Preserved historical biome behavior

- Generated chunks retain surface and vertical biome assignments
- Floating biome assignments persist through terrain transitions
- Archived pack definitions remain available for saved biome rules
- Spawns and effects use each saved biome environment
- Saved biome lookups load asynchronously with bounded caches
- Missing biome identities recover only from exact recorded evidence
- Effects and entity placement follow existing world surfaces
- New pack fingerprints ignore Finder metadata
- Tests cover biome storage, recovery, consumers, and scoped definitions

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/CustomBiomeSource.java

@@ -333,6 +333,10 @@ public class CustomBiomeSource extends BiomeSource {
         return Collections.unmodifiableMap(spawnBiomes);
     }
 
+    Holder<Biome> getRetainedVanillaSpawnBiome(String derivativeKey) {
+        return resolveBiomeHolder(biomeRegistry, derivativeKey);
+    }
+
     Holder<Biome> getVanillaSpawnBiome(Holder<Biome> biome) {
         if (biome == null) {
             return null;

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/IrisChunkGenerator.java

@@ -18,6 +18,7 @@ import art.arcane.iris.engine.framework.NativeStructureGenerationPolicy;
 import art.arcane.iris.engine.IrisEngine;
 import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
 import art.arcane.iris.engine.history.NativeTerrainReceipt;
+import art.arcane.iris.engine.history.NativeBiomeSpawnSelection;
 import art.arcane.iris.engine.history.SavedTerrainChunk;
 import org.bukkit.NamespacedKey;
 import org.bukkit.persistence.PersistentDataType;
@@ -929,11 +930,17 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
 
     @Override
     public WeightedList<MobSpawnSettings.SpawnerData> getMobsAt(Holder<Biome> holder, StructureManager structuremanager, MobCategory enumcreaturetype, BlockPos blockposition) {
+        NativeBiomeSpawnSelection selection = NativeBiomeSpawnSelection.at(
+                engine, blockposition.getX(), blockposition.getY(), blockposition.getZ(),
+                holder.unwrapKey().map(key -> key.identifier().toString()).orElse(""));
+        if (selection.mode() == NativeBiomeSpawnSelection.Mode.LOADING) {
+            return WeightedList.of(List.of());
+        }
         try (GenerationHistoryRuntimeRouter.CoordinateScope route = openHistoryCoordinateScope(
                      blockposition.getX(), blockposition.getZ(), "bukkit_nms_mob_spawns");
              GenerationSessionLease lease = requireGenerationLease("bukkit_nms_mob_spawns");
              IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
-            return getMobsAtWithActiveRuntime(holder, structuremanager, enumcreaturetype, blockposition);
+            return getMobsAtWithActiveRuntime(holder, structuremanager, enumcreaturetype, blockposition, selection);
         }
     }
 
@@ -941,9 +948,14 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
             Holder<Biome> holder,
             StructureManager structuremanager,
             MobCategory enumcreaturetype,
-            BlockPos blockposition
+            BlockPos blockposition,
+            NativeBiomeSpawnSelection selection
     ) {
-        Holder<Biome> vanillaSpawnBiome = customBiomeSource.getVanillaSpawnBiome(holder);
+        Holder<Biome> vanillaSpawnBiome = switch (selection.mode()) {
+            case RETAINED -> customBiomeSource.getRetainedVanillaSpawnBiome(selection.derivativeKey());
+            case CURRENT -> customBiomeSource.getVanillaSpawnBiome(holder);
+            case NONE, LOADING -> null;
+        };
         if (vanillaSpawnBiome == null) {
             return delegate.getMobsAt(holder, structuremanager, enumcreaturetype, blockposition);
         }
@@ -964,7 +976,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
         }
 
         int spawnRuntimeId = engine.getCacheID();
-        SpawnTableKey key = new SpawnTableKey(spawnRuntimeId, holder.value(), enumcreaturetype);
+        SpawnTableKey key = new SpawnTableKey(spawnRuntimeId, holder.value(), vanillaSpawnBiome.value(), enumcreaturetype);
         return mergedSpawnTables.computeIfAbsent(key, ignored -> mergeSpawnTables(vanillaSpawns, explicitSpawns));
     }
 
@@ -1611,7 +1623,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
         }
     }
 
-    private record SpawnTableKey(int runtimeId, Biome biome, MobCategory category) {
+    private record SpawnTableKey(int runtimeId, Biome biome, Biome vanillaBiome, MobCategory category) {
     }
 
     private record ReachableStructureCache(IrisDimension dimension, int runtimeId, Set<String> keys) {

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

@@ -600,7 +600,8 @@ public final class IrisWorldGeneratorResolver {
             throw new IllegalStateException("Immutable Iris generation pack at " + packRoot
                     + " does not contain dimension " + dimensionKey + ".");
         }
-        GenerationEpoch.DimensionContract actualContract = captureDimensionContract(data, dimension);
+        GenerationEpoch.DimensionContract actualContract = GenerationEpochContractFactory.createForEpoch(
+                dimension, dimensionTypeKey(data, dimension), epoch);
         if (!epoch.dimensionContract().equals(actualContract)) {
             throw new IllegalStateException("Immutable Iris generation pack dimension contract changed at "
                     + packRoot + ".");
@@ -612,7 +613,15 @@ public final class IrisWorldGeneratorResolver {
             IrisData data,
             IrisDimension dimension
     ) {
-        String dimensionTypeKey = IrisPlatforms.get()
+        return GenerationEpochContractFactory.create(
+                dimension,
+                dimension.getLoadKey(),
+                dimensionTypeKey(data, dimension)
+        );
+    }
+
+    private static String dimensionTypeKey(IrisData data, IrisDimension dimension) {
+        return IrisPlatforms.get()
                 .registries()
                 .generationRegistry()
                 .dimensionTypeResourceKey(
@@ -620,11 +629,6 @@ public final class IrisWorldGeneratorResolver {
                         dimension.getLoadKey(),
                         dimension.getDimensionTypeKey()
                 );
-        return GenerationEpochContractFactory.create(
-                dimension,
-                dimension.getLoadKey(),
-                dimensionTypeKey
-        );
     }
 
     private static File requireActivePack(GenerationHistory history) {

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

@@ -31,6 +31,7 @@ import art.arcane.iris.engine.framework.GenerationSessionLease;
 import art.arcane.iris.engine.platform.EngineBukkitOps;
 import art.arcane.iris.util.project.context.IrisContext;
 import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.history.SavedBiomeUnavailableException;
 import art.arcane.iris.engine.object.IrisRegion;
 import art.arcane.iris.util.common.director.DirectorExecutor;
 import art.arcane.iris.util.common.plugin.VolmitSender;
@@ -82,21 +83,36 @@ public class CommandWhat implements DirectorExecutor {
 
     @Director(description = "What biome am i in?", descriptionKey = "iris.director.commandwhat.director.what_biome_am_i", origin = DirectorOrigin.PLAYER)
     public void biome() {
+        VolmitSender commandSender = sender();
+        Player player = player();
+        onPlayerThread(player, () -> describeBiome(commandSender, player));
+    }
+
+    private void describeBiome(VolmitSender commandSender, Player player) {
+        Location location = player.getLocation();
         try {
-            IrisBiome b = engine().getBiome(player().getLocation().getBlockX(), player().getLocation().getBlockY() - player().getWorld().getMinHeight(), player().getLocation().getBlockZ());
+            Engine engine = IrisToolbelt.access(location.getWorld()).getEngine();
+            IrisBiome b = engine.getBiome(location.getBlockX(), location.getBlockY() - location.getWorld().getMinHeight(), location.getBlockZ());
             Biome derivative = b.getDerivative();
             NamespacedKey derivativeKey = resolveBiomeKey(derivative);
-            sender().sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_IBIOME, MessageArgument.untrusted("value", b.getLoadKey()), MessageArgument.untrusted("value2", derivativeKey == null ? IrisLanguage.plain(RuntimeUiMessages.STATUS_UNREGISTERED) : derivativeKey.getKey())));
+            commandSender.sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_IBIOME, MessageArgument.untrusted("value", b.getLoadKey()), MessageArgument.untrusted("value2", derivativeKey == null ? IrisLanguage.plain(RuntimeUiMessages.STATUS_UNREGISTERED) : derivativeKey.getKey())));
 
+        } catch (SavedBiomeUnavailableException e) {
+            if (e.getCause() != null) {
+                Iris.reportError(e);
+            }
+            commandSender.sendMessage(e.isLoading()
+                    ? "Saved biome data is loading. Run the command again shortly."
+                    : e.getMessage());
         } catch (Throwable e) {
             Iris.reportError(e);
-            Biome biome = player().getLocation().getBlock().getBiome();
+            Biome biome = location.getBlock().getBiome();
             NamespacedKey key = resolveBiomeKey(biome);
-            sender().sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_NON_IRIS_BIOME, MessageArgument.untrusted("value", key == null ? IrisLanguage.plain(RuntimeUiMessages.STATUS_UNREGISTERED) : key)));
+            commandSender.sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_NON_IRIS_BIOME, MessageArgument.untrusted("value", key == null ? IrisLanguage.plain(RuntimeUiMessages.STATUS_UNREGISTERED) : key)));
 
             if (key == null || key.getKey().equals("custom")) {
                 try {
-                    sender().sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_DATA_PACK_BIOME_ID, MessageArgument.untrusted("value", INMS.get().getTrueBiomeBaseKey(player().getLocation())), MessageArgument.untrusted("value2", INMS.get().getTrueBiomeBaseId(INMS.get().getTrueBiomeBase(player().getLocation())))));
+                    commandSender.sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_DATA_PACK_BIOME_ID, MessageArgument.untrusted("value", INMS.get().getTrueBiomeBaseKey(location)), MessageArgument.untrusted("value2", INMS.get().getTrueBiomeBaseId(INMS.get().getTrueBiomeBase(location)))));
                 } catch (Throwable ee) {
                     Iris.reportError(ee);
                 }
@@ -118,6 +134,13 @@ public class CommandWhat implements DirectorExecutor {
                 IrisRegion r = EngineBukkitOps.getRegion(engine, chunk);
                 commandSender.sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_IREGION, MessageArgument.untrusted("value", r.getLoadKey()), MessageArgument.untrusted("value2", r.getName())));
 
+            } catch (SavedBiomeUnavailableException e) {
+                if (e.getCause() != null) {
+                    Iris.reportError(e);
+                }
+                commandSender.sendMessage(e.isLoading()
+                        ? "Saved biome data is loading. Run the command again shortly."
+                        : e.getMessage());
             } catch (Throwable e) {
                 Iris.reportError(e);
                 commandSender.sendMessage(IrisLanguage.text(BukkitCommandMessagesExtended.COMMAND_WHAT_IRIS_WORLDS_ONLY));

adapters/bukkit/plugin/src/test/java/art/arcane/iris/core/commands/CommandWhatBiomeTest.java

@@ -0,0 +1,80 @@
+package art.arcane.iris.core.commands;
+
+import art.arcane.iris.core.localization.BukkitCommandMessagesExtended;
+import art.arcane.iris.core.localization.IrisLanguage;
+import art.arcane.iris.core.localization.RuntimeUiMessages;
+import art.arcane.iris.core.tools.IrisToolbelt;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.platform.PlatformChunkGenerator;
+import art.arcane.iris.util.common.director.DirectorContext;
+import art.arcane.iris.util.common.plugin.VolmitSender;
+import art.arcane.iris.util.common.scheduling.J;
+import art.arcane.volmlib.util.localization.MessageArgument;
+import org.bukkit.Location;
+import org.bukkit.World;
+import org.bukkit.entity.Player;
+import org.junit.Test;
+import org.mockito.MockedStatic;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+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.mockStatic;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
+import static org.mockito.Mockito.when;
+
+public class CommandWhatBiomeTest {
+    @Test
+    public void scheduledBiomeDescriptionUsesThePlayersWorldAfterCommandContextClears() {
+        VolmitSender sender = mock(VolmitSender.class);
+        Player player = mock(Player.class);
+        World world = mock(World.class);
+        Engine engine = mock(Engine.class);
+        PlatformChunkGenerator generator = mock(PlatformChunkGenerator.class);
+        IrisBiome biome = mock(IrisBiome.class);
+        AtomicReference<Runnable> scheduled = new AtomicReference<>();
+        when(sender.player()).thenReturn(player);
+        when(sender.isPlayer()).thenReturn(true);
+        when(player.getWorld()).thenReturn(world);
+        when(player.getLocation()).thenReturn(new Location(world, 19, 70, -3));
+        when(world.getMinHeight()).thenReturn(-64);
+        when(generator.getEngine()).thenReturn(engine);
+        when(engine.getBiome(19, 134, -3)).thenReturn(biome);
+        when(biome.getLoadKey()).thenReturn("retained-flat");
+        try (MockedStatic<J> scheduler = mockStatic(J.class);
+             MockedStatic<IrisToolbelt> toolbelt = mockStatic(IrisToolbelt.class);
+             MockedStatic<IrisLanguage> language = mockStatic(IrisLanguage.class)) {
+            scheduler.when(() -> J.runEntity(eq(player), any(Runnable.class))).thenAnswer(invocation -> {
+                scheduled.set(invocation.getArgument(1));
+                return true;
+            });
+            toolbelt.when(() -> IrisToolbelt.access(world)).thenReturn(generator);
+            language.when(() -> IrisLanguage.plain(RuntimeUiMessages.STATUS_UNREGISTERED)).thenReturn("unregistered");
+            language.when(() -> IrisLanguage.text(eq(BukkitCommandMessagesExtended.COMMAND_WHAT_IBIOME),
+                    any(MessageArgument.class), any(MessageArgument.class))).thenReturn("Iris biome: retained-flat");
+            DirectorContext.touch(sender);
+            try {
+                new CommandWhat().biome();
+            } finally {
+                DirectorContext.remove();
+            }
+            assertNull(DirectorContext.get());
+            assertNotNull(scheduled.get());
+            verifyNoInteractions(engine);
+
+            scheduled.get().run();
+
+            verify(engine).getBiome(19, 134, -3);
+            verify(sender).sendMessage("Iris biome: retained-flat");
+            verify(world, never()).getBlockAt(anyInt(), anyInt(), anyInt());
+        }
+    }
+}

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

@@ -20,6 +20,7 @@ package art.arcane.iris.modded;
 
 import art.arcane.iris.nativegen.NativeTransitionColumn;
 import art.arcane.iris.engine.history.TerrainBoundarySignature;
+import art.arcane.iris.engine.history.NativeBiomeSpawnSelection;
 import java.util.Optional;
 import art.arcane.iris.nativegen.NativeGenerationWriteGuard;
 import art.arcane.iris.core.loader.IrisData;
@@ -920,6 +921,11 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
     public WeightedList<MobSpawnSettings.SpawnerData> getMobsAt(
             Holder<Biome> biome, StructureManager structureManager, MobCategory category, BlockPos pos) {
         Engine current = engine();
+        NativeBiomeSpawnSelection selection = NativeBiomeSpawnSelection.at(current, pos.getX(), pos.getY(), pos.getZ(),
+                biome.unwrapKey().map(key -> key.identifier().toString()).orElse(""));
+        if (selection.mode() == NativeBiomeSpawnSelection.Mode.LOADING) {
+            return WeightedList.of(List.of());
+        }
         try (GenerationHistoryRuntimeRouter.CoordinateScope historyScope = openHistoryCoordinateScope(
                      current, pos.getX(), pos.getZ(), "modded_mob_spawn_table");
              GenerationSessionLease lease = requireGenerationLease(current, "modded_mob_spawn_table");
@@ -933,8 +939,15 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
             }
 
             Registry<Biome> registry = structureManager.registryAccess().lookupOrThrow(Registries.BIOME);
-            spawnTables.initializeVanillaSpawnBiomes(registry);
-            Holder<Biome> vanillaSpawnBiome = spawnTables.vanillaSpawnBiome(biome.value());
+            Holder<Biome> vanillaSpawnBiome;
+            if (selection.mode() == NativeBiomeSpawnSelection.Mode.RETAINED) {
+                vanillaSpawnBiome = spawnTables.resolveBiomeHolder(registry, selection.derivativeKey());
+            } else if (selection.mode() == NativeBiomeSpawnSelection.Mode.CURRENT) {
+                spawnTables.initializeVanillaSpawnBiomes(registry);
+                vanillaSpawnBiome = spawnTables.vanillaSpawnBiome(biome.value());
+            } else {
+                vanillaSpawnBiome = null;
+            }
             if (vanillaSpawnBiome == null) {
                 return explicitSpawns;
             }
@@ -951,6 +964,7 @@ public final class IrisModdedChunkGenerator extends ChunkGenerator {
             return spawnTables.mergedSpawnTable(
                     current,
                     biome.value(),
+                    vanillaSpawnBiome.value(),
                     category,
                     vanillaSpawns,
                     explicitSpawns

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

@@ -20,13 +20,13 @@ package art.arcane.iris.modded;
 
 import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.framework.BiomeEnvironment;
+import art.arcane.iris.engine.history.SavedBiomeUnavailableException;
 import art.arcane.iris.engine.framework.EngineAssignedComponent;
 import art.arcane.iris.engine.framework.EngineEffects;
-import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisCommand;
 import art.arcane.iris.engine.object.IrisCommandRegistry;
 import art.arcane.iris.engine.object.IrisEffect;
-import art.arcane.iris.engine.object.IrisRegion;
 import art.arcane.iris.spi.IrisLogging;
 import art.arcane.volmlib.util.math.RNG;
 import net.minecraft.core.Holder;
@@ -36,6 +36,7 @@ import net.minecraft.core.registries.BuiltInRegistries;
 import net.minecraft.network.protocol.game.ClientboundSoundPacket;
 import net.minecraft.resources.Identifier;
 import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.levelgen.Heightmap;
 import net.minecraft.server.level.ServerPlayer;
 import net.minecraft.sounds.SoundEvent;
 import net.minecraft.sounds.SoundSource;
@@ -193,18 +194,21 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
             return;
         }
 
-        samplePlayer(state);
+        try {
+            samplePlayer(state);
+        } catch (SavedBiomeUnavailableException unavailable) {
+            state.environment = null;
+            state.sampled(false);
+            return;
+        }
         if (!IrisSettings.get().getWorld().isEffectSystem()) {
             return;
         }
 
-        IrisRegion region = state.region();
-        if (region != null) {
-            applyEffects(level, player, region.getEffects());
-        }
-        IrisBiome biome = state.biome();
-        if (biome != null) {
-            applyEffects(level, player, biome.getEffects());
+        BiomeEnvironment environment = state.environment;
+        try (BiomeEnvironment.Scope ignored = getEngine().openBiomeEnvironmentScope(environment)) {
+            applyEffects(level, player, environment.region().getEffects());
+            applyEffects(level, player, environment.biome().getEffects());
         }
     }
 
@@ -219,14 +223,14 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
             return;
         }
 
-        state.sampled(true);
-        state.lastPosition(player.getX(), player.getY(), player.getZ());
-        state.lastSample(now);
         int blockX = floor(player.getX());
         int blockY = floor(player.getY()) - getEngine().getWorld().minHeight();
         int blockZ = floor(player.getZ());
-        state.biome(getEngine().getBiome(blockX, blockY, blockZ));
-        state.region(getEngine().getRegion(blockX, blockY, blockZ));
+        BiomeEnvironment environment = getEngine().getBiomeEnvironment(blockX, blockY, blockZ);
+        state.environment = environment;
+        state.sampled(true);
+        state.lastPosition(player.getX(), player.getY(), player.getZ());
+        state.lastSample(now);
     }
 
     private void applyEffects(ServerLevel level, ServerPlayer player, Iterable<IrisEffect> effects) {
@@ -299,9 +303,12 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
         double sideways = RNG.r.d(-effect.getParticleDistanceWidth(), effect.getParticleDistanceWidth());
         double surfaceX = player.getX() + direction.x * forward + direction.z * sideways;
         double surfaceZ = player.getZ() + direction.z * forward - direction.x * sideways;
-        int surfaceY = getEngine().getHeight(floor(surfaceX), floor(surfaceZ));
+        if (level.getChunkSource().getChunkNow(floor(surfaceX) >> 4, floor(surfaceZ) >> 4) == null) {
+            return;
+        }
+        int surfaceY = level.getHeight(Heightmap.Types.OCEAN_FLOOR, floor(surfaceX), floor(surfaceZ));
         double x = surfaceX + RNG.r.d();
-        double y = surfaceY + 1.0D + level.getMinY() + RNG.r.i(effect.getParticleOffset());
+        double y = surfaceY + RNG.r.i(effect.getParticleOffset());
         double z = surfaceZ + RNG.r.d();
         double altX = randomized(effect.getParticleAltX(), effect.isRandomAltX());
         double altY = randomized(effect.getParticleAltY(), effect.isRandomAltY());
@@ -421,8 +428,7 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
 
     private static final class PlayerState {
         private ServerPlayer player;
-        private IrisBiome biome;
-        private IrisRegion region;
+        private BiomeEnvironment environment;
         private double lastX;
         private double lastY;
         private double lastZ;
@@ -446,22 +452,6 @@ public final class ModdedEngineEffects extends EngineAssignedComponent implement
             this.player = player;
         }
 
-        private IrisBiome biome() {
-            return biome;
-        }
-
-        private void biome(IrisBiome biome) {
-            this.biome = biome;
-        }
-
-        private IrisRegion region() {
-            return region;
-        }
-
-        private void region(IrisRegion region) {
-            this.region = region;
-        }
-
         private double lastX() {
             return lastX;
         }

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

@@ -175,7 +175,7 @@ final class ModdedGenerationHistoryStorage {
         GenerationEpoch epoch = history.activeEpoch();
         Path packRoot = normalize(history.activePackRoot());
         validatePack(packRoot, epoch.packFingerprint());
-        validateDimension(packRoot, epoch.dimensionContract());
+        validateDimension(packRoot, epoch);
         return new ActivePack(
                 packRoot,
                 epoch.dimensionContract().dimensionKey(),
@@ -194,8 +194,9 @@ final class ModdedGenerationHistoryStorage {
 
     private static void validateDimension(
             Path packRoot,
-            GenerationEpoch.DimensionContract recordedContract
+            GenerationEpoch epoch
     ) throws IOException {
+        GenerationEpoch.DimensionContract recordedContract = epoch.dimensionContract();
         IrisData data = IrisData.openDatapackCompiler(packRoot.toFile());
         try {
             IrisDimension dimension = data.getDimensionLoader().load(recordedContract.dimensionKey(), false);
@@ -203,10 +204,10 @@ final class ModdedGenerationHistoryStorage {
                 throw new IOException("Immutable generation pack does not contain dimension '"
                         + recordedContract.dimensionKey() + "'.");
             }
-            GenerationEpoch.DimensionContract actualContract = GenerationEpochContractFactory.create(
+            GenerationEpoch.DimensionContract actualContract = GenerationEpochContractFactory.createForEpoch(
                     dimension,
-                    dimension.getLoadKey(),
-                    recordedContract.dimensionTypeKey()
+                    recordedContract.dimensionTypeKey(),
+                    epoch
             );
             if (!recordedContract.equals(actualContract)) {
                 throw new IOException("Immutable generation pack no longer matches its recorded dimension contract.");

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

@@ -137,10 +137,11 @@ final class ModdedSpawnTableMerger {
     WeightedList<MobSpawnSettings.SpawnerData> mergedSpawnTable(
             Engine engine,
             Biome biome,
+            Biome vanillaBiome,
             MobCategory category,
             WeightedList<MobSpawnSettings.SpawnerData> vanillaSpawns,
             WeightedList<MobSpawnSettings.SpawnerData> explicitSpawns) {
-        SpawnTableKey key = new SpawnTableKey(engine.getCacheID(), biome, category);
+        SpawnTableKey key = new SpawnTableKey(engine.getCacheID(), biome, vanillaBiome, category);
         return mergedSpawnTables.computeIfAbsent(
                 key,
                 ignored -> NativeSpawnTableMerger.merge(vanillaSpawns, explicitSpawns)
@@ -153,7 +154,7 @@ final class ModdedSpawnTableMerger {
         initializedRuntimeIdentities.remove(runtimeIdentity);
     }
 
-    private Holder<Biome> resolveBiomeHolder(Registry<Biome> registry, String key) {
+    Holder<Biome> resolveBiomeHolder(Registry<Biome> registry, String key) {
         if (key == null || key.isBlank()) {
             return null;
         }
@@ -176,6 +177,6 @@ final class ModdedSpawnTableMerger {
     private record SpawnBiomeKey(int runtimeIdentity, Biome biome) {
     }
 
-    private record SpawnTableKey(int runtimeIdentity, Biome biome, MobCategory category) {
+    private record SpawnTableKey(int runtimeIdentity, Biome biome, Biome vanillaBiome, MobCategory category) {
     }
 }

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

@@ -22,6 +22,8 @@ import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.core.gui.PregeneratorJob;
 import art.arcane.iris.engine.IrisComplex;
 import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.framework.BiomeEnvironment;
+import art.arcane.iris.engine.history.SavedBiomeUnavailableException;
 import art.arcane.iris.engine.framework.EngineLifecycleTasks;
 import art.arcane.iris.engine.framework.EngineWorldManager;
 import art.arcane.iris.engine.framework.LootResolver;
@@ -32,7 +34,6 @@ import art.arcane.iris.engine.object.IrisEntitySpawn;
 import art.arcane.iris.engine.object.IrisMarker;
 import art.arcane.iris.engine.object.IrisPosition;
 import art.arcane.iris.engine.object.IrisRange;
-import art.arcane.iris.engine.object.IrisRegion;
 import art.arcane.iris.engine.object.IrisSpawnGroup;
 import art.arcane.iris.engine.object.IrisSpawner;
 import art.arcane.iris.engine.object.IrisSurface;
@@ -53,12 +54,16 @@ import net.minecraft.world.entity.Entity;
 import net.minecraft.world.entity.LivingEntity;
 import net.minecraft.world.entity.MobCategory;
 import net.minecraft.world.level.NaturalSpawner;
+import net.minecraft.world.level.levelgen.Heightmap;
 import net.minecraft.world.level.block.Blocks;
 import net.minecraft.world.level.block.state.BlockState;
 import net.minecraft.world.level.chunk.LevelChunk;
 import net.minecraft.world.phys.AABB;
 
 import java.util.HashSet;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.BlockingQueue;
@@ -152,7 +157,7 @@ public final class ModdedWorldManager implements EngineWorldManager {
     }
 
     private void recoverLoadedInitialSpawns(ServerLevel level) {
-        if (!markerSystemEnabled()) {
+        if (!markerSystemEnabled() && !ambientSystemEnabled()) {
             return;
         }
         long now = System.currentTimeMillis();
@@ -194,7 +199,7 @@ public final class ModdedWorldManager implements EngineWorldManager {
         if (initialSpawnQueue.isEmpty()) {
             return;
         }
-        if (!markerSystemEnabled()) {
+        if (!markerSystemEnabled() && !ambientSystemEnabled()) {
             initialSpawnQueue.clear();
             return;
         }
@@ -217,6 +222,8 @@ public final class ModdedWorldManager implements EngineWorldManager {
                     retry = true;
                     warmupMantleChunkAsync(key, chunkX, chunkZ);
                 }
+            } catch (SavedBiomeUnavailableException unavailable) {
+                retry = unavailable.isLoading();
             } catch (Throwable e) {
                 IrisLogging.reportError(e);
                 retry = true;
@@ -230,7 +237,7 @@ public final class ModdedWorldManager implements EngineWorldManager {
         }
     }
 
-    private boolean initialSpawnChunk(ServerLevel level, int chunkX, int chunkZ) {
+    boolean initialSpawnChunk(ServerLevel level, int chunkX, int chunkZ) {
         if (!ModdedEntitySpawner.chunksSafe(level, chunkX, chunkZ)) {
             return false;
         }
@@ -244,9 +251,15 @@ public final class ModdedWorldManager implements EngineWorldManager {
 
         MantleChunk<Matter> chunk = mantle.getChunk(chunkX, chunkZ).use();
         try {
+            List<PreparedMarkerSpawn> markers = markerSystemEnabled()
+                    ? prepareMarkerSpawns(level, chunkX, chunkZ, chunk) : List.of();
+            Optional<BiomeEnvironment> environment = ambientSystemEnabled()
+                    ? resolveSurfaceEnvironment(chunkX, chunkZ) : Optional.empty();
             chunk.raiseFlagUnchecked(INITIAL_SPAWN_COMPLETION_FLAG, () -> {
-                spawnMarkerSpawners(level, chunkX, chunkZ, chunk, true);
-                scheduleInitialFollowUp(level, chunkX, chunkZ);
+                spawnPreparedMarkers(level, markers, true);
+                if (environment.isPresent()) {
+                    scheduleInitialFollowUp(level, chunkX, chunkZ, environment.get());
+                }
             });
         } finally {
             chunk.release();
@@ -254,7 +267,7 @@ public final class ModdedWorldManager implements EngineWorldManager {
         return true;
     }
 
-    private void scheduleInitialFollowUp(ServerLevel level, int chunkX, int chunkZ) {
+    private void scheduleInitialFollowUp(ServerLevel level, int chunkX, int chunkZ, BiomeEnvironment environment) {
         ModdedScheduler scheduler = ModdedEngineBootstrap.schedulerOrNull();
         if (scheduler == null) {
             IrisLogging.error("Iris could not schedule the initial entity-spawn follow-up because the modded scheduler is unavailable.");
@@ -264,11 +277,11 @@ public final class ModdedWorldManager implements EngineWorldManager {
                 () -> EngineLifecycleTasks.run(
                         engine,
                         "modded_world_manager_initial_spawn_followup",
-                        () -> runInitialFollowUp(level, chunkX, chunkZ)),
+                        () -> runInitialFollowUp(level, chunkX, chunkZ, environment)),
                 RNG.r.i(5, 200));
     }
 
-    private void runInitialFollowUp(ServerLevel level, int chunkX, int chunkZ) {
+    private void runInitialFollowUp(ServerLevel level, int chunkX, int chunkZ, BiomeEnvironment environment) {
         Mantle<Matter> mantle = engine.getMantle().getMantle();
         if (closed || engine.isClosed() || mantle.isClosed() || !isEntitySpawningEnabledForCurrentWorld()) {
             return;
@@ -280,16 +293,10 @@ public final class ModdedWorldManager implements EngineWorldManager {
             return;
         }
 
-        MantleChunk<Matter> chunk = mantle.getChunk(chunkX, chunkZ).use();
-        try {
-            if (markerSystemEnabled()) {
-                spawnMarkerSpawners(level, chunkX, chunkZ, chunk, false);
+        if (ambientSystemEnabled()) {
+            try (BiomeEnvironment.Scope ignored = engine.openBiomeEnvironmentScope(environment)) {
+                spawnAmbient(level, chunkX, chunkZ, true, environment);
             }
-            if (ambientSystemEnabled()) {
-                spawnAmbient(level, chunkX, chunkZ, true);
-            }
-        } finally {
-            chunk.release();
         }
     }
 
@@ -360,6 +367,8 @@ public final class ModdedWorldManager implements EngineWorldManager {
             long key = ambientChunkSample[RNG.r.nextInt(sampled)];
             try {
                 ambientSpawnChunk(level, unpackX(key), unpackZ(key));
+            } catch (SavedBiomeUnavailableException unavailable) {
+                continue;
             } catch (Throwable e) {
                 IrisLogging.reportError(e);
             }
@@ -378,7 +387,7 @@ public final class ModdedWorldManager implements EngineWorldManager {
         MantleChunk<Matter> chunk = mantle.getChunk(chunkX, chunkZ).use();
         try {
             if (markerSystemEnabled()) {
-                spawnMarkerSpawners(level, chunkX, chunkZ, chunk, false);
+                spawnPreparedMarkers(level, prepareMarkerSpawns(level, chunkX, chunkZ, chunk), false);
             }
             if (ambientSystemEnabled()) {
                 spawnAmbient(level, chunkX, chunkZ, false);
@@ -388,29 +397,38 @@ public final class ModdedWorldManager implements EngineWorldManager {
         }
     }
 
-    private void spawnMarkerSpawners(ServerLevel level, int chunkX, int chunkZ, MantleChunk<Matter> chunk, boolean initial) {
+    private List<PreparedMarkerSpawn> prepareMarkerSpawns(ServerLevel level, int chunkX, int chunkZ, MantleChunk<Matter> chunk) {
         int minHeight = engine.getWorld().minHeight();
         KList<IrisPosition> obstructed = new KList<>();
+        List<PreparedMarkerSpawn> prepared = new ArrayList<>();
         chunk.iterate(MatterMarker.class, (Integer x, Integer yf, Integer z, MatterMarker marker) -> {
             String tag = marker.getTag();
             if (tag.equals("cave_floor") || tag.equals("cave_ceiling")) {
                 return;
             }
-            IrisMarker resolved = engine.getData().getMarkerLoader().load(tag);
-            if (resolved == null) {
-                return;
-            }
-
             int worldX = (chunkX << 4) + (x & 15);
             int worldZ = (chunkZ << 4) + (z & 15);
             int worldY = yf + minHeight;
+            BiomeEnvironment environment;
+            try {
+                environment = engine.getBiomeEnvironment(worldX, yf, worldZ);
+            } catch (SavedBiomeUnavailableException unavailable) {
+                if (unavailable.isLoading()) {
+                    throw unavailable;
+                }
+                return;
+            }
+            IrisMarker resolved = environment.data().getMarkerLoader().load(tag);
+            if (resolved == null) {
+                return;
+            }
 
             if (resolved.isEmptyAbove() && aboveObstructed(level, worldX, worldY, worldZ)) {
                 obstructed.add(new IrisPosition(worldX, yf, worldZ));
                 return;
             }
 
-            KList<IrisSpawner> spawners = resolveMarkerSpawners(resolved);
+            KList<IrisSpawner> spawners = resolveMarkerSpawners(resolved, environment);
             if (spawners.isEmpty()) {
                 return;
             }
@@ -418,18 +436,19 @@ public final class ModdedWorldManager implements EngineWorldManager {
             if (chosen == null) {
                 return;
             }
-            spawnFromSpawner(level, new IrisPosition(worldX, worldY, worldZ), chosen, initial);
+            prepared.add(new PreparedMarkerSpawn(new IrisPosition(worldX, worldY, worldZ), chosen, environment));
         });
         Mantle<Matter> mantle = engine.getMantle().getMantle();
         for (IrisPosition position : obstructed) {
             mantle.remove(position.getX(), position.getY(), position.getZ(), MatterMarker.class);
         }
+        return List.copyOf(prepared);
     }
 
-    private KList<IrisSpawner> resolveMarkerSpawners(IrisMarker marker) {
+    private KList<IrisSpawner> resolveMarkerSpawners(IrisMarker marker, BiomeEnvironment environment) {
         KList<IrisSpawner> spawners = new KList<>();
         for (String key : marker.getSpawners()) {
-            IrisSpawner spawner = engine.getData().getSpawnerLoader().load(key);
+            IrisSpawner spawner = environment.data().getSpawnerLoader().load(key);
             if (spawner == null) {
                 IrisLogging.error("Cannot load spawner: " + key + " for marker on " + engine.getName());
                 continue;
@@ -440,6 +459,14 @@ public final class ModdedWorldManager implements EngineWorldManager {
         return spawners;
     }
 
+    private void spawnPreparedMarkers(ServerLevel level, List<PreparedMarkerSpawn> markers, boolean initial) {
+        for (PreparedMarkerSpawn marker : markers) {
+            try (BiomeEnvironment.Scope ignored = engine.openBiomeEnvironmentScope(marker.environment())) {
+                spawnFromSpawner(level, marker.position(), marker.spawner(), initial);
+            }
+        }
+    }
+
     private void spawnFromSpawner(ServerLevel level, IrisPosition position, IrisSpawner spawner, boolean initial) {
         KList<IrisEntitySpawn> spawns = initial ? spawner.getInitialSpawns() : spawner.getSpawns();
         if (spawns.isEmpty()) {
@@ -465,22 +492,41 @@ public final class ModdedWorldManager implements EngineWorldManager {
         }
     }
 
+    private Optional<BiomeEnvironment> resolveSurfaceEnvironment(int chunkX, int chunkZ) {
+        try {
+            return Optional.of(engine.getSurfaceBiomeEnvironment((chunkX << 4) + 8, (chunkZ << 4) + 8));
+        } catch (SavedBiomeUnavailableException unavailable) {
+            if (unavailable.isLoading()) {
+                throw unavailable;
+            }
+            return Optional.empty();
+        }
+    }
+
     private void spawnAmbient(ServerLevel level, int chunkX, int chunkZ, boolean initial) {
+        Optional<BiomeEnvironment> resolved = resolveSurfaceEnvironment(chunkX, chunkZ);
+        if (resolved.isEmpty()) {
+            return;
+        }
+        BiomeEnvironment environment = resolved.get();
+        try (BiomeEnvironment.Scope ignored = engine.openBiomeEnvironmentScope(environment)) {
+            spawnAmbient(level, chunkX, chunkZ, initial, environment);
+        }
+    }
+
+    private void spawnAmbient(ServerLevel level, int chunkX, int chunkZ, boolean initial, BiomeEnvironment environment) {
         IrisComplex complex = engine.getComplex();
         if (complex == null) {
             return;
         }
 
-        int blockX = chunkX << 4;
-        int blockZ = chunkZ << 4;
-        IrisBiome biome = engine.getSurfaceBiome(blockX, blockZ);
-        IrisRegion region = engine.getRegion(blockX, blockZ);
+        IrisBiome biome = environment.biome();
         int chunkMobs = countChunkLivingEntities(level, chunkX, chunkZ);
 
         KList<IrisEntitySpawn> pool = new KList<>();
-        collectSpawns(pool, engine.getData().getSpawnerLoader().loadAll(engine.getDimension().getEntitySpawners()), biome, chunkX, chunkZ, chunkMobs, initial);
-        collectSpawns(pool, engine.getData().getSpawnerLoader().loadAll(region.getEntitySpawners()), null, chunkX, chunkZ, chunkMobs, initial);
-        collectSpawns(pool, engine.getData().getSpawnerLoader().loadAll(biome.getEntitySpawners()), null, chunkX, chunkZ, chunkMobs, initial);
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(environment.dimension().getEntitySpawners()), biome, chunkX, chunkZ, chunkMobs, initial);
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(environment.region().getEntitySpawners()), null, chunkX, chunkZ, chunkMobs, initial);
+        collectSpawns(pool, environment.data().getSpawnerLoader().loadAll(biome.getEntitySpawners()), null, chunkX, chunkZ, chunkMobs, initial);
         if (pool.isEmpty()) {
             return;
         }
@@ -556,8 +602,8 @@ public final class ModdedWorldManager implements EngineWorldManager {
             } else {
                 worldX = (chunkX << 4) + RNG.r.i(16);
                 worldZ = (chunkZ << 4) + RNG.r.i(16);
-                int surfaceY = level.getMinY() + engine.getHeight(worldX, worldZ, false);
-                int solidY = level.getMinY() + engine.getHeight(worldX, worldZ, true);
+                int surfaceY = level.getHeight(Heightmap.Types.WORLD_SURFACE, worldX, worldZ) - 1;
+                int solidY = level.getHeight(Heightmap.Types.OCEAN_FLOOR, worldX, worldZ) - 1;
                 worldY = group == IrisSpawnGroup.NORMAL
                         ? surfaceY + 1
                         : RNG.r.i(solidY + 1, surfaceY);
@@ -864,4 +910,8 @@ public final class ModdedWorldManager implements EngineWorldManager {
         }
         return failure;
     }
+
+    private record PreparedMarkerSpawn(IrisPosition position, IrisSpawner spawner, BiomeEnvironment environment) {
+    }
+
 }

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

@@ -24,6 +24,7 @@ import art.arcane.iris.core.localization.IrisMessages;
 import art.arcane.iris.core.localization.ModdedCommandMessages;
 import art.arcane.iris.core.localization.RuntimeUiMessages;
 import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.history.SavedBiomeUnavailableException;
 import art.arcane.iris.engine.framework.GenerationSessionException;
 import art.arcane.iris.engine.framework.GenerationSessionLease;
 import art.arcane.iris.engine.object.IrisBiome;
@@ -194,6 +195,13 @@ public final class ModdedWhatCommands {
                     MessageArgument.untrusted("biome", nativeBiome.key()),
                     MessageArgument.trusted("id", nativeBiome.id())));
             return 1;
+        } catch (SavedBiomeUnavailableException error) {
+            if (error.getCause() != null) {
+                ModdedIrisLog.error("Iris saved biome lookup failed in {}",
+                        source.getLevel().dimension().identifier(), error);
+            }
+            IrisModdedCommands.fail(source, error.getMessage());
+            return 0;
         } catch (Throwable error) {
             logLookupFailure(source, "biome", error,
                     ModdedCommandMessages.IRIS_MODDED_COMMANDS_BIOME_LOOKUP_FAILED_2);
@@ -223,6 +231,13 @@ public final class ModdedWhatCommands {
                     MessageArgument.untrusted("region", region.getLoadKey()),
                     MessageArgument.untrusted("name", region.getName())));
             return 1;
+        } catch (SavedBiomeUnavailableException error) {
+            if (error.getCause() != null) {
+                ModdedIrisLog.error("Iris saved biome lookup failed in {}",
+                        source.getLevel().dimension().identifier(), error);
+            }
+            IrisModdedCommands.fail(source, error.getMessage());
+            return 0;
         } catch (Throwable error) {
             logLookupFailure(source, "region", error,
                     ModdedCommandMessages.IRIS_MODDED_COMMANDS_REGION_LOOKUP_FAILED_2);

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

@@ -0,0 +1,131 @@
+package art.arcane.iris.modded;
+
+import art.arcane.iris.core.IrisSettings;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.framework.BiomeEnvironment;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.iris.engine.history.SavedBiomeUnavailableException;
+import art.arcane.iris.engine.mantle.EngineMantle;
+import art.arcane.iris.engine.object.IrisBiome;
+import art.arcane.iris.engine.object.IrisDimension;
+import art.arcane.iris.engine.object.IrisRegion;
+import art.arcane.iris.engine.object.IrisWorld;
+import art.arcane.volmlib.util.mantle.runtime.Mantle;
+import art.arcane.volmlib.util.mantle.runtime.MantleChunk;
+import art.arcane.volmlib.util.matter.Matter;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.SharedConstants;
+import net.minecraft.server.Bootstrap;
+import net.minecraft.server.level.ServerPlayer;
+import org.junit.Test;
+import org.junit.BeforeClass;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.util.common.data.B;
+import org.mockito.MockedStatic;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+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.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class ModdedSavedBiomeConsumersTest {
+    @BeforeClass
+    public static void initializeRuntimeTypes() throws Exception {
+        SharedConstants.tryDetectVersion();
+        Bootstrap.bootStrap();
+        PlatformBlockState air = mock(PlatformBlockState.class);
+        try (MockedStatic<B> blocks = mockStatic(B.class)) {
+            blocks.when(() -> B.getState("AIR")).thenReturn(air);
+            Class.forName(EngineMantle.class.getName());
+        }
+    }
+
+    @Test
+    @SuppressWarnings("unchecked")
+    public void loadingDefinitionsLeaveInitialFlagUnclaimed() {
+        Engine engine = mock(Engine.class);
+        EngineMantle engineMantle = mock(EngineMantle.class);
+        Mantle<Matter> mantle = mock(Mantle.class);
+        MantleChunk<Matter> chunk = mock(MantleChunk.class);
+        ServerLevel level = mock(ServerLevel.class);
+        when(engine.getMantle()).thenReturn(engineMantle);
+        when(engine.getWorld()).thenReturn(mock(IrisWorld.class));
+        when(engineMantle.getMantle()).thenReturn(mantle);
+        when(mantle.isChunkLoaded(2, -1)).thenReturn(true);
+        when(mantle.getChunk(2, -1)).thenReturn(chunk);
+        when(chunk.use()).thenReturn(chunk);
+        BiomeEnvironment environment = environment();
+        when(engine.getSurfaceBiomeEnvironment(40, -8))
+                .thenThrow(new SavedBiomeUnavailableException("Loading", true))
+                .thenReturn(environment);
+        IrisSettings settings = new IrisSettings();
+        settings.getWorld().setAmbientEntitySpawningSystem(true);
+        settings.getWorld().setMarkerEntitySpawningSystem(false);
+        ModdedWorldManager manager = new ModdedWorldManager(engine);
+
+        try (MockedStatic<IrisSettings> configured = mockStatic(IrisSettings.class);
+             MockedStatic<ModdedEntitySpawner> spawns = mockStatic(ModdedEntitySpawner.class)) {
+            configured.when(IrisSettings::get).thenReturn(settings);
+            spawns.when(() -> ModdedEntitySpawner.chunksSafe(level, 2, -1)).thenReturn(true);
+            assertThrows(SavedBiomeUnavailableException.class, () -> manager.initialSpawnChunk(level, 2, -1));
+            verify(chunk, never()).raiseFlagUnchecked(eq(ModdedWorldManager.INITIAL_SPAWN_COMPLETION_FLAG), any());
+            assertTrue(manager.initialSpawnChunk(level, 2, -1));
+            verify(chunk).raiseFlagUnchecked(eq(ModdedWorldManager.INITIAL_SPAWN_COMPLETION_FLAG), any());
+        } finally {
+            manager.close();
+        }
+    }
+
+    @Test
+    public void playerEffectsRetryLoadingWithoutMovement() throws Exception {
+        Engine engine = mock(Engine.class);
+        IrisWorld world = mock(IrisWorld.class);
+        ServerLevel level = mock(ServerLevel.class);
+        ServerPlayer player = mock(ServerPlayer.class);
+        when(engine.getWorld()).thenReturn(world);
+        when(world.minHeight()).thenReturn(-64);
+        when(player.isAlive()).thenReturn(true);
+        when(player.level()).thenReturn(level);
+        BiomeEnvironment environment = environment();
+        when(engine.getBiomeEnvironment(0, 64, 0))
+                .thenThrow(new SavedBiomeUnavailableException("Loading", true))
+                .thenReturn(environment);
+        IrisSettings settings = new IrisSettings();
+        settings.getWorld().setEffectSystem(false);
+        Class<?> stateType = Class.forName(ModdedEngineEffects.class.getName() + "$PlayerState");
+        Constructor<?> constructor = stateType.getDeclaredConstructor(ServerPlayer.class);
+        constructor.setAccessible(true);
+        Object state = constructor.newInstance(player);
+        Method tick = ModdedEngineEffects.class.getDeclaredMethod("tickPlayer", ServerLevel.class, stateType);
+        tick.setAccessible(true);
+        Field selected = stateType.getDeclaredField("environment");
+        selected.setAccessible(true);
+
+        try (MockedStatic<IrisSettings> configured = mockStatic(IrisSettings.class)) {
+            configured.when(IrisSettings::get).thenReturn(settings);
+            ModdedEngineEffects effects = new ModdedEngineEffects(engine);
+            tick.invoke(effects, level, state);
+            assertNull(selected.get(state));
+            tick.invoke(effects, level, state);
+            assertSame(environment, selected.get(state));
+            verify(engine, times(2)).getBiomeEnvironment(0, 64, 0);
+        }
+    }
+
+    private static BiomeEnvironment environment() {
+        return new BiomeEnvironment(3L, mock(IrisBiome.class), mock(IrisRegion.class),
+                mock(IrisDimension.class), mock(IrisData.class));
+    }
+}