underworld beta packs and any distinct configured default |
 | `primaryWorld` | string | `""` | Primary Iris dimension id for player routing |
 | `routePlayersToPrimaryWorld` | boolean | `true` | Route players to primary when set |
 | `mainWorldPack` | string | `""` | Pack (or `pack:dimensionKey`) for main-world preset |

docs/04 - Commands & Permissions.md

@@ -79,7 +79,7 @@ Tree feller on mod loaders uses platform permission APIs (`irisworldgen:treefell
 | `load` | `import` | **Bukkit** | `<world>` | Load managed Iris world |
 | `unload` | | **Bukkit** | `<world>` | Unload Iris world |
 | `debug` | | Both | — | Toggle `general.debug` and save settings |
-| `download` | `dl` | Both | `<pack> [branch=stable] [overwrite=false]` (`overwrite` alias `force`) | Download pack project |
+| `download` | `dl` | Both | `<pack> [branch=stable] [overwrite=false]` (`overwrite` alias `force`) | Download a pack; `overworld` and `underworld` resolve to managed beta release ZIPs |
 | `metrics` | `measure` | Both | — | Generation metrics (player / current Iris level) |
 | `reload` | | Both | — | Reload `settings.json` and locale; modded also schedules forced datapack regeneration |
 | `seed` | | **Modded** | — | Print world/engine seeds (gamemaster) |
@@ -102,7 +102,7 @@ Tree feller on mod loaders uses platform permission APIs (`irisworldgen:treefell
 
 ---
 
-On Bukkit, `overwrite=true` is deliberately restart-only. The name may resolve to a safe `iris:*` world or exactly the configured main, `_nether`, or `_the_end` alias; arbitrary `minecraft:*` and foreign namespaces are rejected. Iris stages and validates a fresh pack snapshot, compare-and-swaps only that world's `bukkit.yml` generator and seed, and retains the existing dimension folder as a rollback backup until the restarted world proves its Iris identity, pack, dimension, environment, and seed. Multiple distinct slots may be staged before one restart. `main=true` is valid with overwrite only when the name is the configured main-world name. Exact vanilla slots preserve the authoritative seed shared by the existing level, regardless of the supplied `seed`; this keeps Overworld/Nether/End coordinate generation aligned. Use ordinary new-main promotion when a new level seed is required.
+On Paper-family servers, `overwrite=true` is deliberately restart-only; Spigot rejects it because it has no pre-registry plugin bootstrap. The exact target dimension folder must already exist; use ordinary `/iris create` for a new world. The name may resolve to a safe `iris:*` world or exactly the configured main, `_nether`, or `_the_end` alias; arbitrary `minecraft:*` and foreign namespaces are rejected. Iris stages and validates a fresh pack snapshot, compare-and-swaps only that world's `bukkit.yml` generator and seed, and retains the existing dimension folder as a rollback backup until the restarted world proves its Iris identity, pack, dimension, environment, and seed. Multiple distinct slots may be staged before one restart. `main=true` is valid with overwrite only when the name is the configured main-world name. Exact vanilla slots preserve the authoritative seed shared by the existing level, regardless of the supplied `seed`; this keeps Overworld/Nether/End coordinate generation aligned. Use ordinary new-main promotion when a new level seed is required.
 
 ---
 

docs/05 - Concepts & Pack Layout.md

@@ -103,7 +103,7 @@ Live overworld also contains authoring-only or empty trees that loaders do not r
 2. `dimensions/` is missing.
 3. `dimensions/` has no `*.json` files.
 
-A downloaded archive is also rejected unless it contains exactly one loadable dimension (install key becomes that dimension’s load key). Presence of a pack on disk is defined as a safe pack directory with at least one non-symlink `dimensions/*.json` file.
+A downloaded archive without an expected key is rejected unless it contains exactly one loadable dimension, whose load key becomes the install key. Managed-release and listing downloads carry an exact expected primary key, so their archive may retain additional dimension resources; the expected dimension selects the folder key and the entire pack still validates before publication. Presence of a pack on disk is defined as a safe pack directory with at least one non-symlink `dimensions/*.json` file.
 
 ## Snippets
 

docs/06 - Worlds & Lifecycle.md

@@ -126,11 +126,11 @@ Runtime world creation is disabled on Folia. `/iris create` instead:
 
 ## Exact world-slot replacement
 
-`overwrite=true` uses lifecycle kind `WORLD_REPLACE` and always stages for restart on Bukkit-family servers, including Paper and Folia. It accepts safe `iris:*` keys and only the three exact vanilla slots resolved from the configured level name: `minecraft:overworld`, `minecraft:the_nether`, and `minecraft:the_end`. A vanilla slot requires a matching pack environment (`NORMAL`, `NETHER`, or `THE_END`), and Nether/End replacement requires the server's matching allow setting to be enabled; foreign namespaces, other `minecraft:*` keys, path traversal, links, and special filesystem entries fail closed. `main=true` may accompany overwrite only for the configured main-world name. Minecraft stores one authoritative seed for the existing level, so all three exact vanilla slots preserve that loaded primary-world seed and report when it differs from the command's `seed`; changing the level seed remains the ordinary new-main promotion workflow.
+`overwrite=true` uses lifecycle kind `WORLD_REPLACE` and always stages for a complete Paper-family restart, including Paper, Purpur, Leaf, and Folia; Spigot has no early registry bootstrap and rejects this mode. The exact target dimension folder must already exist; ordinary create remains the path for a new world. It accepts safe `iris:*` keys and only the three exact vanilla slots resolved from the configured level name: `minecraft:overworld`, `minecraft:the_nether`, and `minecraft:the_end`. A vanilla slot requires a matching pack environment (`NORMAL`, `NETHER`, or `THE_END`), and Nether/End replacement requires the server's matching allow setting to be enabled; foreign namespaces, other `minecraft:*` keys, path traversal, links, and special filesystem entries fail closed. `main=true` may accompany overwrite only for the configured main-world name. Minecraft stores one authoritative seed for the existing level, so all three exact vanilla slots preserve that loaded primary-world seed and report when it differs from the command's `seed`; changing the level seed remains the ordinary new-main promotion workflow.
 
-The transaction copies and validates a fresh frozen pack under a same-filesystem sibling stage, fingerprints it, journals the original target state and `bukkit.yml` generator/seed, then compare-and-swaps that one configuration entry. Distinct slots can be queued before one restart. During Iris `STARTUP`, before Bukkit loads worlds, each authorized transaction atomically moves the old exact dimension directory to a retained sibling backup and publishes its stage. There is no chunk merge: old region, entity, POI, and Iris data remain only in the backup, while the target starts with the staged pack snapshot.
+The transaction copies and validates a fresh frozen pack under a same-filesystem sibling stage, fingerprints it, binds its journal to the canonical level root and logical world name, records the original target and `bukkit.yml` generator/seed, then compare-and-swaps that one configuration entry. Distinct slots can be queued before one restart. Paper bootstrap reconciles each authorized transaction before Iris compiles its aggregate datapack or Minecraft builds registries: it atomically moves the old exact dimension directory to a retained sibling backup and publishes the stage. The filesystem must support atomic replacement for the world directories, journal, and `bukkit.yml`; Iris refuses the operation without falling back to a non-atomic destructive move. Publication retains Paper's per-world `data/paper/metadata.dat`, `data/paper/level_overrides.dat`, and `data/minecraft/world_gen_settings.dat` so the replacement keeps the exact slot metadata and authoritative seed. Old `region`, `entities`, `poi`, and Iris runtime data are never merged; they remain only in the backup while the target starts with the staged pack snapshot.
 
-The backup is deleted only after `WorldLoad` proves the exact namespaced identity, Iris generator, selected dimension, seed, vanilla-slot environment, and unchanged pack fingerprint. A failed runtime check journals rollback, restores the prior `bukkit.yml` generator/seed with compare-and-swap semantics, requests another restart, and restores the retained directory before that restart loads worlds. A crash between either atomic move or journal write is retried idempotently. Conflicting manual configuration, changed staged bytes, unsafe storage, or corrupt journals block Iris world admission and preserve the stage/backup for operator recovery instead of guessing or deleting.
+The backup is eligible for asynchronous deletion only after `WorldLoad` proves the exact namespaced identity, Iris generator, selected dimension, seed, vanilla-slot environment, and unchanged pack fingerprint; cleanup failure retains its committed journal and retries without rolling back a verified world. A failed runtime check journals rollback and requests another restart, then cold bootstrap restores the retained directory and prior `bukkit.yml` generator/seed before registry or world loading. A crash between any move, configuration write, or journal phase is retried idempotently. Conflicting manual configuration, changed roots or logical names, changed staged bytes, unsafe storage, duplicate/corrupt journals, or irreconcilable transaction state abort the early bootstrap and preserve recoverable artifacts instead of guessing or deleting.
 
 ## Studio create
 

docs/25 - Pack Management.md

@@ -34,13 +34,13 @@ If validation reports a missing edge, restore or repair that resource before pac
 | Command | Behavior |
 |---------|----------|
 | Bukkit: `/iris download <pack> [branch=stable] [overwrite=false]`; modded: `/iris download <pack> [branch] [force]` | Download into packs root |
-| Default overworld special case | Pack name `overworld` uses IrisDimensions overworld **beta release zip** (`…/releases/download/beta/overworld.zip`), not an arbitrary branch zip |
+| Managed beta packs | Pack names `overworld` and `underworld` use their IrisDimensions **beta release ZIPs**, not arbitrary branch archives |
 | Other packs | `IrisDimensions/<pack>/<branch>` GitHub archive search via `StudioSVC.downloadSearch` |
 
 | Param | Default | Notes |
 |-------|---------|-------|
 | `pack` | required | Folder/key or repo short name |
-| `branch` | `stable` | GitHub ref when not default overworld |
+| `branch` | `stable` | GitHub ref when the pack is not a managed beta |
 | `overwrite` | `false` | Force replace existing present pack |
 
 ### Install pipeline (`PackDownloader`)
@@ -48,12 +48,12 @@ If validation reports a missing edge, restore or repair that resource before pac
 1. Per-key/ref download lock (concurrent startup and commands do not double-fetch).
 2. If pack present and not force → skip network.
 3. Download zip (size/entry limits: archive ≤512MiB, ≤100k entries, total uncompressed budget, per-file cap).
-4. Unpack to temp; require single pack home directory.
-5. Open as datapack-compiler `IrisData`; require **exactly one** dimension; key = that dimension load key.
+4. Unpack to temp; require a single pack home directory.
+5. Open as datapack-compiler `IrisData`. A normal download without an expected key requires exactly one dimension; a managed or listing download selects its exact expected dimension while retaining and validating any additional dimension resources in the same pack.
 6. Run `PackValidator.validate`; blocking errors abort install.
 7. Publish into `packs/<key>/` with conflict checks (refuses symlink targets; detects dimension-key conflicts with other folders).
 
-Default overworld repository constant: `IrisDimensions/overworld`.
+Managed beta sources are `IrisDimensions/overworld` (`overworld.zip`) and `IrisDimensions/underworld` (`underworld.zip`). Startup treats their ownership independently: an operator-edited or linked pack is preserved without preventing the other managed pack from updating.
 
 ## Validate
 

docs/27 - Example - Configuring Overworld.md

@@ -109,7 +109,7 @@ Worlds created from a pack store a **copy** at:
 
 `StudioSVC.installIntoWorld` and `replaceIntoWorld` copy the source pack tree into that directory. Runtime generation for a normal world reads the world copy, not the global `packs/` tree. Studio worlds hotload the pack under `packs/` directly.
 
-First install often downloads the default overworld release into `packs/` (`downloadDefaultOverworld` / `/iris download` flows — see `02 - Getting Started.md`, `25 - Pack Management.md`).
+First install downloads the managed Overworld and Underworld beta releases into `packs/`; `/iris download overworld` uses the same Overworld asset (see `02 - Getting Started.md`, `25 - Pack Management.md`).
 
 ## High-level layout (shipping overworld)
 

docs/30 - Platform Differences.md

@@ -41,7 +41,7 @@ Hotload: Bukkit file-watch engine; modded 3s poll. Same invalidate/reload/locale
 
 | Concern | Bukkit | Modded |
 |---------|--------|--------|
-| Create | `/iris create` → managed world name, generator Iris, optional main-world; `overwrite=true` stages exact Iris/vanilla-slot replacement for restart | `/iris create` or `/iris world enable` → dimension id + pack injection |
+| Create | `/iris create` → managed world name, generator Iris, optional main-world; on Paper-family servers `overwrite=true` stages replacement of an existing exact Iris/vanilla slot for restart | `/iris create` or `/iris world enable` → dimension id + pack injection |
 | Load / unload | `/iris load` (`import`), `/iris unload` | `/iris world disable` unloads; no separate load command |
 | Remove / delete | `/iris remove` optional folder delete | `/iris world delete` wipes chunk/mantle data |
 | Primary / main world | create `main=true` for a new level root, or name the configured main with `overwrite=true` for journaled in-place dimension replacement | `modded.json` primary + `routePlayersToPrimaryWorld`; `/iris world mainworld`, `replace-overworld` |
@@ -49,7 +49,7 @@ Hotload: Bukkit file-watch engine; modded 3s poll. Same invalidate/reload/locale
 | Studio world | Transient studio world via StudioSVC; `/iris jigsaw` can select the Jigsaw Studio generator for one activation | Studio dimension under `irisworldgen:studio_*`; no Jigsaw Studio authoring command tree |
 | Folia | Regionized schedulers; pregen `runtimeSchedulerMode` forces `FOLIA` when regionized | N/A (not Bukkit Folia) |
 
-Default pack bootstrap still downloads the IrisDimensions overworld release into `packs/overworld` when missing (shared provisioner).
+Startup installs the IrisDimensions Overworld and Underworld beta releases into `packs/overworld` and `packs/underworld` when missing. Paper bootstrap publishes both in one rollback scope before compiling the aggregate datapack; legacy Bukkit and modded startup use the same managed release sources.
 
 Modded startup quarantines a corrupt persistent-dimension registry as `iris-dimensions.json.broken-<timestamp>` and continues without those dynamic worlds. Recovery details are in `06 - Worlds & Lifecycle.md`.
 
@@ -76,7 +76,7 @@ Jigsaw pack resources are shared runtime data, but in-game Jigsaw Studio is not
 | Jigsaw Studio create/grid/marker capture/rules/export | yes | no | no | no |
 | Saved planar/spatial Iris jigsaw runtime | yes | yes | yes | yes |
 | Pack validate / cleanup / download | yes | yes | yes | yes |
-| Exact restart replacement of configured Overworld/Nether/End slots | yes | no | no | no |
+| Exact restart replacement of configured Overworld/Nether/End slots | Paper/Purpur/Leaf/Folia | no | no | no |
 | Pregen | yes (Paper-like / Folia modes) | yes (`moddedPregenInFlight`) | yes | yes |
 | Studio open/close/vscode/package | yes | yes | yes | yes |
 | Object wand / paste / save / undo | yes | yes | yes | yes |

docs/31 - Operator Runbooks & Smoke Tests.md

@@ -25,7 +25,7 @@ GoldenHash details and file layout: `32 - Determinism & Goldenhash.md`.
 ## A. Fresh install and first world (Bukkit-family)
 
 1. Install the CraftBukkit-family jar into `plugins/` (Paper, Purpur, Folia, Spigot, Leaf, Canvas as advertised). Require Java 25. See `01 - Installation & Platforms.md`.
-2. Start the server once. Confirm Iris enables, default pack download completes when no pack is present, and `settings.json` is written under the Iris data directory.
+2. Start the server once. Confirm Iris enables, managed `overworld` and `underworld` beta downloads complete when absent, and `settings.json` is written under the Iris data directory.
 3. Create a world with a fixed seed and teleport into it:
 
 ```
@@ -36,21 +36,21 @@ GoldenHash details and file layout: `32 - Determinism & Goldenhash.md`.
 4. Join or teleport into the world. Confirm non-empty terrain, surface biomes, and no repeating console stack traces on first chunks.
 5. Gate: world is loaded as an Iris world; chunks generate without enable-time crash; console shows no fatal engine init failure.
 
-## A.1 Exact vanilla-slot replacement (Bukkit-family)
+## A.1 Exact vanilla-slot replacement (Paper-family)
 
-Use a disposable server whose configured level name is `world`, with a valid `NETHER` Iris pack and a generated vanilla Nether containing a unique marker chunk. Record hashes of the old Nether `region`, `entities`, and `poi` files before staging.
+Use a disposable server whose configured level name is `world`, with a valid `NETHER` Iris pack and a generated vanilla Nether containing a unique marker chunk. Record hashes of the old Nether `region`, `entities`, and `poi` files, and retain copies of `data/paper/metadata.dat`, `data/paper/level_overrides.dat`, and `data/minecraft/world_gen_settings.dat` for comparison before staging.
 
 1. Run `/iris create world_nether type=<nether-pack> seed=1337 overwrite=true`. Gate: the command says the replacement is staged, the loaded Nether and its files remain unchanged, `bukkit.yml` now names `Iris:<dimension>`, and one pending replacement journal plus one sibling stage exists.
 2. Optionally stage the configured main name with a `NORMAL` pack and the End alias with a `THE_END` pack. Gate: each distinct slot gets its own transaction and no live dimension folder is moved.
-3. Restart normally. Gate: Iris publishes before Bukkit world loading; `minecraft:the_nether` loads with the Iris generator, requested dimension and seed; its frozen `iris/pack` exists; no old `region`, `entities`, or `poi` file was merged into the target; and the marker chunk is absent.
-4. Gate after `WorldLoad`: the retained sibling backup and journal disappear only after identity, environment, seed, dimension, and pack-fingerprint verification succeeds.
+3. Restart normally. Gate: Iris publishes before aggregate-datapack compilation and Bukkit world loading; `minecraft:the_nether` loads with the selected Iris dimension, the prior Paper per-world metadata files, and the authoritative shared level seed; its frozen `iris/pack` exists; no old `region`, `entities`, or `poi` file was merged into the target; and the marker chunk is absent.
+4. Gate after `WorldLoad`: the journal advances to committed cleanup only after identity, environment, seed, dimension, and pack-fingerprint verification succeeds; asynchronous cleanup then removes the retained sibling backup and journal without stalling the world thread.
 5. Restart again and generate fresh Nether chunks. Gate: the exact vanilla identity and Iris generator persist, ordinary Nether portals still target `minecraft:the_nether`, and no pending stage/backup/journal returns.
-6. Repeat once with a deliberately changed staged pack or conflicting `bukkit.yml` value before restart. Gate: Iris refuses publication or world admission, preserves recoverable artifacts, and never guesses a target. For a post-publication verification failure, gate that Iris restores the prior configuration, requests the controlled rollback restart, and restores the retained original directory before world load.
+6. Repeat once with a deliberately changed staged pack or conflicting `bukkit.yml` value before restart. Gate: early Paper bootstrap aborts before registry/world loading, preserves recoverable artifacts, and never guesses a target. For a post-publication verification failure, gate that Iris journals rollback and requests the controlled restart, then restores the retained original directory and prior configuration before datapack compilation or world loading.
 
 ## B. Fresh install and first world (Fabric / Forge / NeoForge)
 
 1. Install the matching mod jar into `mods/`. Fabric requires Loader ≥ declared floor; Forge/NeoForge require their declared floors. See `01 - Installation & Platforms.md` and `30 - Platform Differences.md`.
-2. Start dedicated server (or integrated singleplayer for client-mod smoke). Confirm Iris boots, default pack installs, and datapack/biome registration completes.
+2. Start dedicated server (or integrated singleplayer for client-mod smoke). Confirm Iris boots, both managed beta packs install, and datapack/biome registration completes.
 3. Create a world with fixed seed (positional mod syntax):
 
 ```

docs/87 - Maintainer - Release Readiness.md

@@ -270,7 +270,7 @@ Gate: every advertised server, loader, client, and content path completes the sa
 
 ### Confirmed release blockers and follow-ups
 
-- [ ] Freeze the default overworld pack to an immutable release input. The runtime downloader currently follows the mutable `master` branch, so any recorded tree checksum remains reproducible only while that upstream content is unchanged. Immutable branch/tag/commit URL resolution is implemented, but published commit `8e32852ee6ecd039fae27a36f701f57cdc02e83f` predates the five local slime-category and biome-tag corrections, the dormant standard entity resource restoration, and removal of the legacy default ambient-spawner attachments; publish those pack edits under a new commit/tag before pinning automatic installs.
+- [ ] Publish and retain anonymously downloadable `beta` assets for both managed pack repositories before shipping a build that requires dual bootstrap. Runtime uses the mutable Overworld and Underworld beta release URLs; record each downloaded asset checksum for a release baseline, and move production installs to immutable release inputs when the beta streams are promoted.
 - [x] Make modded GoldenHash metadata use the active Iris engine seed. Fabric, Forge, and NeoForge generated identical output from Iris seed `1337`, but filenames and headers recorded each vanilla level seed, preventing one captured baseline file from being reused directly across loaders.
 - [x] Correct the default overworld pack's slime spawn category from implicit `MISC` to explicit `MONSTER` in `biomes/vanilla/mangrove_swamp.json`, `biomes/swamp/cambian-drift.json`, `biomes/swamp/cambian-drift-extended.json`, `biomes/swamp/marsh.json`, and `biomes/swamp/marsh-rotten.json`. NeoForge exposes the bad category at startup; all loaders generate the same bad datapack entry, which can affect natural slime spawning and mob-cap accounting.
 - [x] Extend `PackValidator` to reject authored custom-biome spawn categories that disagree with the live entity category instead of allowing the bad datapack to reach loader validation.

docs/94 - API - Modded.md

@@ -248,7 +248,7 @@ Paths relative to the loader config directory (`config/`):
 
 Pack install root is `config/irisworldgen/packs`, not `config/iris`. Missing pack at world open is a hard failure with the expected absolute path (no silent vanilla terrain).
 
-Async default-pack prefetch at boot when `autoDownloadDefaultPack` is set and `defaultPack` is missing (`IrisDimensions/<pack>` from `master`). Failures log a pointer to `/iris download <pack>`.
+When `autoDownloadDefaultPack` is enabled, startup asynchronously installs missing `overworld` and `underworld` managed beta releases, plus a distinct configured non-managed `defaultPack`. Failures log a pointer to `/iris download <pack>`; managed beta names always resolve to their release assets rather than the requested branch.
 
 Forced datapack contributes presets, dimension types, and biomes under `irisworldgen` (ids from pack/dimension names). Regenerated on pack change / studio hotload. Failure to inject (mixin/event not applied) logs once: