Fix seeded entity spawning

- Derive initial chunk and marker populations from stable location seeds.
- Persist initial spawn completion after region-owned application.
- Seed entity loot and attribute modifier identities.
- Cover spawning, reload completion, and registry fixtures with regression tests.

core/src/main/java/art/arcane/iris/generation/runtime/MarkerSpawnScanner.java

@@ -67,6 +67,7 @@ final class MarkerSpawnScanner {
         J.a(manager.managedTask("bukkit_world_manager_marker_scan", () -> {
             try {
                 if (initialOnly && manager.getMantle().hasFlag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER)) {
+                    manager.chunkMaintenance.recordInitialSpawnCompletion(chunkX, chunkZ);
                     markerScanQueue.remove(key);
                     return;
                 }
@@ -91,7 +92,7 @@ final class MarkerSpawnScanner {
                                         continue;
                                     }
                                     prepared.add(new PreparedMarkerSpawn(new IrisPosition(position.getX(),
-                                            position.getY() + minimumY, position.getZ()), data.spawners, data.environment));
+                                            position.getY() + minimumY, position.getZ()), data.spawners, data.environment, data.marker));
                                 }
                                 consumer.accept(List.copyOf(prepared));
                             } finally {
@@ -137,7 +138,7 @@ final class MarkerSpawnScanner {
             }
 
             IrisPosition position = new IrisPosition((chunkX << 4) + x, y, (chunkZ << 4) + z);
-            MarkerSpawnData data = markerData.computeIfAbsent(position, k -> new MarkerSpawnData(environment));
+            MarkerSpawnData data = markerData.computeIfAbsent(position, k -> new MarkerSpawnData(environment, mark));
             data.requiresEmptyAbove = data.requiresEmptyAbove || mark.isEmptyAbove();
 
             for (String i : mark.getSpawners()) {
@@ -149,7 +150,6 @@ final class MarkerSpawnScanner {
                 if (spawner.isCompatExcluded()) {
                     continue;
                 }
-                spawner.setReferenceMarker(mark);
                 data.spawners.add(spawner);
             }
         });
@@ -184,16 +184,18 @@ final class MarkerSpawnScanner {
         }));
     }
 
-    record PreparedMarkerSpawn(IrisPosition position, KSet<IrisSpawner> spawners, BiomeEnvironment environment) {
+    record PreparedMarkerSpawn(IrisPosition position, KSet<IrisSpawner> spawners, BiomeEnvironment environment, IrisMarker marker) {
     }
 
     private static final class MarkerSpawnData {
         private final KSet<IrisSpawner> spawners = new KSet<>();
         private final BiomeEnvironment environment;
+        private final IrisMarker marker;
         private boolean requiresEmptyAbove;
 
-        private MarkerSpawnData(BiomeEnvironment environment) {
+        private MarkerSpawnData(BiomeEnvironment environment, IrisMarker marker) {
             this.environment = environment;
+            this.marker = marker;
         }
     }
 }

core/src/main/java/art/arcane/iris/generation/runtime/WorldChunkMaintenance.java

@@ -20,6 +20,7 @@ package art.arcane.iris.generation.runtime;
 
 import art.arcane.iris.configuration.IrisSettings;
 import art.arcane.iris.generation.cache.Cache;
+import art.arcane.iris.generation.context.IrisContext;
 import art.arcane.iris.world.history.GenerationHistoryRuntimeRouter;
 import art.arcane.iris.world.history.GenerationHistoryRuntimeRouter.SavedChunkMantle;
 import art.arcane.iris.world.history.SavedBiomeUnavailableException;
@@ -35,6 +36,7 @@ import art.arcane.volmlib.util.math.PowerOfTwoCoordinates;
 import art.arcane.volmlib.util.math.Position2;
 import art.arcane.volmlib.util.matter.Matter;
 import org.bukkit.Chunk;
+import org.bukkit.GameRules;
 import org.bukkit.World;
 import org.bukkit.entity.Player;
 
@@ -44,6 +46,7 @@ import java.util.Comparator;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ForkJoinPool;
@@ -59,10 +62,12 @@ final class WorldChunkMaintenance {
     private static final int MAX_FORCED_CHUNK_UPDATES = 128;
     private static final int MAX_PENDING_MATERIALIZATIONS = 128;
     private static final int MAX_COMPLETED_MATERIALIZATIONS = 8192;
+    private static final int MAX_COMPLETED_INITIAL_SPAWNS = 8192;
 
     private final IrisWorldManager manager;
     private final Set<Long> mantleWarmupQueue = ConcurrentHashMap.newKeySet();
     private final Set<Long> markerFlagQueue = ConcurrentHashMap.newKeySet();
+    private final Set<Long> completedInitialSpawns = Collections.synchronizedSet(new LinkedHashSet<>());
     private final Set<Long> discoveredFlagQueue = ConcurrentHashMap.newKeySet();
     private final Set<Long> chunkUpdateQueue = ConcurrentHashMap.newKeySet();
     private final Map<Long, RoutedMaterialization> materializations = new ConcurrentHashMap<>();
@@ -85,6 +90,7 @@ final class WorldChunkMaintenance {
             }
         }
         completedMaterializations.clear();
+        completedInitialSpawns.clear();
     }
 
     void invalidateMaterialization(int chunkX, int chunkZ) {
@@ -375,46 +381,107 @@ final class WorldChunkMaintenance {
         }
     }
 
-    void raiseInitialSpawnMarkerFlag(World world, int chunkX, int chunkZ, Runnable onFirstRaise) {
-        if (world == null || onFirstRaise == null) {
+    boolean isInitialSpawnComplete(int chunkX, int chunkZ) {
+        return completedInitialSpawns.contains(Cache.key(chunkX, chunkZ));
+    }
+
+    void recordInitialSpawnCompletion(int chunkX, int chunkZ) {
+        rememberInitialSpawn(Cache.key(chunkX, chunkZ));
+    }
+
+    void runInitialSpawn(World world, int chunkX, int chunkZ, Runnable initialSpawn) {
+        if (world == null || initialSpawn == null || closed) {
             return;
         }
-
-        if (!J.isFolia()) {
-            manager.getMantle().raiseFlag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER, onFirstRaise);
-            return;
-        }
-
         long key = Cache.key(chunkX, chunkZ);
-        if (!markerFlagQueue.add(key)) {
+        if (completedInitialSpawns.contains(key) || !markerFlagQueue.add(key)) {
             return;
         }
+        J.a(manager.managedTask("bukkit_world_manager_initial_spawn_prepare",
+                () -> prepareInitialSpawnFlag(world, chunkX, chunkZ, key, initialSpawn),
+                () -> markerFlagQueue.remove(key)));
+    }
 
-        J.a(manager.managedTask("bukkit_world_manager_spawn_marker_flag", () -> {
-            boolean raised = false;
-            try {
-                Mantle<Matter> mantle = manager.getMantle();
-                if (!mantle.hasFlag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER)) {
-                    mantle.flag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER, true);
-                    raised = true;
-                }
-            } catch (Throwable e) {
-                IrisLogging.reportError(e);
-            } finally {
+    private void prepareInitialSpawnFlag(World world, int chunkX, int chunkZ, long key, Runnable initialSpawn) {
+        try {
+            if (manager.getMantle().hasFlag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER)) {
+                rememberInitialSpawn(key);
                 markerFlagQueue.remove(key);
-            }
-
-            if (!raised) {
                 return;
             }
+            boolean accepted = J.runRegion(world, chunkX, chunkZ,
+                    manager.managedTask("bukkit_world_manager_initial_spawn_apply",
+                            () -> applyInitialSpawn(world, chunkX, chunkZ, key, initialSpawn),
+                            () -> markerFlagQueue.remove(key)));
+            if (!accepted) {
+                markerFlagQueue.remove(key);
+            }
+        } catch (Throwable failure) {
+            markerFlagQueue.remove(key);
+            IrisLogging.reportError("Failed to prepare initial Iris entities in chunk " + chunkX + "," + chunkZ + ".", failure);
+        }
+    }
 
-            J.runRegion(world, chunkX, chunkZ, manager.managedTask("bukkit_world_manager_spawn_marker_callback", () -> {
-                if (!world.isChunkLoaded(chunkX, chunkZ) || !Chunks.isSafe(world, chunkX, chunkZ)) {
-                    return;
-                }
-                onFirstRaise.run();
-            }));
-        }, () -> markerFlagQueue.remove(key)));
+    private void applyInitialSpawn(World world, int chunkX, int chunkZ, long key, Runnable initialSpawn) {
+        GenerationSessionLease completionLease = null;
+        try {
+            if (closed || !world.isChunkLoaded(chunkX, chunkZ) || !Chunks.isSafe(world, chunkX, chunkZ)
+                    || !Boolean.TRUE.equals(world.getGameRuleValue(GameRules.SPAWN_MOBS))) {
+                markerFlagQueue.remove(key);
+                return;
+            }
+            Optional<GenerationSessionLease> admitted = initialCompletionLease();
+            if (admitted.isEmpty()) {
+                markerFlagQueue.remove(key);
+                return;
+            }
+            completionLease = admitted.get();
+            initialSpawn.run();
+            rememberInitialSpawn(key);
+            completionLease.detachThread();
+            GenerationSessionLease transferred = completionLease;
+            J.a(() -> persistInitialSpawnFlag(chunkX, chunkZ, key, transferred));
+            completionLease = null;
+        } catch (Throwable failure) {
+            markerFlagQueue.remove(key);
+            IrisLogging.reportError("Failed to apply initial Iris entities in chunk " + chunkX + "," + chunkZ + ".", failure);
+        } finally {
+            if (completionLease != null) {
+                completionLease.close();
+            }
+        }
+    }
+
+    private Optional<GenerationSessionLease> initialCompletionLease() throws GenerationSessionException {
+        Engine engine = manager.getEngine();
+        GenerationSessionManager sessions = engine.getGenerationSessions();
+        if (sessions == null) {
+            return Optional.of(GenerationSessionLease.noop());
+        }
+        IrisContext context = IrisContext.get();
+        if (context != null && context.getEngine() == engine && context.getGenerationSessionId() != 0L) {
+            return Optional.of(sessions.continueSession("bukkit_world_manager_initial_spawn_complete", context.getGenerationSessionId()));
+        }
+        return sessions.tryAcquireForEngine(engine, "bukkit_world_manager_initial_spawn_complete");
+    }
+
+    private void persistInitialSpawnFlag(int chunkX, int chunkZ, long key, GenerationSessionLease lease) {
+        try (lease; IrisContext.Scope ignored = IrisContext.open(manager.getEngine(), lease.sessionId(), null)) {
+            manager.getMantle().flag(chunkX, chunkZ, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+        } catch (Throwable failure) {
+            IrisLogging.reportError("Failed to save initial Iris entity completion in chunk " + chunkX + "," + chunkZ + ".", failure);
+        } finally {
+            markerFlagQueue.remove(key);
+        }
+    }
+
+    private void rememberInitialSpawn(long key) {
+        synchronized (completedInitialSpawns) {
+            completedInitialSpawns.add(key);
+            if (completedInitialSpawns.size() > MAX_COMPLETED_INITIAL_SPAWNS) {
+                completedInitialSpawns.remove(completedInitialSpawns.iterator().next());
+            }
+        }
     }
 
     void warmupMantleChunkAsync(int chunkX, int chunkZ) {

core/src/main/java/art/arcane/iris/generation/runtime/WorldEntitySpawner.java

@@ -25,6 +25,9 @@ import art.arcane.volmlib.util.math.Rarity;
 import art.arcane.iris.world.history.SavedBiomeUnavailableException;
 import art.arcane.iris.generation.biome.IrisBiome;
 import art.arcane.iris.world.entity.IrisEntitySpawn;
+import art.arcane.iris.world.entity.EntitySpawnSeed;
+import art.arcane.iris.world.entity.IrisMarker;
+import art.arcane.iris.world.entity.IrisEntitySpawn.SpawnContext;
 import art.arcane.iris.pack.value.IrisPosition;
 import art.arcane.iris.world.entity.IrisSpawner;
 import art.arcane.iris.platform.bukkit.BukkitWorldBinding;
@@ -47,6 +50,9 @@ import org.bukkit.entity.LivingEntity;
 import org.bukkit.entity.Player;
 
 import java.util.List;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.Objects;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Optional;
@@ -57,10 +63,7 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
-import java.util.function.Predicate;
 import java.util.function.Supplier;
-import java.util.stream.Collectors;
-import java.util.stream.Stream;
 
 /**
  * Ambient and marker entity spawning for a Bukkit Iris world. Every count and spawn hops onto the
@@ -308,12 +311,15 @@ final class WorldEntitySpawner {
             return;
         }
 
+        if (!manager.chunkMaintenance.isInitialSpawnComplete(c.getX(), c.getZ())) {
+            spawnInitially(c);
+            return;
+        }
         if (IrisSettings.get().getWorld().isMarkerEntitySpawningSystem()) {
             manager.markerScanner.scanMarkerSpawners(c, false, markers -> {
                 for (PreparedMarkerSpawn marker : markers) {
                     spawnPreparedMarker(marker, false);
                 }
-                prepareInitialSpawn(c, markers);
             });
         }
 
@@ -333,7 +339,9 @@ final class WorldEntitySpawner {
     }
 
     void spawnInitially(Chunk chunk) {
-        if (manager.getEngine().isClosed() || !isEntitySpawningEnabledForCurrentWorld()) {
+        if (manager.getEngine().isClosed() || !isEntitySpawningEnabledForCurrentWorld()
+                || manager.chunkMaintenance.isInitialSpawnComplete(chunk.getX(), chunk.getZ())
+                || !Boolean.TRUE.equals(chunk.getWorld().getGameRuleValue(GameRules.SPAWN_MOBS))) {
             return;
         }
         if (IrisSettings.get().getWorld().isMarkerEntitySpawningSystem()) {
@@ -356,71 +364,94 @@ final class WorldEntitySpawner {
             }
         }
         Optional<BiomeEnvironment> preparedEnvironment = environment;
-        manager.chunkMaintenance.raiseInitialSpawnMarkerFlag(chunk.getWorld(), chunk.getX(), chunk.getZ(), () -> {
-            for (PreparedMarkerSpawn marker : markers) {
+        List<PreparedMarkerSpawn> ordered = new ArrayList<>(markers);
+        ordered.sort(Comparator.comparingInt((PreparedMarkerSpawn marker) -> marker.position().getX())
+                .thenComparingInt(marker -> marker.position().getY())
+                .thenComparingInt(marker -> marker.position().getZ()));
+        manager.chunkMaintenance.runInitialSpawn(chunk.getWorld(), chunk.getX(), chunk.getZ(), () -> {
+            for (PreparedMarkerSpawn marker : ordered) {
                 spawnPreparedMarker(marker, true);
             }
             if (preparedEnvironment.isPresent()) {
-                J.runRegion(chunk.getWorld(), chunk.getX(), chunk.getZ(),
-                        manager.managedTask("bukkit_world_manager_initial_spawn_followup", () -> {
-                            if (!chunk.getWorld().isChunkLoaded(chunk.getX(), chunk.getZ())) {
-                                return;
-                            }
-                            BiomeEnvironment selected = preparedEnvironment.get();
-                            try (BiomeEnvironment.Scope ignored = manager.getEngine().openBiomeEnvironmentScope(selected)) {
-                                spawnAmbient(chunk, true, selected);
-                            }
-                        }), RNG.r.i(5, 200));
+                BiomeEnvironment selected = preparedEnvironment.get();
+                try (BiomeEnvironment.Scope ignored = manager.getEngine().openBiomeEnvironmentScope(selected)) {
+                    spawnAmbient(chunk, true, selected);
+                }
             }
         });
     }
 
     private void spawnPreparedMarker(PreparedMarkerSpawn marker, boolean initial) {
-        IrisSpawner spawner = new KList<>(marker.spawners()).getRandom();
-        if (spawner == null || manager.getEngine().isClosed()) {
+        if (manager.getEngine().isClosed()) {
+            return;
+        }
+        RNG random = initial ? EntitySpawnSeed.marker(manager.getEngine().getSeedManager().getEntity(),
+                marker.position().getX(), marker.position().getY(), marker.position().getZ()) : new RNG(RNG.r.nextLong());
+        KList<IrisSpawner> spawners = new KList<>(marker.spawners());
+        if (initial) {
+            spawners.sort(Comparator.comparing(spawner -> Objects.toString(spawner.getLoadKey(), "")));
+        }
+        IrisSpawner spawner = spawners.getRandom(random);
+        if (spawner == null) {
             return;
         }
         try (BiomeEnvironment.Scope ignored = manager.getEngine().openBiomeEnvironmentScope(marker.environment())) {
-            spawnMarker(marker.position(), spawner, initial);
+            spawnMarker(marker.position(), spawner, marker.marker(), random, initial);
         }
     }
 
-    private void spawnAmbient(Chunk c, boolean initial, BiomeEnvironment environment) {
-        if (!ambientAllowed(c, initial)) {
+    private void spawnAmbient(Chunk chunk, boolean initial, BiomeEnvironment environment) {
+        if (!ambientAllowed(chunk, initial)) {
             return;
         }
-        //@builder
-        // Excluded spawners cannot spawn anything on this Minecraft version, so they never enter the selection pool.
-        Predicate<IrisSpawner> filter = i -> !i.isCompatExcluded() && i.canSpawn(manager.getEngine(), c.getX(), c.getZ());
-        ChunkCounter counter = new ChunkCounter(c.getEntities());
-
-        IrisBiome biome = environment.biome();
-        IrisEntitySpawn v = spawnRandomly(Stream.concat(environment.data().getSpawnerLoader()
-                                .loadAll(environment.dimension().getEntitySpawners())
-                                .shuffleCopy(RNG.r)
-                                .stream()
-                                .filter(filter)
-                                .filter((i) -> i.isValid(biome)),
-                        Stream.concat(environment.data()
-                                        .getSpawnerLoader()
-                                        .loadAll(environment.region().getEntitySpawners())
-                                        .shuffleCopy(RNG.r)
-                                        .stream()
-                                        .filter(filter),
-                                environment.data().getSpawnerLoader()
-                                        .loadAll(environment.biome().getEntitySpawners())
-                                        .shuffleCopy(RNG.r)
-                                        .stream()
-                                        .filter(filter)))
-                .flatMap((i) -> stream(i, initial))
-                .filter(entry -> counter.remainingCapacity(entry, manager.getEngine()) > 0)
-                .collect(Collectors.toList()))
-                .getRandom();
-        //@done
-        if (v == null || v.getReferenceSpawner() == null)
+        ChunkCounter counter = initial ? null : new ChunkCounter(chunk.getEntities());
+        List<SpawnSelection> pool = new ArrayList<>();
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(environment.dimension().getEntitySpawners()),
+                environment.biome(), chunk, initial, counter);
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(environment.region().getEntitySpawners()),
+                null, chunk, initial, counter);
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(environment.biome().getEntitySpawners()),
+                null, chunk, initial, counter);
+        RNG random = initial ? EntitySpawnSeed.chunk(manager.getEngine().getSeedManager().getEntity(),
+                chunk.getX(), chunk.getZ()) : new RNG(RNG.r.nextLong());
+        SpawnSelection selected = selectSpawn(pool, random);
+        if (selected == null) {
             return;
+        }
+        int capacity = initial ? Math.max(0, selected.spawner().getMaxEntitiesPerChunk())
+                : counter.remainingCapacity(selected.entry(), selected.spawner(), manager.getEngine());
+        int spawned = selected.entry().spawn(manager.getEngine(), chunk, random, capacity,
+                new SpawnContext(selected.spawner(), null, initial));
+        actuallySpawned.addAndGet(spawned);
+        if (spawned > 0 && !initial) {
+            selected.spawner().spawn(manager.getEngine(), chunk.getX(), chunk.getZ());
+        }
+    }
 
-        spawn(c, v, counter.remainingCapacity(v, manager.getEngine()));
+    private void collectSpawns(List<SpawnSelection> pool, KList<IrisSpawner> spawners, IrisBiome biome,
+                               Chunk chunk, boolean initial, ChunkCounter counter) {
+        for (IrisSpawner spawner : spawners) {
+            if (!spawnerAllowed(spawner, manager.getEngine(), chunk.getX(), chunk.getZ(), initial)
+                    || biome != null && !spawner.isValid(biome)) {
+                continue;
+            }
+            for (IrisEntitySpawn entry : initial ? spawner.getInitialSpawns() : spawner.getSpawns()) {
+                IrisEntity entity = entry.getRealEntity(manager.getEngine());
+                int capacity = initial ? Math.max(0, spawner.getMaxEntitiesPerChunk())
+                        : counter.remainingCapacity(entry, spawner, manager.getEngine());
+                if (entity != null && capacity > 0) {
+                    pool.add(new SpawnSelection(entry, spawner));
+                }
+            }
+        }
+    }
+
+    static boolean spawnerAllowed(IrisSpawner spawner, Engine engine, int chunkX, int chunkZ, boolean initial) {
+        return !spawner.isCompatExcluded() && (initial || spawner.canSpawn(engine, chunkX, chunkZ));
+    }
+
+    static SpawnSelection selectSpawn(List<SpawnSelection> pool, RNG random) {
+        return Rarity.expandWeighted(pool).getRandom(random);
     }
 
     static boolean ambientAllowed(Chunk chunk, boolean initial) {
@@ -428,46 +459,22 @@ final class WorldEntitySpawner {
                 && (initial || chunk.getLoadLevel() == Chunk.LoadLevel.ENTITY_TICKING);
     }
 
-    private void spawn(Chunk c, IrisEntitySpawn i, int remainingCapacity) {
-        IrisSpawner ref = i.getReferenceSpawner();
-        int s = i.spawn(manager.getEngine(), c, RNG.r, remainingCapacity);
-        actuallySpawned.addAndGet(s);
-        if (s > 0) {
-            ref.spawn(manager.getEngine(), c.getX(), c.getZ());
-        }
-    }
-
-    private void spawn(IrisPosition pos, IrisEntitySpawn i) {
-        IrisSpawner ref = i.getReferenceSpawner();
-        if (!ref.canSpawn(manager.getEngine(), PowerOfTwoCoordinates.blockToChunkFloor(pos.getX()), PowerOfTwoCoordinates.blockToChunkFloor(pos.getZ())))
-            return;
-
-        int s = i.spawn(manager.getEngine(), pos, RNG.r);
-        actuallySpawned.addAndGet(s);
-        if (s > 0) {
-            ref.spawn(manager.getEngine(), PowerOfTwoCoordinates.blockToChunkFloor(pos.getX()), PowerOfTwoCoordinates.blockToChunkFloor(pos.getZ()));
-        }
-    }
-
-    private void spawnMarker(IrisPosition block, IrisSpawner spawner, boolean initial) {
-        KList<IrisEntitySpawn> s = initial ? spawner.getInitialSpawns() : spawner.getSpawns();
-        if (s.isEmpty()) {
+    private void spawnMarker(IrisPosition position, IrisSpawner spawner, IrisMarker marker, RNG random, boolean initial) {
+        int chunkX = PowerOfTwoCoordinates.blockToChunkFloor(position.getX());
+        int chunkZ = PowerOfTwoCoordinates.blockToChunkFloor(position.getZ());
+        if (!spawnerAllowed(spawner, manager.getEngine(), chunkX, chunkZ, initial)) {
             return;
         }
-
-        IrisEntitySpawn ss = spawnRandomly(s).getRandom();
-        ss.setReferenceSpawner(spawner);
-        ss.setReferenceMarker(spawner.getReferenceMarker());
-        spawn(block, ss);
-    }
-
-    private Stream<IrisEntitySpawn> stream(IrisSpawner s, boolean initial) {
-        for (IrisEntitySpawn i : initial ? s.getInitialSpawns() : s.getSpawns()) {
-            i.setReferenceSpawner(s);
-            i.setReferenceMarker(s.getReferenceMarker());
+        KList<IrisEntitySpawn> entries = initial ? spawner.getInitialSpawns() : spawner.getSpawns();
+        IrisEntitySpawn selected = Rarity.expandWeighted(entries).getRandom(random);
+        if (selected == null) {
+            return;
+        }
+        int spawned = selected.spawn(manager.getEngine(), position, random, new SpawnContext(spawner, marker, initial));
+        actuallySpawned.addAndGet(spawned);
+        if (spawned > 0 && !initial) {
+            spawner.spawn(manager.getEngine(), chunkX, chunkZ);
         }
-
-        return (initial ? s.getInitialSpawns() : s.getSpawns()).stream();
     }
 
     boolean isEntitySpawningEnabledForCurrentWorld() {
@@ -478,8 +485,11 @@ final class WorldEntitySpawner {
         return IrisSettings.get().getStudio().isEntitySpawning();
     }
 
-    private KList<IrisEntitySpawn> spawnRandomly(List<IrisEntitySpawn> types) {
-        return Rarity.expandWeighted(types);
+    record SpawnSelection(IrisEntitySpawn entry, IrisSpawner spawner) implements Rarity {
+        @Override
+        public int getRarity() {
+            return entry.getRarity();
+        }
     }
 
     static final class ChunkCounter {
@@ -493,9 +503,9 @@ final class WorldEntitySpawner {
             }
         }
 
-        int remainingCapacity(IrisEntitySpawn entry, Engine engine) {
+        int remainingCapacity(IrisEntitySpawn entry, IrisSpawner spawner, Engine engine) {
             IrisEntity entity = entry.getRealEntity(engine);
-            return entry.getReferenceSpawner().remainingCapacity(entity, counts);
+            return spawner.remainingCapacity(entity, counts);
         }
     }
 }

core/src/main/java/art/arcane/iris/world/entity/EntitySpawnSeed.java

@@ -0,0 +1,35 @@
+package art.arcane.iris.world.entity;
+
+import art.arcane.volmlib.util.math.RNG;
+
+public final class EntitySpawnSeed {
+    private static final long CHUNK_SALT = 0xA0761D6478BD642FL;
+    private static final long MARKER_SALT = 0xE7037ED1A0B428DBL;
+    private static final long ENTITY_SALT = 0x8EBC6AF09C88C6E3L;
+
+    private EntitySpawnSeed() {
+    }
+
+    public static RNG chunk(long entitySeed, int x, int z) {
+        long seed = mix(entitySeed ^ CHUNK_SALT);
+        seed = mix(seed ^ (long) x * 0xD6E8FEB86659FD93L);
+        return new RNG(mix(seed ^ (long) z * 0x8CB92BA72F3D8DD7L));
+    }
+
+    public static RNG marker(long entitySeed, int x, int y, int z) {
+        long seed = mix(entitySeed ^ MARKER_SALT);
+        seed = mix(seed ^ (long) x * 0xD6E8FEB86659FD93L);
+        seed = mix(seed ^ (long) y * 0xA5A3564E27F886A7L);
+        return new RNG(mix(seed ^ (long) z * 0x8CB92BA72F3D8DD7L));
+    }
+
+    public static RNG entity(long batchSeed, int ordinal) {
+        return new RNG(mix(batchSeed ^ ENTITY_SALT ^ (long) ordinal * 0x9E3779B97F4A7C15L));
+    }
+
+    private static long mix(long seed) {
+        seed = (seed ^ seed >>> 30) * 0xBF58476D1CE4E5B9L;
+        seed = (seed ^ seed >>> 27) * 0x94D049BB133111EBL;
+        return seed ^ seed >>> 31;
+    }
+}

core/src/main/java/art/arcane/iris/world/entity/IrisAttributeModifier.java

@@ -40,7 +40,6 @@ import org.bukkit.inventory.meta.ItemMeta;
 
 import java.util.Locale;
 import java.util.Set;
-import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 
 @Snippet("attribute-modifier")
@@ -134,12 +133,12 @@ public class IrisAttributeModifier {
         }
     }
 
-    private AttributeModifier createModifier(RNG rng) {
-        NamespacedKey key = NamespacedKey.minecraft(generateModifierKey());
+    AttributeModifier createModifier(RNG rng) {
+        NamespacedKey key = NamespacedKey.minecraft(generateModifierKey(rng));
         return new AttributeModifier(key, getAmount(rng), resolveOperation(), EquipmentSlotGroup.ANY);
     }
 
-    private String generateModifierKey() {
+    private String generateModifierKey(RNG rng) {
         String source = getName() == null ? "modifier" : getName();
         String normalized = source.toLowerCase(Locale.ROOT).replaceAll("[^a-z0-9/._-]", "_");
         if (normalized.isBlank()) {
@@ -148,7 +147,8 @@ public class IrisAttributeModifier {
         if (normalized.length() > 32) {
             normalized = normalized.substring(0, 32);
         }
-        String random = UUID.randomUUID().toString().replace("-", "");
+        String random = Long.toUnsignedString(rng.nextLong(), 16)
+                + "_" + Long.toUnsignedString(rng.nextLong(), 16);
         return normalized + "_" + random;
     }
 }

core/src/main/java/art/arcane/iris/world/entity/IrisEntity.java

@@ -268,7 +268,8 @@ public class IrisEntity extends IrisRegistrant implements NativeEntityOptions {
     }
 
     public Entity spawn(Engine gen, Location at) {
-        return spawn(gen, at, new RNG(at.hashCode()));
+        return spawn(gen, at, EntitySpawnSeed.marker(gen.getSeedManager().getEntity(),
+                at.getBlockX(), at.getBlockY(), at.getBlockZ()));
     }
 
     public Entity spawn(Engine gen, Location at, RNG rng) {
@@ -531,17 +532,19 @@ public class IrisEntity extends IrisRegistrant implements NativeEntityOptions {
         return specialType != null && !specialType.equals("");
     }
 
-    private static final class BukkitOps {
+    static final class BukkitOps {
         private static void persistVillager(Villager villager) {
             villager.setPersistent(true);
         }
 
-        private static void bindLoot(IrisEntity entity, Engine gen, Lootable l, Location finalAt, RNG rng) {
+        static void bindLoot(IrisEntity entity, Engine gen, Lootable l, Location finalAt, RNG rng) {
             IrisData definitions = gen.getData();
+            long lootSeed = EntitySpawnSeed.entity(rng.getSeed(), -2).getSeed();
+            NamespacedKey lootKey = new NamespacedKey("iris", "loot-" + Long.toUnsignedString(lootSeed, 16));
             l.setLootTable(new LootTable() {
                 @Override
                 public NamespacedKey getKey() {
-                    return new NamespacedKey("iris", "loot-" + entity.hashCode());
+                    return lootKey;
                 }
 
                 @Override

core/src/main/java/art/arcane/iris/world/entity/IrisEntitySpawn.java

@@ -54,6 +54,9 @@ import org.bukkit.World;
 import org.bukkit.block.Block;
 import org.bukkit.entity.Entity;
 
+import java.util.Objects;
+import java.util.Comparator;
+
 @Snippet("entity-spawn")
 @Accessors(chain = true)
 @NoArgsConstructor
@@ -63,7 +66,6 @@ import org.bukkit.entity.Entity;
 public class IrisEntitySpawn implements Rarity {
     private static final int CAVE_COLUMN_ATTEMPTS = 8;
     private static final int CAVE_VERTICAL_ATTEMPTS = 128;
-    private final transient AtomicCache<RNG> rng = new AtomicCache<>();
     private final transient AtomicCache<IrisEntity> ent = new AtomicCache<>();
     @RegistryListResource(IrisEntity.class)
     @Required
@@ -78,10 +80,8 @@ public class IrisEntitySpawn implements Rarity {
     @MinNumber(1)
     @Description("The max of this entity to spawn")
     private int maxSpawns = 1;
-    private transient IrisSpawner referenceSpawner;
-    private transient IrisMarker referenceMarker;
 
-    public int spawn(Engine gen, Chunk c, RNG rng, int remainingCapacity) {
+    public int spawn(Engine gen, Chunk chunk, RNG rng, int remainingCapacity, SpawnContext context) {
         if (remainingCapacity <= 0) {
             return 0;
         }
@@ -89,42 +89,39 @@ public class IrisEntitySpawn implements Rarity {
         if (definition == null) {
             return 0;
         }
-        int spawns = Math.min(remainingCapacity, LootResolver.inclusive(rng, minSpawns, maxSpawns));
-        int s = 0;
-
-        if (spawns > 0) {
-            for (int id = 0; id < spawns; id++) {
-                IrisSpawnGroup group = getReferenceSpawner().getGroup();
-                Location l;
-                if (group == IrisSpawnGroup.CAVE) {
-                    l = findCaveSpawnLocation(gen, c, rng, definition.getSurface());
-                } else {
-                    int x = (c.getX() << 4) + rng.i(16);
-                    int z = (c.getZ() << 4) + rng.i(16);
-                    World world = c.getWorld();
-                    int h = world.getHighestBlockYAt(x, z, HeightMap.OCEAN_FLOOR);
-                    int hf = world.getHighestBlockYAt(x, z, HeightMap.WORLD_SURFACE);
-                    Integer y = selectSurfaceSpawnY(group, definition.getSurface(), h, hf, rng);
-                    l = y == null ? null : new Location(world, x, y, z);
-                }
-
-                if (l != null) {
-                    if (referenceSpawner.getAllowedLightLevels().getMin() > 0 || referenceSpawner.getAllowedLightLevels().getMax() < 15) {
-                        if (referenceSpawner.getAllowedLightLevels().contains(l.getBlock().getLightLevel())) {
-                            if (spawn100(gen, l) != null) {
-                                s++;
-                            }
-                        }
-                    } else {
-                        if (spawn100(gen, l) != null) {
-                            s++;
-                        }
-                    }
-                }
+        long batchSeed = rng.getSeed();
+        int count = Math.min(remainingCapacity, LootResolver.inclusive(rng, minSpawns, maxSpawns));
+        IrisSpawnGroup group = context.spawner().getGroup();
+        KList<IrisPosition> caveMarkers = null;
+        if (context.initial() && group == IrisSpawnGroup.CAVE && !J.isFolia()) {
+            caveMarkers = gen.getMantle().findMarkers(chunk.getX(), chunk.getZ(), MarkerMatter.CAVE_FLOOR);
+            caveMarkers.sort(Comparator.comparingInt(IrisPosition::getX)
+                    .thenComparingInt(IrisPosition::getY).thenComparingInt(IrisPosition::getZ));
+        }
+        int spawned = 0;
+        for (int ordinal = 0; ordinal < count; ordinal++) {
+            RNG attemptRng = EntitySpawnSeed.entity(batchSeed, ordinal);
+            RNG positionRng = attemptRng.nextParallelRNG(0x632BE59BD9B4E019L);
+            Location location;
+            if (group == IrisSpawnGroup.CAVE) {
+                location = caveMarkers == null
+                        ? findCaveSpawnLocation(gen, chunk, positionRng, definition.getSurface())
+                        : selectCaveSpawnLocation(caveMarkers, chunk.getWorld(), positionRng);
+            } else {
+                int x = (chunk.getX() << 4) + positionRng.i(16);
+                int z = (chunk.getZ() << 4) + positionRng.i(16);
+                World world = chunk.getWorld();
+                int floor = world.getHighestBlockYAt(x, z, HeightMap.OCEAN_FLOOR);
+                int top = world.getHighestBlockYAt(x, z, HeightMap.WORLD_SURFACE);
+                Integer y = selectSurfaceSpawnY(group, definition.getSurface(), floor, top, positionRng);
+                location = y == null ? null : new Location(world, x, y, z);
+            }
+            if (location != null && lightAllowed(location, context)
+                    && spawn100(gen, location, attemptRng, false) != null) {
+                spawned++;
             }
         }
-
-        return s;
+        return spawned;
     }
 
     public static Integer selectSurfaceSpawnY(IrisSpawnGroup group, IrisSurface surface,
@@ -138,43 +135,41 @@ public class IrisEntitySpawn implements Rarity {
         return top > floor ? LootResolver.inclusive(rng, floor + 1, top) : null;
     }
 
-    public int spawn(Engine gen, IrisPosition c, RNG rng) {
-        int spawns = LootResolver.inclusive(rng, minSpawns, maxSpawns);
-        int s = 0;
-
-        if (!BukkitWorldBinding.tryBind(gen.getWorld())) {
+    public int spawn(Engine gen, IrisPosition position, RNG rng, SpawnContext context) {
+        long batchSeed = rng.getSeed();
+        int count = LootResolver.inclusive(rng, minSpawns, maxSpawns);
+        if (count <= 0 || !BukkitWorldBinding.tryBind(gen.getWorld())) {
             return 0;
         }
-
         World world = BukkitWorldBinding.world(gen.getWorld());
-        if (spawns > 0) {
-
-            if (referenceMarker != null && referenceMarker.shouldExhaust()) {
-                if (J.isFolia()) {
-                    J.a(() -> gen.getMantle().getMantle().remove(c.getX(), c.getY() - gen.getWorld().minHeight(), c.getZ(), MatterMarker.class));
-                } else {
-                    gen.getMantle().getMantle().remove(c.getX(), c.getY() - gen.getWorld().minHeight(), c.getZ(), MatterMarker.class);
-                }
-            }
-
-            for (int id = 0; id < spawns; id++) {
-                Location l = BukkitPlatform.toLocation(c, world).add(0, 1, 0);
-
-                if (referenceSpawner.getAllowedLightLevels().getMin() > 0 || referenceSpawner.getAllowedLightLevels().getMax() < 15) {
-                    if (referenceSpawner.getAllowedLightLevels().contains(l.getBlock().getLightLevel())) {
-                        if (spawn100(gen, l, true) != null) {
-                            s++;
-                        }
-                    }
-                } else {
-                    if (spawn100(gen, l, true) != null) {
-                        s++;
-                    }
-                }
+        if (context.marker() != null && context.marker().shouldExhaust(EntitySpawnSeed.entity(batchSeed, -1))) {
+            if (J.isFolia()) {
+                J.a(() -> gen.getMantle().getMantle().remove(position.getX(),
+                        position.getY() - gen.getWorld().minHeight(), position.getZ(), MatterMarker.class));
+            } else {
+                gen.getMantle().getMantle().remove(position.getX(),
+                        position.getY() - gen.getWorld().minHeight(), position.getZ(), MatterMarker.class);
             }
         }
+        int spawned = 0;
+        for (int ordinal = 0; ordinal < count; ordinal++) {
+            Location location = BukkitPlatform.toLocation(position, world).add(0, 1, 0);
+            if (lightAllowed(location, context)
+                    && spawn100(gen, location, EntitySpawnSeed.entity(batchSeed, ordinal), true) != null) {
+                spawned++;
+            }
+        }
+        return spawned;
+    }
 
-        return s;
+    private static boolean lightAllowed(Location location, SpawnContext context) {
+        if (context.initial()) {
+            return true;
+        }
+        IrisSpawner spawner = context.spawner();
+        return spawner.getAllowedLightLevels().getMin() <= 0
+                && spawner.getAllowedLightLevels().getMax() >= 15
+                || spawner.getAllowedLightLevels().contains(location.getBlock().getLightLevel());
     }
 
     public IrisEntity getRealEntity(Engine g) {
@@ -191,16 +186,13 @@ public class IrisEntitySpawn implements Rarity {
         return entity == null || entity.isCompatExcluded() ? null : entity;
     }
 
-    public Entity spawn(Engine g, Location at) {
-        if (getRealEntity(g) == null) {
+    public Entity spawn(Engine engine, Location at) {
+        if (getRealEntity(engine) == null) {
             return null;
         }
-
-        if (LootResolver.oneIn(rng.aquire(() -> new RNG(g.getSeedManager().getEntity())), getRarity())) {
-            return spawn100(g, at);
-        }
-
-        return null;
+        RNG rng = EntitySpawnSeed.marker(engine.getSeedManager().getEntity(),
+                at.getBlockX(), at.getBlockY(), at.getBlockZ());
+        return LootResolver.oneIn(rng, getRarity()) ? spawn100(engine, at, rng, false) : null;
     }
 
     static Location findCaveSpawnLocation(Engine engine, Chunk chunk, RNG rng, IrisSurface surface) {
@@ -249,11 +241,7 @@ public class IrisEntitySpawn implements Rarity {
         return marker == null ? null : BukkitPlatform.toLocation(marker, world);
     }
 
-    private Entity spawn100(Engine g, Location at) {
-        return spawn100(g, at, false);
-    }
-
-    private Entity spawn100(Engine g, Location at, boolean ignoreSurfaces) {
+    private Entity spawn100(Engine g, Location at, RNG rng, boolean ignoreSurfaces) {
         try {
             IrisEntity irisEntity = getRealEntity(g);
             if (irisEntity == null) { // No entity
@@ -278,7 +266,7 @@ public class IrisEntitySpawn implements Rarity {
                 }
             }
 
-            Entity e = irisEntity.spawn(g, at.clone().add(0.5, surface.isFluid() ? 0.5 : 0, 0.5), rng.aquire(() -> new RNG(g.getSeedManager().getEntity())));
+            Entity e = irisEntity.spawn(g, at.clone().add(0.5, surface.isFluid() ? 0.5 : 0, 0.5), rng);
             if (e != null) {
                 IrisLogging.debug("Spawned " + C.DARK_AQUA + "Entity<" + getEntity() + "> " + C.GREEN + e.getType() + C.LIGHT_PURPLE + " @ " + C.GRAY + e.getLocation().getX() + ", " + e.getLocation().getY() + ", " + e.getLocation().getZ());
             }
@@ -291,6 +279,12 @@ public class IrisEntitySpawn implements Rarity {
         }
     }
 
+    public record SpawnContext(IrisSpawner spawner, IrisMarker marker, boolean initial) {
+        public SpawnContext {
+            Objects.requireNonNull(spawner, "spawner");
+        }
+    }
+
     private static boolean isAir(Material material) {
         return material == Material.AIR || material == Material.CAVE_AIR || material == Material.VOID_AIR;
     }

core/src/main/java/art/arcane/iris/world/entity/IrisMarker.java

@@ -51,8 +51,8 @@ public class IrisMarker extends IrisRegistrant {
     @Description("If this marker is used, what is the chance it removes itself. For example 25% (0.25) would mean that on average 4 uses will remove a specific marker. Set this below 0 (-1) to never exhaust & set this to 1 or higher to always exhaust on first use.")
     private double exhaustionChance = 0;
 
-    public boolean shouldExhaust() {
-        return exhaustionChance > RNG.r.nextDouble();
+    public boolean shouldExhaust(RNG rng) {
+        return exhaustionChance > rng.nextDouble();
     }
 
     @Override

core/src/main/java/art/arcane/iris/world/entity/IrisSpawner.java

@@ -49,36 +49,33 @@ import java.util.Map;
 @Accessors(chain = true)
 @NoArgsConstructor
 @AllArgsConstructor
-@Description("Represents an entity spawn during initial chunk generation")
+@Description("Configures initial inhabitants and ongoing entity spawning")
 @Data
 public class IrisSpawner extends IrisRegistrant {
-
-    private transient IrisMarker referenceMarker;
-
     @ArrayType(min = 1, type = IrisEntitySpawn.class)
     @Description("The entity spawns to add")
     private KList<IrisEntitySpawn> spawns = new KList<>();
 
     @ArrayType(min = 1, type = IrisEntitySpawn.class)
-    @Description("The entity spawns to add initially. EXECUTES PER CHUNK!")
+    @Description("The seeded entity pool used once per chunk or marker for initial inhabitants. Live time, weather, light, population, and cooldowns do not restrict this pool.")
     private KList<IrisEntitySpawn> initialSpawns = new KList<>();
 
-    @Description("Maximum ambient entities of the selected entity's native spawn category in one chunk. Spawn groups are limited to the remaining capacity.")
+    @Description("Maximum ambient entities of the selected entity's native spawn category in one chunk. Ongoing groups use remaining capacity; initial groups use this limit independently of existing mobs.")
     private int maxEntitiesPerChunk = 1;
 
-    @Description("The block of 24 hour time to contain this spawn in.")
+    @Description("The 24-hour time window for ongoing spawns.")
     private IrisTimeBlock timeBlock = new IrisTimeBlock();
 
-    @Description("The weather condition required for this spawner to fire.")
+    @Description("The weather condition required for ongoing spawns.")
     private IrisWeather weather = IrisWeather.ANY;
 
-    @Description("The maximum rate this spawner can fire")
+    @Description("The maximum ongoing spawn rate")
     private IrisRate maximumRate = new IrisRate();
 
-    @Description("The maximum rate this spawner can fire on a specific chunk")
+    @Description("The maximum ongoing spawn rate in a specific chunk")
     private IrisRate maximumRatePerChunk = new IrisRate();
 
-    @Description("The light levels this spawn is allowed to run in (0-15 inclusive)")
+    @Description("The light levels allowed for ongoing spawns (0-15 inclusive). Initial inhabitants ignore light.")
     private IrisRange allowedLightLevels = new IrisRange(0, 15);
 
     @Description("Where should these spawns be placed")

core/src/test/java/art/arcane/iris/generation/chunk/LinkedTerrainChunkTest.java

@@ -20,7 +20,6 @@ import org.mockito.MockedStatic;
 import java.lang.reflect.Proxy;
 import java.util.Random;
 import java.util.concurrent.atomic.AtomicInteger;
-import java.util.concurrent.atomic.AtomicReference;
 
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
@@ -35,18 +34,7 @@ public class LinkedTerrainChunkTest {
     @BeforeClass
     public static void initializeBiomes() throws Exception {
         BukkitTestServer.install();
-        AtomicReference<Object> registered = new AtomicReference<>();
-        RegistryAccess access = mock(RegistryAccess.class, invocation -> {
-            if (invocation.getMethod().getName().equals("getRegistry")) {
-                if (registered.get() == null) {
-                    registered.set(Proxy.newProxyInstance(Registry.class.getClassLoader(), new Class<?>[]{Registry.class},
-                            (proxy, method, arguments) -> method.getName().equals("getOrThrow")
-                                    ? biome((Key) arguments[0]) : null));
-                }
-                return registered.get();
-            }
-            return RETURNS_DEFAULTS.answer(invocation);
-        });
+        RegistryAccess access = BukkitTestServer.registryAccess();
         InternalAPIBridge bridge = mock(InternalAPIBridge.class, invocation ->
                 invocation.getMethod().getName().equals("constructLegacyCustomBiome")
                         ? biome(Key.key("minecraft:custom")) : RETURNS_DEFAULTS.answer(invocation));

core/src/test/java/art/arcane/iris/generation/runtime/WorldChunkMaintenanceInitialSpawnTest.java

@@ -0,0 +1,211 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.platform.bukkit.plugin.Chunks;
+import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.world.task.J;
+import art.arcane.volmlib.util.mantle.flag.MantleFlag;
+import art.arcane.volmlib.util.mantle.runtime.Mantle;
+import art.arcane.volmlib.util.matter.Matter;
+import org.bukkit.GameRules;
+import org.bukkit.World;
+import org.junit.Test;
+import org.junit.BeforeClass;
+import org.mockito.MockedStatic;
+
+import java.util.ArrayDeque;
+import java.util.Queue;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class WorldChunkMaintenanceInitialSpawnTest {
+    @BeforeClass
+    public static void initializeGameRules() throws Exception {
+        WorldEntitySpawnerAdmissionTest.initializeGameRules();
+    }
+
+    @Test
+    public void completionIsWrittenAfterOwningRegionWorkAndDuplicateRequestsAreSkipped() {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger attempts = new AtomicInteger();
+            fixture.start(attempts::incrementAndGet);
+            fixture.start(attempts::incrementAndGet);
+            assertEquals(1, fixture.async.size());
+            fixture.async.remove().run();
+            verify(fixture.world, never()).isChunkLoaded(2, -1);
+            verify(fixture.mantle, never()).flag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+
+            fixture.regions.remove().run();
+
+            assertEquals(1, attempts.get());
+            assertTrue(fixture.maintenance.isInitialSpawnComplete(2, -1));
+            verify(fixture.mantle, never()).flag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+            fixture.start(attempts::incrementAndGet);
+            assertTrue(fixture.regions.isEmpty());
+            fixture.async.remove().run();
+            verify(fixture.mantle).flag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+        }
+    }
+
+    @Test
+    public void unloadAndDisabledMobSpawningRemainEligibleForRetry() {
+        for (boolean unloaded : new boolean[]{true, false}) {
+            try (Fixture fixture = new Fixture()) {
+                AtomicInteger attempts = new AtomicInteger();
+                when(fixture.world.isChunkLoaded(2, -1)).thenReturn(!unloaded);
+                when(fixture.world.getGameRuleValue(GameRules.SPAWN_MOBS)).thenReturn(unloaded);
+                fixture.start(attempts::incrementAndGet);
+                fixture.async.remove().run();
+                fixture.regions.remove().run();
+                assertFalse(fixture.maintenance.isInitialSpawnComplete(2, -1));
+                assertEquals(0, attempts.get());
+                verify(fixture.mantle, never()).flag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+
+                when(fixture.world.isChunkLoaded(2, -1)).thenReturn(true);
+                when(fixture.world.getGameRuleValue(GameRules.SPAWN_MOBS)).thenReturn(true);
+                fixture.start(attempts::incrementAndGet);
+                fixture.async.remove().run();
+                fixture.regions.remove().run();
+                assertEquals(1, attempts.get());
+            }
+        }
+    }
+
+    @Test
+    public void persistedCompletionAndRegionRejectionDoNotRunInitialWork() {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger attempts = new AtomicInteger();
+            when(fixture.mantle.hasFlag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER)).thenReturn(true);
+            fixture.start(attempts::incrementAndGet);
+            fixture.async.remove().run();
+            assertTrue(fixture.maintenance.isInitialSpawnComplete(2, -1));
+            assertTrue(fixture.regions.isEmpty());
+            assertEquals(0, attempts.get());
+        }
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger attempts = new AtomicInteger();
+            fixture.acceptRegions = false;
+            fixture.start(attempts::incrementAndGet);
+            fixture.async.remove().run();
+            fixture.start(attempts::incrementAndGet);
+            assertEquals(1, fixture.async.size());
+            assertFalse(fixture.maintenance.isInitialSpawnComplete(2, -1));
+            assertEquals(0, attempts.get());
+        }
+    }
+
+    @Test
+    public void shutdownDrainWaitsForTheReservedCompletionWriteAfterInitialEntitiesWereEmitted() throws Exception {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger spawned = new AtomicInteger();
+            fixture.start(spawned::incrementAndGet);
+            fixture.async.remove().run();
+            try (GenerationSessionLease owner = fixture.sessions.acquire("initial_spawn_owner");
+                 IrisContext.Scope ignored = IrisContext.open(fixture.engine, owner.sessionId(), null)) {
+                fixture.regions.remove().run();
+            }
+            assertEquals(1, spawned.get());
+            assertEquals(1, fixture.sessions.activeLeases());
+            when(fixture.engine.isClosing()).thenReturn(true);
+            ExecutorService closer = Executors.newSingleThreadExecutor();
+            try {
+                CountDownLatch sealing = new CountDownLatch(1);
+                Future<?> drained = closer.submit(() -> {
+                    sealing.countDown();
+                    fixture.sessions.sealAndAwait("shutdown", 2_000L, true);
+                    return null;
+                });
+                assertTrue(sealing.await(1L, TimeUnit.SECONDS));
+                assertFalse(drained.isDone());
+                fixture.async.remove().run();
+                drained.get(1L, TimeUnit.SECONDS);
+
+                verify(fixture.mantle).flag(2, -1, MantleFlag.INITIAL_SPAWNED_MARKER, true);
+                assertEquals(0, fixture.sessions.activeLeases());
+            } finally {
+                closer.shutdownNow();
+            }
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private static final class Fixture implements AutoCloseable {
+        private final Engine engine = mock(Engine.class);
+        private final GenerationSessionManager sessions = new GenerationSessionManager();
+        private final IrisWorldManager manager = mock(IrisWorldManager.class);
+        private final World world = mock(World.class);
+        private final Mantle<Matter> mantle = mock(Mantle.class);
+        private final WorldChunkMaintenance maintenance = new WorldChunkMaintenance(manager);
+        private final Queue<Runnable> async = new ArrayDeque<>();
+        private final Queue<Runnable> regions = new ArrayDeque<>();
+        private final MockedStatic<J> scheduler;
+        private final MockedStatic<Chunks> chunks;
+        private boolean acceptRegions = true;
+
+        private Fixture() {
+            scheduler = mockStatic(J.class);
+            try {
+                chunks = mockStatic(Chunks.class);
+            } catch (RuntimeException | Error failure) {
+                scheduler.close();
+                throw failure;
+            }
+            try {
+                configure();
+            } catch (RuntimeException | Error failure) {
+                close();
+                throw failure;
+            }
+        }
+
+        private void configure() {
+            when(manager.getEngine()).thenReturn(engine);
+            when(engine.getGenerationSessions()).thenReturn(sessions);
+            when(manager.getMantle()).thenReturn(mantle);
+            when(manager.managedTask(anyString(), any(Runnable.class), any(Runnable.class)))
+                    .thenAnswer(invocation -> invocation.getArgument(1));
+            when(world.isChunkLoaded(2, -1)).thenReturn(true);
+            when(world.getGameRuleValue(GameRules.SPAWN_MOBS)).thenReturn(true);
+            chunks.when(() -> Chunks.isSafe(world, 2, -1)).thenReturn(true);
+            scheduler.when(() -> J.a(any(Runnable.class))).thenAnswer(invocation -> {
+                async.add(invocation.getArgument(0));
+                return null;
+            });
+            scheduler.when(() -> J.runRegion(eq(world), eq(2), eq(-1), any(Runnable.class)))
+                    .thenAnswer(invocation -> {
+                        if (acceptRegions) {
+                            regions.add(invocation.getArgument(3));
+                        }
+                        return acceptRegions;
+                    });
+        }
+
+        private void start(Runnable work) {
+            maintenance.runInitialSpawn(world, 2, -1, work);
+        }
+
+        @Override
+        public void close() {
+            while (!async.isEmpty()) {
+                async.remove().run();
+            }
+            chunks.close();
+            scheduler.close();
+        }
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/WorldEntitySpawnerAdmissionTest.java

@@ -4,20 +4,15 @@ import art.arcane.iris.testsupport.BukkitTestServer;
 import io.papermc.paper.InternalAPIBridge;
 import io.papermc.paper.registry.RegistryAccess;
 import org.bukkit.Chunk;
-import org.bukkit.GameRule;
 import org.bukkit.GameRules;
-import org.bukkit.Registry;
 import org.bukkit.World;
 import org.junit.BeforeClass;
 import org.junit.Test;
 import org.mockito.MockedStatic;
 
-import java.lang.reflect.Proxy;
-import java.util.concurrent.atomic.AtomicReference;
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
-import static org.mockito.Mockito.RETURNS_DEFAULTS;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
 import static org.mockito.Mockito.when;
@@ -26,19 +21,7 @@ public class WorldEntitySpawnerAdmissionTest {
     @BeforeClass
     public static void initializeGameRules() throws Exception {
         BukkitTestServer.install();
-        AtomicReference<Object> registered = new AtomicReference<>();
-        RegistryAccess access = mock(RegistryAccess.class, invocation -> {
-            if (invocation.getMethod().getName().equals("getRegistry")) {
-                if (registered.get() == null) {
-                    registered.set(Proxy.newProxyInstance(Registry.class.getClassLoader(),
-                            new Class<?>[]{Registry.class},
-                            (proxy, method, arguments) -> method.getName().equals("getOrThrow")
-                                    ? mock(GameRule.class) : null));
-                }
-                return registered.get();
-            }
-            return RETURNS_DEFAULTS.answer(invocation);
-        });
+        RegistryAccess access = BukkitTestServer.registryAccess();
         InternalAPIBridge bridge = mock(InternalAPIBridge.class);
         try (MockedStatic<RegistryAccess> registries = mockStatic(RegistryAccess.class);
              MockedStatic<InternalAPIBridge> internals = mockStatic(InternalAPIBridge.class)) {

core/src/test/java/art/arcane/iris/generation/runtime/WorldEntitySpawnerBiomeTest.java

@@ -7,6 +7,7 @@ import art.arcane.iris.world.history.SavedBiomeUnavailableException;
 import art.arcane.iris.generation.biome.IrisBiome;
 import art.arcane.iris.generation.terrain.IrisDimension;
 import art.arcane.iris.world.entity.IrisEntitySpawn;
+import art.arcane.iris.world.entity.IrisEntitySpawn.SpawnContext;
 import art.arcane.iris.pack.value.IrisPosition;
 import art.arcane.iris.generation.terrain.IrisRegion;
 import art.arcane.iris.world.entity.IrisSpawner;
@@ -54,9 +55,9 @@ public class WorldEntitySpawnerBiomeTest {
             configured.when(IrisSettings::get).thenReturn(settings);
             WorldEntitySpawner spawner = new WorldEntitySpawner(manager);
             spawner.prepareInitialSpawn(chunk, List.of());
-            verify(maintenance, never()).raiseInitialSpawnMarkerFlag(any(), anyInt(), anyInt(), any());
+            verify(maintenance, never()).runInitialSpawn(any(), anyInt(), anyInt(), any());
             spawner.prepareInitialSpawn(chunk, List.of());
-            verify(maintenance).raiseInitialSpawnMarkerFlag(eq(world), eq(2), eq(-1), any());
+            verify(maintenance).runInitialSpawn(eq(world), eq(2), eq(-1), any());
         }
     }
 
@@ -76,11 +77,10 @@ public class WorldEntitySpawnerBiomeTest {
         when(definition.getInitialSpawns()).thenReturn(new KList<>(entity));
         when(definition.canSpawn(engine, 0, 0)).thenReturn(true);
         when(entity.getRarity()).thenReturn(1);
-        when(entity.getReferenceSpawner()).thenReturn(definition);
-        when(entity.spawn(eq(engine), eq(position), any(RNG.class))).thenReturn(1);
+        when(entity.spawn(eq(engine), eq(position), any(RNG.class), any(SpawnContext.class))).thenReturn(1);
         KSet<IrisSpawner> definitions = new KSet<>();
         definitions.add(definition);
-        PreparedMarkerSpawn marker = new PreparedMarkerSpawn(position, definitions, environment);
+        PreparedMarkerSpawn marker = new PreparedMarkerSpawn(position, definitions, environment, null);
         IrisSettings settings = new IrisSettings();
         settings.getWorld().setAmbientEntitySpawningSystem(false);
 
@@ -88,11 +88,11 @@ public class WorldEntitySpawnerBiomeTest {
             configured.when(IrisSettings::get).thenReturn(settings);
             new WorldEntitySpawner(manager).prepareInitialSpawn(chunk, List.of(marker));
             ArgumentCaptor<Runnable> callback = ArgumentCaptor.forClass(Runnable.class);
-            verify(manager.chunkMaintenance).raiseInitialSpawnMarkerFlag(eq(world), eq(0), eq(0), callback.capture());
+            verify(manager.chunkMaintenance).runInitialSpawn(eq(world), eq(0), eq(0), callback.capture());
             doThrow(new SavedBiomeUnavailableException("Evicted", true)).when(engine)
                     .getBiomeEnvironment(anyInt(), anyInt(), anyInt());
             callback.getValue().run();
-            verify(entity).spawn(eq(engine), eq(position), any(RNG.class));
+            verify(entity).spawn(eq(engine), eq(position), any(RNG.class), any(SpawnContext.class));
             verify(engine).openBiomeEnvironmentScope(environment);
             verify(scope).close();
             verify(engine, never()).getBiomeEnvironment(anyInt(), anyInt(), anyInt());
@@ -102,6 +102,7 @@ public class WorldEntitySpawnerBiomeTest {
     private static IrisWorldManager manager(Engine engine) throws Exception {
         IrisWorldManager manager = mock(IrisWorldManager.class);
         when(manager.getEngine()).thenReturn(engine);
+        when(engine.getSeedManager()).thenReturn(mock(SeedManager.class));
         Field field = IrisWorldManager.class.getDeclaredField("chunkMaintenance");
         field.setAccessible(true);
         field.set(manager, mock(WorldChunkMaintenance.class));

core/src/test/java/art/arcane/iris/generation/runtime/WorldEntitySpawnerInitialSeedTest.java

@@ -0,0 +1,145 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.generation.biome.IrisBiome;
+import art.arcane.iris.generation.terrain.IrisDimension;
+import art.arcane.iris.generation.terrain.IrisRegion;
+import art.arcane.iris.pack.loading.IrisData;
+import art.arcane.iris.pack.loading.ResourceLoader;
+import art.arcane.iris.world.entity.IrisEntity;
+import art.arcane.iris.world.entity.IrisEntitySpawn;
+import art.arcane.iris.world.entity.IrisEntitySpawn.SpawnContext;
+import art.arcane.iris.world.entity.IrisSpawner;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.math.RNG;
+import org.bukkit.Chunk;
+import org.bukkit.GameRules;
+import org.bukkit.World;
+import org.junit.Test;
+import org.junit.BeforeClass;
+
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class WorldEntitySpawnerInitialSeedTest {
+    @BeforeClass
+    public static void initializeGameRules() throws Exception {
+        WorldEntitySpawnerAdmissionTest.initializeGameRules();
+    }
+
+    @Test
+    public void initialPopulationUsesAuthoredCapacityWithoutLiveGatesOrPopulationReads() throws Exception {
+        Fixture fixture = new Fixture();
+        fixture.spawn(2, -3);
+
+        assertEquals(1, fixture.observed.size());
+        Observation observation = fixture.observed.get("2,-3");
+        assertEquals(4, observation.capacity());
+        assertTrue(observation.initial());
+        verify(fixture.definition, never()).canSpawn(any(Engine.class), anyInt(), anyInt());
+        verify(fixture.definition, never()).spawn(any(Engine.class), anyInt(), anyInt());
+        verify(fixture.lastChunk, never()).getEntities();
+    }
+
+    @Test
+    public void chunkBatchSeedsAreIndependentOfVisitOrderAndUnrelatedRandomActivity() throws Exception {
+        Fixture fixture = new Fixture();
+        for (int coordinate = -16; coordinate < 16; coordinate++) {
+            fixture.spawn(coordinate, -coordinate - 1);
+        }
+        Map<String, Observation> forward = new HashMap<>(fixture.observed);
+        fixture.observed.clear();
+        for (int coordinate = 15; coordinate >= -16; coordinate--) {
+            for (int draw = 0; draw < 19; draw++) {
+                RNG.r.nextLong();
+            }
+            fixture.spawn(coordinate, -coordinate - 1);
+        }
+        assertEquals(forward, fixture.observed);
+        assertEquals(32, forward.values().stream().map(Observation::seed).distinct().count());
+    }
+
+    @Test
+    public void initialSpawnerIgnoresCooldownButExcludedContentIsNeverAdmitted() {
+        Engine engine = mock(Engine.class);
+        IrisSpawner definition = mock(IrisSpawner.class);
+        when(definition.canSpawn(engine, 1, 2)).thenReturn(false);
+        assertTrue(WorldEntitySpawner.spawnerAllowed(definition, engine, 1, 2, true));
+        assertFalse(WorldEntitySpawner.spawnerAllowed(definition, engine, 1, 2, false));
+        when(definition.isCompatExcluded()).thenReturn(true);
+        assertFalse(WorldEntitySpawner.spawnerAllowed(definition, engine, 1, 2, true));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static final class Fixture {
+        private final Engine engine = mock(Engine.class);
+        private final IrisWorldManager manager = mock(IrisWorldManager.class);
+        private final World world = mock(World.class);
+        private final IrisSpawner definition = mock(IrisSpawner.class);
+        private final IrisEntitySpawn entry = mock(IrisEntitySpawn.class);
+        private final Map<String, Observation> observed = new HashMap<>();
+        private final BiomeEnvironment environment;
+        private final Method spawn;
+        private final WorldEntitySpawner spawner;
+        private Chunk lastChunk;
+
+        private Fixture() throws Exception {
+            SeedManager seeds = mock(SeedManager.class);
+            when(seeds.getEntity()).thenReturn(918273645L);
+            when(engine.getSeedManager()).thenReturn(seeds);
+            when(manager.getEngine()).thenReturn(engine);
+            IrisData data = mock(IrisData.class);
+            ResourceLoader<IrisSpawner> loader = mock(ResourceLoader.class);
+            when(data.getSpawnerLoader()).thenReturn(loader);
+            KList<String> authored = new KList<>("qa/initial");
+            when(loader.loadAll(any(KList.class))).thenReturn(new KList<>());
+            when(loader.loadAll(authored)).thenReturn(new KList<>(definition));
+            IrisBiome biome = mock(IrisBiome.class);
+            IrisRegion region = mock(IrisRegion.class);
+            IrisDimension dimension = mock(IrisDimension.class);
+            when(biome.getEntitySpawners()).thenReturn(authored);
+            when(region.getEntitySpawners()).thenReturn(new KList<>());
+            when(dimension.getEntitySpawners()).thenReturn(new KList<>());
+            when(definition.getInitialSpawns()).thenReturn(new KList<>(entry));
+            when(definition.getMaxEntitiesPerChunk()).thenReturn(4);
+            when(entry.getRarity()).thenReturn(1);
+            when(entry.getRealEntity(engine)).thenReturn(mock(IrisEntity.class));
+            when(world.getGameRuleValue(GameRules.SPAWN_MOBS)).thenReturn(true);
+            when(entry.spawn(eq(engine), any(Chunk.class), any(RNG.class), anyInt(), any(SpawnContext.class)))
+                    .thenAnswer(invocation -> {
+                        Chunk chunk = invocation.getArgument(1);
+                        RNG rng = invocation.getArgument(2);
+                        int capacity = invocation.getArgument(3);
+                        SpawnContext context = invocation.getArgument(4);
+                        observed.put(chunk.getX() + "," + chunk.getZ(), new Observation(rng.getSeed(), capacity, context.initial()));
+                        return 4;
+                    });
+            environment = new BiomeEnvironment(1L, biome, region, dimension, data);
+            spawner = new WorldEntitySpawner(manager);
+            spawn = WorldEntitySpawner.class.getDeclaredMethod("spawnAmbient", Chunk.class, boolean.class, BiomeEnvironment.class);
+            spawn.setAccessible(true);
+        }
+
+        private void spawn(int x, int z) throws Exception {
+            lastChunk = mock(Chunk.class);
+            when(lastChunk.getWorld()).thenReturn(world);
+            when(lastChunk.getX()).thenReturn(x);
+            when(lastChunk.getZ()).thenReturn(z);
+            spawn.invoke(spawner, lastChunk, true, environment);
+        }
+    }
+
+    private record Observation(long seed, int capacity, boolean initial) {
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/WorldEntitySpawnerMarkerSeedTest.java

@@ -0,0 +1,98 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.generation.biome.IrisBiome;
+import art.arcane.iris.generation.runtime.MarkerSpawnScanner.PreparedMarkerSpawn;
+import art.arcane.iris.generation.terrain.IrisDimension;
+import art.arcane.iris.generation.terrain.IrisRegion;
+import art.arcane.iris.pack.loading.IrisData;
+import art.arcane.iris.pack.value.IrisPosition;
+import art.arcane.iris.world.entity.IrisEntitySpawn;
+import art.arcane.iris.world.entity.IrisEntitySpawn.SpawnContext;
+import art.arcane.iris.world.entity.IrisMarker;
+import art.arcane.iris.world.entity.IrisSpawner;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.collection.KSet;
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.Test;
+
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class WorldEntitySpawnerMarkerSeedTest {
+    @Test
+    public void initialMarkersIgnoreHashSetInsertionOrderAndOtherChunksRandomDraws() throws Exception {
+        Engine engine = mock(Engine.class);
+        SeedManager seeds = mock(SeedManager.class);
+        when(engine.getSeedManager()).thenReturn(seeds);
+        when(seeds.getEntity()).thenReturn(145398276L);
+        IrisWorldManager manager = mock(IrisWorldManager.class);
+        when(manager.getEngine()).thenReturn(engine);
+        BiomeEnvironment environment = new BiomeEnvironment(1L, mock(IrisBiome.class), mock(IrisRegion.class),
+                mock(IrisDimension.class), mock(IrisData.class));
+        when(engine.openBiomeEnvironmentScope(environment)).thenReturn(mock(BiomeEnvironment.Scope.class));
+        Map<String, Observation> observed = new HashMap<>();
+        IrisMarker marker = new IrisMarker();
+        IrisSpawner first = definition("qa/a", engine, marker, observed);
+        IrisSpawner second = definition("qa/b", engine, marker, observed);
+        KSet<IrisSpawner> forward = new KSet<>();
+        forward.add(first);
+        forward.add(second);
+        KSet<IrisSpawner> reversed = new KSet<>();
+        reversed.add(second);
+        reversed.add(first);
+        Method spawn = WorldEntitySpawner.class.getDeclaredMethod("spawnPreparedMarker", PreparedMarkerSpawn.class, boolean.class);
+        spawn.setAccessible(true);
+        WorldEntitySpawner spawner = new WorldEntitySpawner(manager);
+        for (int x = -32; x < 32; x++) {
+            IrisPosition position = new IrisPosition(x * 16 + 1, -49, -x * 16 + 3);
+            spawn.invoke(spawner, new PreparedMarkerSpawn(position, forward, environment, marker), true);
+        }
+        Map<String, Observation> expected = new HashMap<>(observed);
+        observed.clear();
+        for (int x = 31; x >= -32; x--) {
+            RNG.r.nextLong();
+            IrisPosition position = new IrisPosition(x * 16 + 1, -49, -x * 16 + 3);
+            spawn.invoke(spawner, new PreparedMarkerSpawn(position, reversed, environment, marker), true);
+        }
+        assertEquals(expected, observed);
+        assertEquals(64, observed.size());
+        assertTrue(observed.values().stream().allMatch(Observation::initial));
+        verify(first, never()).canSpawn(any(Engine.class), anyInt(), anyInt());
+        verify(second, never()).canSpawn(any(Engine.class), anyInt(), anyInt());
+        verify(first, never()).spawn(any(Engine.class), anyInt(), anyInt());
+        verify(second, never()).spawn(any(Engine.class), anyInt(), anyInt());
+    }
+
+    private static IrisSpawner definition(String key, Engine engine, IrisMarker marker, Map<String, Observation> observed) {
+        IrisSpawner spawner = mock(IrisSpawner.class);
+        IrisEntitySpawn entry = mock(IrisEntitySpawn.class);
+        when(spawner.getLoadKey()).thenReturn(key);
+        when(spawner.getInitialSpawns()).thenReturn(new KList<>(entry));
+        when(entry.getRarity()).thenReturn(1);
+        when(entry.spawn(eq(engine), any(IrisPosition.class), any(RNG.class), any(SpawnContext.class)))
+                .thenAnswer(invocation -> {
+                    IrisPosition position = invocation.getArgument(1);
+                    RNG rng = invocation.getArgument(2);
+                    SpawnContext context = invocation.getArgument(3);
+                    assertEquals(marker, context.marker());
+                    observed.put(position.getX() + "," + position.getY() + "," + position.getZ(),
+                            new Observation(context.spawner().getLoadKey(), rng.getSeed(), context.initial()));
+                    return 1;
+                });
+        return spawner;
+    }
+
+    private record Observation(String definition, long seed, boolean initial) {
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/WorldEntitySpawnerPopulationTest.java

@@ -35,15 +35,15 @@ public class WorldEntitySpawnerPopulationTest {
         IrisEntity definition = mock(IrisEntity.class);
         when(definition.spawnCategory()).thenReturn("monster");
         when(entry.getRealEntity(engine)).thenReturn(definition);
-        when(entry.getReferenceSpawner()).thenReturn(new IrisSpawner().setMaxEntitiesPerChunk(2));
+        IrisSpawner spawner = new IrisSpawner().setMaxEntitiesPerChunk(2);
         try (MockedStatic<BukkitEntityType> types = mockStatic(BukkitEntityType.class)) {
             types.when(() -> BukkitEntityType.of(EntityType.FROG)).thenReturn(creatureType);
             types.when(() -> BukkitEntityType.of(EntityType.ZOMBIE)).thenReturn(monsterType);
             WorldEntitySpawner.ChunkCounter counter = new WorldEntitySpawner.ChunkCounter(
                     new Entity[]{frog, frog, frog, zombie, player});
-            assertEquals(1, counter.remainingCapacity(entry, engine));
+            assertEquals(1, counter.remainingCapacity(entry, spawner, engine));
             when(definition.spawnCategory()).thenReturn("creature");
-            assertEquals(0, counter.remainingCapacity(entry, engine));
+            assertEquals(0, counter.remainingCapacity(entry, spawner, engine));
         }
     }
 

core/src/test/java/art/arcane/iris/studio/generation/ObjectStudioGeometryTest.java

@@ -1,6 +1,7 @@
 package art.arcane.iris.studio.generation;
 
 import art.arcane.iris.pack.loading.IrisData;
+import art.arcane.iris.testsupport.BukkitTestServer;
 import art.arcane.iris.pack.loading.ResourceLoader;
 import art.arcane.iris.studio.object.ObjectStudioLayout;
 import art.arcane.iris.studio.object.ObjectStudioSaveService;
@@ -38,7 +39,6 @@ import java.nio.file.Files;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.atomic.AtomicReference;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
@@ -59,19 +59,9 @@ public class ObjectStudioGeometryTest {
     public final TemporaryFolder temporary = new TemporaryFolder();
 
     @BeforeClass
-    @SuppressWarnings("unchecked")
     public static void initializeBukkit() throws Exception {
         PreObjectMatterTest.setUpBukkit();
-        AtomicReference<Registry<Biome>> registered = new AtomicReference<>();
-        RegistryAccess access = mock(RegistryAccess.class, invocation -> {
-            if (invocation.getMethod().getName().equals("getRegistry")) {
-                if (registered.get() == null) {
-                    registered.set(biomeRegistry());
-                }
-                return registered.get();
-            }
-            return RETURNS_DEFAULTS.answer(invocation);
-        });
+        RegistryAccess access = BukkitTestServer.registryAccess();
         InternalAPIBridge bridge = mock(InternalAPIBridge.class, invocation -> {
             if (invocation.getMethod().getName().equals("constructLegacyCustomBiome")) {
                 return biome(Key.key("minecraft:custom"));
@@ -89,20 +79,6 @@ public class ObjectStudioGeometryTest {
         }
     }
 
-    @SuppressWarnings("unchecked")
-    private static Registry<Biome> biomeRegistry() {
-        return (Registry<Biome>) Proxy.newProxyInstance(Registry.class.getClassLoader(),
-                new Class<?>[]{Registry.class},
-                (proxy, method, arguments) -> switch (method.getName()) {
-                    case "getOrThrow" -> biome((Key) arguments[0]);
-                    case "hashCode" -> System.identityHashCode(proxy);
-                    case "equals" -> proxy == arguments[0];
-                    case "toString" -> "iris-object-studio-biome-registry";
-                    case "size" -> 0;
-                    case "hasTag" -> false;
-                    default -> null;
-                });
-    }
 
     @Test
     public void jungleClutterSignedBoundsStayInsideTheEditableCellAboveTheFloor() throws Exception {

core/src/test/java/art/arcane/iris/world/entity/EntitySpawnSeedTest.java

@@ -0,0 +1,63 @@
+package art.arcane.iris.world.entity;
+
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+
+public class EntitySpawnSeedTest {
+    @Test
+    public void coordinateAndAttemptSeedsRemainPinned() {
+        assertEquals(-4130335506775599505L, EntitySpawnSeed.chunk(0L, 0, 0).getSeed());
+        assertEquals(8337887711347547470L, EntitySpawnSeed.chunk(1337L, -2, 3).getSeed());
+        assertEquals(7508281853961706446L,
+                EntitySpawnSeed.chunk(-1L, Integer.MAX_VALUE, Integer.MIN_VALUE).getSeed());
+        assertEquals(-2305592493329120404L, EntitySpawnSeed.marker(0L, 0, 0, 0).getSeed());
+        assertEquals(8373759002518926811L, EntitySpawnSeed.marker(1337L, -32, -64, 48).getSeed());
+        assertEquals(-8713573891296014035L,
+                EntitySpawnSeed.marker(-1L, Integer.MAX_VALUE, Integer.MIN_VALUE, 1).getSeed());
+        assertEquals(-6302458730787398690L, EntitySpawnSeed.entity(1337L, -1).getSeed());
+        assertEquals(858402071020777840L, EntitySpawnSeed.entity(1337L, 0).getSeed());
+        assertEquals(-632476273033191086L, EntitySpawnSeed.entity(1337L, 1).getSeed());
+        assertEquals(4034378215462583149L, EntitySpawnSeed.entity(1337L, 17).getSeed());
+    }
+
+    @Test
+    public void consumptionAndReverseOrderDoNotChangeSiblingAttempts() {
+        long batchSeed = EntitySpawnSeed.chunk(1337L, -2, 3).getSeed();
+        List<Long> forward = new ArrayList<>();
+        for (int ordinal = 0; ordinal < 16; ordinal++) {
+            forward.add(EntitySpawnSeed.entity(batchSeed, ordinal).nextLong());
+        }
+        for (int ordinal = 15; ordinal >= 0; ordinal--) {
+            RNG sibling = EntitySpawnSeed.entity(batchSeed, ordinal - 1);
+            for (int draw = 0; draw < 100; draw++) {
+                sibling.nextLong();
+            }
+            assertEquals(forward.get(ordinal).longValue(), EntitySpawnSeed.entity(batchSeed, ordinal).nextLong());
+        }
+        assertNotEquals(EntitySpawnSeed.marker(1337L, -32, -64, 48).getSeed(),
+                EntitySpawnSeed.marker(1337L, -32, -63, 48).getSeed());
+        assertNotEquals(EntitySpawnSeed.chunk(1337L, -2, 3).getSeed(),
+                EntitySpawnSeed.chunk(1338L, -2, 3).getSeed());
+    }
+
+    @Test
+    public void concurrentCoordinateEvaluationMatchesSerialEvaluation() {
+        List<CompletableFuture<Long>> results = new ArrayList<>();
+        for (int ordinal = 0; ordinal < 64; ordinal++) {
+            int index = ordinal;
+            results.add(CompletableFuture.supplyAsync(() -> EntitySpawnSeed.marker(
+                    42L, index - 32, -64 + index, -index).nextLong()));
+        }
+        for (int ordinal = 63; ordinal >= 0; ordinal--) {
+            assertEquals(EntitySpawnSeed.marker(42L, ordinal - 32, -64 + ordinal, -ordinal).nextLong(),
+                    results.get(ordinal).join().longValue());
+        }
+    }
+}

core/src/test/java/art/arcane/iris/world/entity/IrisAttributeModifierSeedTest.java

@@ -0,0 +1,47 @@
+package art.arcane.iris.world.entity;
+
+import art.arcane.volmlib.util.math.RNG;
+import org.bukkit.attribute.AttributeModifier;
+import org.bukkit.inventory.meta.ItemMeta;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+public class IrisAttributeModifierSeedTest {
+    @Test
+    public void repeatedSeedProducesIdenticalModifierKeyAmountAndOperation() {
+        IrisAttributeModifier definition = new IrisAttributeModifier().setName("Custom Bonus")
+                .setMinAmount(-3).setMaxAmount(7).setOperation("ADD_SCALAR");
+        AttributeModifier first = definition.createModifier(new RNG(1337L));
+        AttributeModifier repeated = definition.createModifier(new RNG(1337L));
+        assertEquals(first.getKey(), repeated.getKey());
+        assertEquals("minecraft:custom_bonus_a8f1288a2c8a93b8_b072351edfd5484c", first.getKey().toString());
+        assertEquals(first.getAmount(), repeated.getAmount(), 0D);
+        assertEquals(5.832726771624211D, first.getAmount(), 0D);
+        assertEquals(AttributeModifier.Operation.ADD_SCALAR, first.getOperation());
+        assertTrue(first.getKey().getKey().startsWith("custom_bonus_"));
+        assertTrue(first.getAmount() >= -3D && first.getAmount() <= 7D);
+    }
+
+    @Test
+    public void sameNamedModifiersRemainDistinctWithinOneSeededSequence() {
+        IrisAttributeModifier definition = new IrisAttributeModifier().setName("Bonus");
+        RNG rng = new RNG(1337L);
+        AttributeModifier first = definition.createModifier(rng);
+        AttributeModifier second = definition.createModifier(rng);
+        assertNotEquals(first.getKey(), second.getKey());
+        assertNotEquals(first.getKey(), definition.createModifier(new RNG(1338L)).getKey());
+    }
+
+    @Test
+    public void zeroChanceNeverAppliesAModifier() {
+        ItemMeta metadata = mock(ItemMeta.class);
+        IrisAttributeModifier definition = new IrisAttributeModifier().setChance(0D);
+        definition.apply(new RNG(1337L), metadata);
+        verifyNoInteractions(metadata);
+    }
+}

core/src/test/java/art/arcane/iris/world/entity/IrisEntityLootSeedTest.java

@@ -0,0 +1,49 @@
+package art.arcane.iris.world.entity;
+
+import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.volmlib.util.math.RNG;
+import org.bukkit.Location;
+import org.bukkit.World;
+import org.bukkit.loot.LootTable;
+import org.bukkit.loot.Lootable;
+import org.junit.Test;
+import org.mockito.ArgumentCaptor;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+public class IrisEntityLootSeedTest {
+    @Test
+    public void lootKeyDependsOnSpawnSeedRatherThanEntityHashOrWorldIdentity() {
+        IrisEntity first = new IrisEntity().setCustomName("First");
+        IrisEntity second = new IrisEntity().setCustomName("Second");
+        assertNotEquals(first.hashCode(), second.hashCode());
+        LootTable firstTable = bind(first, new RNG(1337L), mock(World.class));
+        LootTable secondTable = bind(second, new RNG(1337L), mock(World.class));
+        assertEquals(firstTable.getKey(), secondTable.getKey());
+        assertEquals("iris:loot-22371b531cd5acd3", firstTable.getKey().toString());
+        assertNotEquals(firstTable.getKey(), bind(first, new RNG(1338L), mock(World.class)).getKey());
+    }
+
+    @Test
+    public void priorCustomizationConsumptionDoesNotChangeTheLootIdentity() {
+        IrisEntity definition = new IrisEntity();
+        RNG consumed = new RNG(1337L);
+        for (int draw = 0; draw < 100; draw++) {
+            consumed.nextLong();
+        }
+        assertEquals(bind(definition, new RNG(1337L), mock(World.class)).getKey(),
+                bind(definition, consumed, mock(World.class)).getKey());
+    }
+
+    private static LootTable bind(IrisEntity definition, RNG rng, World world) {
+        Engine engine = mock(Engine.class);
+        Lootable lootable = mock(Lootable.class);
+        IrisEntity.BukkitOps.bindLoot(definition, engine, lootable, new Location(world, -31.5, -63.5, 48.5), rng);
+        ArgumentCaptor<LootTable> table = ArgumentCaptor.forClass(LootTable.class);
+        verify(lootable).setLootTable(table.capture());
+        return table.getValue();
+    }
+}

core/src/test/java/art/arcane/iris/world/entity/IrisEntitySpawnTest.java

@@ -2,6 +2,7 @@ package art.arcane.iris.world.entity;
 
 import art.arcane.iris.generation.decoration.IrisSurface;
 import art.arcane.iris.pack.value.IrisPosition;
+import art.arcane.iris.pack.value.IrisRange;
 
 import art.arcane.iris.generation.runtime.Engine;
 import art.arcane.iris.generation.runtime.SeedManager;
@@ -29,12 +30,14 @@ import java.util.ArrayList;
 import java.util.List;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
 import static org.mockito.Mockito.spy;
@@ -381,9 +384,8 @@ public class IrisEntitySpawnTest {
                         IrisPosition position = invocation.getArgument(0);
                         return new Location(fixture.world, position.getX(), position.getY(), position.getZ());
                     });
-            fixture.entry.setReferenceSpawner(new IrisSpawner());
-            assertEquals(0, fixture.entry.spawn(fixture.engine, new IrisPosition(0, 78, 0), new RNG(1L)));
-            assertEquals(3, fixture.entry.spawn(fixture.engine, new IrisPosition(0, 77, 0), new RNG(1L)));
+            assertEquals(0, fixture.entry.spawn(fixture.engine, new IrisPosition(0, 78, 0), new RNG(1L), new IrisEntitySpawn.SpawnContext(new IrisSpawner(), null, false)));
+            assertEquals(3, fixture.entry.spawn(fixture.engine, new IrisPosition(0, 77, 0), new RNG(1L), new IrisEntitySpawn.SpawnContext(new IrisSpawner(), null, false)));
         }
     }
 
@@ -399,6 +401,96 @@ public class IrisEntitySpawnTest {
         }
     }
 
+    @Test
+    public void failedFirstSiblingDoesNotChangeLaterPositionsOrCustomizationSeeds() {
+        try (SpawnFixture fixture = new SpawnFixture(new SpawnOptions(IrisSurface.LAND, Material.WATER, 78, 78))) {
+            assertEquals(3, fixture.spawn(IrisSpawnGroup.NORMAL));
+            List<Location> expectedPositions = new ArrayList<>(fixture.positions.subList(1, 3));
+            List<Long> expectedSeeds = new ArrayList<>(fixture.customizationSeeds.subList(1, 3));
+            Location rejected = fixture.positions.getFirst();
+            Block obstruction = mock(Block.class);
+            when(obstruction.getType()).thenReturn(Material.STONE);
+            when(obstruction.isSolid()).thenReturn(true);
+            when(fixture.world.getBlockAt(rejected.getBlockX(), rejected.getBlockY(), rejected.getBlockZ()))
+                    .thenReturn(obstruction);
+            fixture.positions.clear();
+            fixture.customizationSeeds.clear();
+            assertEquals(2, fixture.spawn(IrisSpawnGroup.NORMAL));
+            assertEquals(expectedPositions, fixture.positions);
+            assertEquals(expectedSeeds, fixture.customizationSeeds);
+        }
+    }
+
+    @Test
+    public void initialCavePopulationDoesNotDependOnMarkerStorageOrder() {
+        try (SpawnFixture fixture = new SpawnFixture(new SpawnOptions(IrisSurface.LAND, Material.WATER, 78, 78));
+             MockedStatic<J> scheduler = mockStatic(J.class)) {
+            scheduler.when(J::isFolia).thenReturn(false);
+            EngineMantle mantle = mock(EngineMantle.class);
+            when(fixture.engine.getMantle()).thenReturn(mantle);
+            IrisPosition first = new IrisPosition(7, 79, 9);
+            IrisPosition second = new IrisPosition(3, 79, 5);
+            when(mantle.findMarkers(0, 0, MarkerMatter.CAVE_FLOOR))
+                    .thenReturn(new KList<>(first, second), new KList<>(second, first));
+            fixture.platform.when(() -> BukkitPlatform.toLocation(any(IrisPosition.class), eq(fixture.world)))
+                    .thenAnswer(invocation -> {
+                        IrisPosition position = invocation.getArgument(0);
+                        return new Location(fixture.world, position.getX(), position.getY(), position.getZ());
+                    });
+            IrisEntitySpawn.SpawnContext context = new IrisEntitySpawn.SpawnContext(
+                    new IrisSpawner().setGroup(IrisSpawnGroup.CAVE), null, true);
+            assertEquals(3, fixture.entry.spawn(fixture.engine, fixture.chunk, new RNG(1337L), 3, context));
+            List<Location> expected = new ArrayList<>(fixture.positions);
+            List<Long> expectedSeeds = new ArrayList<>(fixture.customizationSeeds);
+            fixture.positions.clear();
+            fixture.customizationSeeds.clear();
+            assertEquals(3, fixture.entry.spawn(fixture.engine, fixture.chunk, new RNG(1337L), 3, context));
+            assertEquals(expected, fixture.positions);
+            assertEquals(expectedSeeds, fixture.customizationSeeds);
+            verify(mantle, times(2)).findMarkers(0, 0, MarkerMatter.CAVE_FLOOR);
+        }
+    }
+
+    @Test
+    public void initialPopulationIgnoresLiveLightButRecurringPopulationHonorsIt() {
+        try (SpawnFixture fixture = new SpawnFixture(new SpawnOptions(IrisSurface.LAND, Material.WATER, 78, 78))) {
+            fixture.lightLevel = 15;
+            IrisSpawner spawner = new IrisSpawner().setAllowedLightLevels(new IrisRange(0, 0));
+            assertEquals(0, fixture.entry.spawn(fixture.engine, fixture.chunk, new RNG(1337L), 3,
+                    new IrisEntitySpawn.SpawnContext(spawner, null, false)));
+            assertEquals(3, fixture.entry.spawn(fixture.engine, fixture.chunk, new RNG(1337L), 3,
+                    new IrisEntitySpawn.SpawnContext(spawner, null, true)));
+        }
+    }
+
+    @Test
+    public void markerExhaustionUsesOnlyTheProvidedSeed() {
+        IrisMarker marker = new IrisMarker().setExhaustionChance(0.5D);
+        for (long seed = 0; seed < 32; seed++) {
+            assertEquals(marker.shouldExhaust(new RNG(seed)), marker.shouldExhaust(new RNG(seed)));
+        }
+        assertTrue(marker.setExhaustionChance(1D).shouldExhaust(new RNG(1337L)));
+        assertFalse(marker.setExhaustionChance(-1D).shouldExhaust(new RNG(1337L)));
+    }
+
+    @Test
+    public void directEntitySeedDoesNotDependOnWorldObjectIdentity() {
+        Engine engine = mock(Engine.class);
+        SeedManager seeds = mock(SeedManager.class);
+        when(engine.getSeedManager()).thenReturn(seeds);
+        when(seeds.getEntity()).thenReturn(1337L);
+        IrisEntity entity = spy(new IrisEntity());
+        List<Long> captured = new ArrayList<>();
+        doAnswer(invocation -> {
+            RNG rng = invocation.getArgument(2);
+            captured.add(rng.getSeed());
+            return null;
+        }).when(entity).spawn(eq(engine), any(Location.class), any(RNG.class));
+        entity.spawn(engine, new Location(mock(World.class), -31.5, -63.5, 48.5));
+        entity.spawn(engine, new Location(mock(World.class), -31.5, -63.5, 48.5));
+        assertEquals(List.of(8373759002518926811L, 8373759002518926811L), captured);
+    }
+
     private record SpawnOptions(IrisSurface surface, Material fluid, int floor, int top) {
     }
 
@@ -408,6 +500,8 @@ public class IrisEntitySpawnTest {
         private final Chunk chunk = mock(Chunk.class);
         private final IrisEntitySpawn entry = spy(new IrisEntitySpawn());
         private final List<Location> positions = new ArrayList<>();
+        private final List<Long> customizationSeeds = new ArrayList<>();
+        private int lightLevel;
         private final MockedStatic<BukkitPlatform> platform = mockStatic(BukkitPlatform.class);
         private Material airMaterial = Material.AIR;
         private int remainingCapacity = Integer.MAX_VALUE;
@@ -430,6 +524,7 @@ public class IrisEntitySpawnTest {
                 Block block = mock(Block.class);
                 when(block.getType()).thenReturn(y <= floor ? Material.STONE : y <= top ? fluid : airMaterial);
                 when(block.isSolid()).thenReturn(y <= floor);
+                when(block.getLightLevel()).thenReturn((byte) lightLevel);
                 return block;
             });
             when(world.getBlockAt(any(Location.class))).thenAnswer(invocation -> {
@@ -439,6 +534,8 @@ public class IrisEntitySpawnTest {
             when(definition.spawn(eq(engine), any(Location.class), any(RNG.class))).thenAnswer(invocation -> {
                 Location location = invocation.getArgument(1);
                 positions.add(location.clone());
+                RNG customization = invocation.getArgument(2);
+                customizationSeeds.add(customization.getSeed());
                 Entity spawned = mock(Entity.class);
                 when(spawned.getLocation()).thenReturn(location.clone());
                 when(spawned.getType()).thenReturn(EntityType.TADPOLE);
@@ -451,8 +548,8 @@ public class IrisEntitySpawnTest {
         }
 
         private int spawn(IrisSpawnGroup group) {
-            entry.setReferenceSpawner(new IrisSpawner().setGroup(group));
-            return entry.spawn(engine, chunk, new RNG(1337L), remainingCapacity);
+            return entry.spawn(engine, chunk, new RNG(1337L), remainingCapacity,
+                    new IrisEntitySpawn.SpawnContext(new IrisSpawner().setGroup(group), null, false));
         }
 
         @Override

core/src/testFixtures/java/art/arcane/iris/testsupport/BukkitTestServer.java

@@ -1,7 +1,14 @@
 package art.arcane.iris.testsupport;
 
 
+import io.papermc.paper.registry.RegistryAccess;
+import io.papermc.paper.registry.RegistryKey;
+import net.kyori.adventure.key.Key;
 import org.bukkit.Bukkit;
+import org.bukkit.GameRule;
+import org.bukkit.NamespacedKey;
+import org.bukkit.Registry;
+import org.bukkit.block.Biome;
 import org.bukkit.Material;
 import org.bukkit.Server;
 import org.bukkit.block.data.BlockData;
@@ -11,7 +18,10 @@ import org.junit.runner.Description;
 import org.junit.runners.model.Statement;
 import org.mockito.invocation.InvocationOnMock;
 
+import java.lang.reflect.Proxy;
 import java.util.Locale;
+import java.util.HashMap;
+import java.util.Map;
 import java.util.logging.Logger;
 
 import static org.mockito.ArgumentMatchers.any;
@@ -20,6 +30,7 @@ import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.RETURNS_DEFAULTS;
 
 public final class BukkitTestServer {
     private static final String DEFAULT_NAME = "IrisTestServer";
@@ -71,6 +82,45 @@ public final class BukkitTestServer {
         }
     }
 
+    public static RegistryAccess registryAccess() {
+        Map<Object, Object> registries = new HashMap<>();
+        return mock(RegistryAccess.class, invocation -> {
+            if (!invocation.getMethod().getName().equals("getRegistry")) {
+                return RETURNS_DEFAULTS.answer(invocation);
+            }
+            Object key = invocation.getArgument(0);
+            return registries.computeIfAbsent(key, registryKey -> registryProxy(registryKey == RegistryKey.GAME_RULE
+                    || registryKey == GameRule.class));
+        });
+    }
+
+    private static Object registryProxy(boolean gameRules) {
+        Map<Key, Object> values = new HashMap<>();
+        return Proxy.newProxyInstance(Registry.class.getClassLoader(), new Class<?>[]{Registry.class},
+                (proxy, method, arguments) -> switch (method.getName()) {
+                    case "getOrThrow", "get" -> values.computeIfAbsent((Key) arguments[0],
+                            key -> gameRules ? mock(GameRule.class) : biome(key));
+                    case "hashCode" -> System.identityHashCode(proxy);
+                    case "equals" -> proxy == arguments[0];
+                    case "toString" -> gameRules ? "iris-test-game-rule-registry" : "iris-test-biome-registry";
+                    case "size" -> values.size();
+                    case "hasTag" -> false;
+                    default -> null;
+                });
+    }
+
+    private static Biome biome(Key key) {
+        NamespacedKey namespaced = new NamespacedKey(key.namespace(), key.value());
+        return (Biome) Proxy.newProxyInstance(Biome.class.getClassLoader(), new Class<?>[]{Biome.class},
+                (proxy, method, arguments) -> switch (method.getName()) {
+                    case "getKey" -> namespaced;
+                    case "hashCode" -> namespaced.hashCode();
+                    case "equals" -> proxy == arguments[0];
+                    case "toString", "name" -> key.value();
+                    default -> null;
+                });
+    }
+
     public static ServerBranding branding() {
         return new ServerBranding();
     }