Gate pools, placements and structures

- Filter excluded biomes, regions, spawners, loot tables, jigsaw pieces and structures out of every pool through CompatPools and cascade empty pools to the container
- Evaluate each object placement per object from the .iob palette header, honouring chance-1 edit rules and reporting every missing palette key
- Treat edit find lists and marker mark lists as match-only
- Exclude jigsaw pieces on missing palette keys, drop them from pools and exclude structures whose start pool is gone
- Drop custom biome spawns whose entity is missing before the datapack JSON is written
- Skip excluded placements in the object, floating object, tree and locator paths while keeping the per-chunk RNG draws unchanged
- Keep the engine focus biome and region null when the gate excluded them

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

@@ -2,6 +2,7 @@ package art.arcane.iris.core;
 
 import art.arcane.iris.BuildConstants;
 import art.arcane.iris.core.lifecycle.BukkitWorldConfiguration.IrisGeneratorBinding;
+import art.arcane.iris.core.compat.ContentGate;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.ResourceLoader;
 import art.arcane.iris.core.nms.datapack.DataVersion;
@@ -235,6 +236,7 @@ public final class IrisDatapackCompiler {
                 fingerprintContent(dimension.getDimensionType().toJson(fixer)));
 
         String namespace = dimension.getLoadKey().toLowerCase(Locale.ROOT);
+        ContentGate contentGate = dimension.getLoader() == null ? null : dimension.getLoader().getContentGate();
         for (IrisBiome biome : dimension.getAllBiomes(dimension::getLoader)) {
             if (biome == null || !biome.isCustom()) {
                 continue;
@@ -250,7 +252,7 @@ public final class IrisDatapackCompiler {
                 }
                 requirements.put(
                         "worldgen/biome/" + biomeKey,
-                        fingerprintContent(customBiome.generateJson(fixer)));
+                        fingerprintContent(customBiome.generateJson(fixer, contentGate)));
                 TreeSet<String> tags = new TreeSet<>();
                 for (String tag : customBiome.getEffectiveTags(derivativeKey)) {
                     String normalizedTag = normalizeRegistryKey(tag);

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

@@ -142,8 +142,19 @@ public class TreeSVC implements IrisService {
             return;
         }
 
+        KList<String> pool = placement.compatPlace(worldAccess.getData());
+
+        if (pool.isEmpty()) {
+            IrisLogging.debug("Tree placement has no object available on this Minecraft version");
+            return;
+        }
+
         saplingPlane.forEach(block -> block.setType(Material.AIR));
-        IrisObject object = worldAccess.getData().getObjectLoader().load(placement.getPlace().getRandom(RNG.r));
+        IrisObject object = worldAccess.getData().getObjectLoader().load(pool.getRandom(RNG.r));
+
+        if (object == null) {
+            return;
+        }
         String treeMarker = object.getLoadKey() + "@" + ThreadLocalRandom.current().nextInt(Integer.MAX_VALUE);
         List<BlockState> blockStateList = new KList<>();
         KMap<Location, BlockData> dataCache = new KMap<>();
@@ -311,12 +322,12 @@ public class TreeSVC implements IrisService {
 
         // Retrieve objectPlacements of type `species` from biome
         IrisBiome biome = worldAccess.getEngine().getBiome(location.getBlockX(), location.getBlockY()-worldAccess.getTarget().getWorld().minHeight(), location.getBlockZ());
-        placements.addAll(matchObjectPlacements(biome.getObjects(), size, type));
+        placements.addAll(matchObjectPlacements(biome.getObjects(), size, type, worldAccess.getData()));
 
         // Add more or find any in the region
         if (isUseAll || placements.isEmpty()) {
             IrisRegion region = worldAccess.getEngine().getRegion(location.getBlockX(), location.getBlockZ());
-            placements.addAll(matchObjectPlacements(region.getObjects(), size, type));
+            placements.addAll(matchObjectPlacements(region.getObjects(), size, type, worldAccess.getData()));
         }
 
         // Check if no matches were found, return a random one if they are
@@ -329,14 +340,15 @@ public class TreeSVC implements IrisService {
      * @param objects The object placements to check
      * @param size    The size of the sapling area to filter with
      * @param type    The type of the tree to filter with
+     * @param data    The pack, for the version-content gate
      * @return A list of objectPlacements that matched. May be empty.
      */
-    private KList<IrisObjectPlacement> matchObjectPlacements(KList<IrisObjectPlacement> objects, IrisTreeSize size, TreeType type) {
+    private KList<IrisObjectPlacement> matchObjectPlacements(KList<IrisObjectPlacement> objects, IrisTreeSize size, TreeType type, IrisData data) {
 
         KList<IrisObjectPlacement> p = new KList<>();
 
         for (IrisObjectPlacement i : objects) {
-            if (i.matches(size, type)) {
+            if (i.matches(size, type) && !i.isCompatExcluded(data)) {
                 p.add(i);
             }
         }

core/src/main/java/art/arcane/iris/engine/IrisComplex.java

@@ -20,6 +20,7 @@ package art.arcane.iris.engine;
 
 import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.data.cache.Cache;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.hydrology.HydrologyColumnLayer;
@@ -178,8 +179,9 @@ public class IrisComplex implements DataProvider {
         fluidHeight = engine.getDimension().getFluidHeight();
         generators = new HashMap<>();
         generatorBiomes = Collections.newSetFromMap(new IdentityHashMap<>());
-        focusBiome = engine.getFocus();
-        focusRegion = engine.getFocusRegion();
+        // A registrant the version-content gate excluded never enters a pool, focus included.
+        focusBiome = compatUsable(engine.getFocus());
+        focusRegion = compatUsable(engine.getFocusRegion());
         Map<InferredType, ProceduralStream<IrisBiome>> inferredStreams = new HashMap<>();
         KList<IrisRegion> preparedRegions = new KList<>();
 
@@ -195,7 +197,7 @@ public class IrisComplex implements DataProvider {
         } else {
             engine.getDimension().getRegions().forEach(regionKey -> {
                 IrisRegion region = data.getRegionLoader().load(regionKey);
-                if (region == null) {
+                if (region == null || region.isCompatExcluded()) {
                     return;
                 }
                 prepareInferredBiomes(region, preparedRegions);
@@ -244,7 +246,7 @@ public class IrisComplex implements DataProvider {
                 ProceduralStream.of((x, z) -> focusRegion,
                         Interpolated.of(a -> 0D, a -> focusRegion))
                 : regionStyleStream
-                .selectRarity(data.getRegionLoader().loadAll(engine.getDimension().getRegions()))
+                .selectRarity(compatRegionPool(engine))
                 .cache2D("regionStream", engine, cacheSize);
         regionStream = focusRegion != null ? proceduralRegionStream : proceduralRegionStream
                 .convertAware2D((region, x, z) -> {
@@ -1083,11 +1085,30 @@ public class IrisComplex implements DataProvider {
     private KList<IrisBiome> loadInferredBiomes(KList<String> keys, InferredType type) {
         KList<IrisBiome> inferred = new KList<>();
         for (IrisBiome biome : data.getBiomeLoader().loadAll(keys)) {
+            // Excluded biomes cannot generate on this Minecraft version, so they leave the selection pool.
+            if (biome.isCompatExcluded()) {
+                continue;
+            }
             inferred.add(biome.withInferredType(type));
         }
         return inferred;
     }
 
+    /** The dimension's region pool with regions the version-content gate excluded removed. */
+    private KList<IrisRegion> compatRegionPool(Engine engine) {
+        KList<IrisRegion> pool = new KList<>();
+        for (IrisRegion region : data.getRegionLoader().loadAll(engine.getDimension().getRegions())) {
+            if (!region.isCompatExcluded()) {
+                pool.add(region);
+            }
+        }
+        return pool;
+    }
+
+    private static <T extends IrisRegistrant> T compatUsable(T registrant) {
+        return registrant == null || registrant.isCompatExcluded() ? null : registrant;
+    }
+
     private void registerGenerators(IrisBiome biome) {
         generatorBiomes.add(biome);
         biome.getGenerators().forEach(c -> registerGenerator(c.getCachedGenerator(this)));

core/src/main/java/art/arcane/iris/engine/MarkerSpawnScanner.java

@@ -83,6 +83,9 @@ final class MarkerSpawnScanner {
                     IrisLogging.error("Cannot load spawner: " + i + " for marker on " + manager.getName());
                     continue;
                 }
+                if (m.isCompatExcluded()) {
+                    continue;
+                }
                 m.setReferenceMarker(mark);
 
                 // This is so fucking incorrect its a joke
@@ -182,6 +185,9 @@ final class MarkerSpawnScanner {
                     IrisLogging.error("Cannot load spawner: " + i + " for marker on " + manager.getName());
                     continue;
                 }
+                if (spawner.isCompatExcluded()) {
+                    continue;
+                }
                 spawner.setReferenceMarker(mark);
                 data.spawners.add(spawner);
             }

core/src/main/java/art/arcane/iris/engine/WorldEntitySpawner.java

@@ -299,7 +299,8 @@ final class WorldEntitySpawner {
         }
 
         //@builder
-        Predicate<IrisSpawner> filter = i -> i.canSpawn(manager.getEngine(), c.getX(), c.getZ());
+        // 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 = EngineBukkitOps.getSurfaceBiome(manager.getEngine(), c);

core/src/main/java/art/arcane/iris/engine/framework/Engine.java

@@ -502,7 +502,7 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         IrisRegion region = getRegion(x, z);
 
         for (IrisObjectPlacement i : region.getObjects()) {
-            if (i.getPlace().contains(object)) {
+            if (i.compatPlace(getData()).contains(object)) {
                 return new PlacedObject(i, getData().getObjectLoader().load(object), id, x, z);
             }
         }
@@ -510,7 +510,7 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
         IrisBiome biome = getSurfaceBiome(x, z);
 
         for (IrisObjectPlacement i : biome.getObjects()) {
-            if (i.getPlace().contains(object)) {
+            if (i.compatPlace(getData()).contains(object)) {
                 return new PlacedObject(i, getData().getObjectLoader().load(object), id, x, z);
             }
         }
@@ -526,19 +526,20 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
             return false;
         }
 
+        IrisData data = getData();
         Set<String> biomeKeys = getDimension().getReachableBiomes(this).stream()
-                .filter((i) -> containsObjectPlacement(i.getObjects(), normalizedObjectKey))
+                .filter((i) -> containsObjectPlacement(i.getObjects(), normalizedObjectKey, data))
                 .map(IrisRegistrant::getLoadKey)
                 .collect(Collectors.toSet());
         Set<String> regionKeys = getDimension().getAllRegions(this).stream()
                 .filter((i) -> i.getAllBiomeIds().stream().anyMatch(biomeKeys::contains)
-                        || containsObjectPlacement(i.getObjects(), normalizedObjectKey))
+                        || containsObjectPlacement(i.getObjects(), normalizedObjectKey, data))
                 .map(IrisRegistrant::getLoadKey)
                 .collect(Collectors.toSet());
         return !regionKeys.isEmpty();
     }
 
-    private static boolean containsObjectPlacement(KList<IrisObjectPlacement> placements, String normalizedObjectKey) {
+    private static boolean containsObjectPlacement(KList<IrisObjectPlacement> placements, String normalizedObjectKey, IrisData data) {
         if (placements == null || placements.isEmpty() || normalizedObjectKey.isBlank()) {
             return false;
         }
@@ -548,7 +549,7 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
                 continue;
             }
 
-            for (String placedObject : placement.getPlace()) {
+            for (String placedObject : placement.compatPlace(data)) {
                 String normalizedPlacedObject = normalizeObjectPlacementKey(placedObject);
                 if (!normalizedPlacedObject.isBlank() && normalizedPlacedObject.equals(normalizedObjectKey)) {
                     return true;

core/src/main/java/art/arcane/iris/engine/framework/HintedLocator.java

@@ -20,6 +20,7 @@ package art.arcane.iris.engine.framework;
 
 import art.arcane.iris.core.IrisSettings;
 import art.arcane.iris.engine.IrisComplex;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisObjectPlacement;
 import art.arcane.iris.engine.object.IrisRegion;
@@ -410,7 +411,7 @@ public final class HintedLocator<T> implements Locator<T> {
                 continue;
             }
 
-            if (placesObject(region.getSurfaceObjects(), objectKey)) {
+            if (placesObject(region.getSurfaceObjects(), objectKey, engine.getData())) {
                 directRegions.add(region.getLoadKey());
             }
 
@@ -511,9 +512,9 @@ public final class HintedLocator<T> implements Locator<T> {
         };
     }
 
-    private static boolean placesObject(KList<IrisObjectPlacement> placements, String objectKey) {
+    private static boolean placesObject(KList<IrisObjectPlacement> placements, String objectKey, IrisData data) {
         for (IrisObjectPlacement placement : placements) {
-            if (placement != null && placement.getPlace().contains(objectKey)) {
+            if (placement != null && placement.compatPlace(data).contains(objectKey)) {
                 return true;
             }
         }
@@ -637,7 +638,7 @@ public final class HintedLocator<T> implements Locator<T> {
                     continue;
                 }
 
-                if (placesObject(biome.getSurfaceObjects(), objectKey)) {
+                if (placesObject(biome.getSurfaceObjects(), objectKey, engine.getData())) {
                     return true;
                 }
 

core/src/main/java/art/arcane/iris/engine/framework/IrisStructureLocator.java

@@ -507,6 +507,10 @@ public final class IrisStructureLocator {
             throw new IllegalStateException("Iris structure placement references missing structure '"
                     + selectedKey + "'");
         }
+        if (structure.isCompatExcluded()) {
+            // Gated out on this Minecraft version; the pack report already says why.
+            return null;
+        }
         StructureGraphCompilation compilation = StructureGraphCatalog.compile(engine.getData(), structure);
         if (!compilation.isAssemblyViable()) {
             throw new IllegalStateException("Iris structure '" + selectedKey

core/src/main/java/art/arcane/iris/engine/framework/StructureAssembler.java

@@ -1179,6 +1179,11 @@ public final class StructureAssembler {
         if (pool.getPieces() == null) {
             throw assemblyFailure("pool has no piece list");
         }
+        // A pool the version-content gate excluded has nothing left to place: treat it as empty so the branch
+        // terminates or falls back, instead of failing the whole assembly.
+        if (pool.isCompatExcluded()) {
+            return List.of();
+        }
         List<IrisJigsawPieceEntry> eligible = new ArrayList<>();
         List<IrisJigsawPieceEntry> required = new ArrayList<>();
         for (IrisJigsawPieceEntry entry : pool.getPieces()) {
@@ -1199,6 +1204,9 @@ public final class StructureAssembler {
             if (piece == null) {
                 throw assemblyFailure("pool references missing piece '" + pieceKey + "'");
             }
+            if (piece.isCompatExcluded()) {
+                continue;
+            }
             int placements = placementCounts.getOrDefault(pieceKey, 0);
             if (!pieceEnabled(piece)
                     || terminalOnly && !piece.resolvedRules().isTerminal()

core/src/main/java/art/arcane/iris/engine/hydrology/policy/RiverPolicyResolver.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.engine.hydrology.policy;
 
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.object.IrisBiome;
 import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisRegion;
@@ -17,7 +19,8 @@ public final class RiverPolicyResolver {
         return resolve(
                 dimension == null ? null : dimension.getRiverPolicy(),
                 region == null ? null : region.getRiverPolicy(),
-                biome == null ? null : biome.getRiverPolicy()
+                biome == null ? null : biome.getRiverPolicy(),
+                loaderOf(biome, region, dimension)
         );
     }
 
@@ -25,14 +28,37 @@ public final class RiverPolicyResolver {
             IrisRiverPolicy dimensionPolicy,
             IrisRiverPolicy regionPolicy,
             IrisRiverPolicy biomePolicy
+    ) {
+        return resolve(dimensionPolicy, regionPolicy, biomePolicy, null);
+    }
+
+    /**
+     * @param data pack the policies came from, used to drop river biome references the version-content gate excluded.
+     *             Null skips that filtering.
+     */
+    public static EffectiveRiverPolicy resolve(
+            IrisRiverPolicy dimensionPolicy,
+            IrisRiverPolicy regionPolicy,
+            IrisRiverPolicy biomePolicy,
+            IrisData data
     ) {
         State state = new State();
-        state.apply(dimensionPolicy);
-        state.apply(regionPolicy);
-        state.apply(biomePolicy);
+        state.apply(dimensionPolicy, data);
+        state.apply(regionPolicy, data);
+        state.apply(biomePolicy, data);
         return state.build();
     }
 
+    private static IrisData loaderOf(IrisRegistrant... registrants) {
+        for (IrisRegistrant registrant : registrants) {
+            if (registrant != null && registrant.getLoader() != null) {
+                return registrant.getLoader();
+            }
+        }
+
+        return null;
+    }
+
     private static final class State {
         private IrisRiverPlacementMode placement = IrisRiverPlacementMode.NATURAL;
         private IrisRiverRoutingMode routing = IrisRiverRoutingMode.ALLOW;
@@ -50,7 +76,7 @@ public final class RiverPolicyResolver {
         private double routingMultiplier = 1D;
         private double bankMultiplier = 1D;
 
-        private void apply(IrisRiverPolicy policy) {
+        private void apply(IrisRiverPolicy policy, IrisData data) {
             if (policy == null) {
                 return;
             }
@@ -67,19 +93,19 @@ public final class RiverPolicyResolver {
                 profiles = List.copyOf(policy.getProfiles());
             }
             if (policy.getSurfaceBiomes() != null) {
-                surfaceBiomes = List.copyOf(policy.getSurfaceBiomes());
+                surfaceBiomes = List.copyOf(policy.compatBiomes(policy.getSurfaceBiomes(), data, "surfaceBiomes"));
             }
             if (policy.getMouthBiomes() != null) {
-                mouthBiomes = List.copyOf(policy.getMouthBiomes());
+                mouthBiomes = List.copyOf(policy.compatBiomes(policy.getMouthBiomes(), data, "mouthBiomes"));
             }
             if (policy.getShoreBiomes() != null) {
-                shoreBiomes = List.copyOf(policy.getShoreBiomes());
+                shoreBiomes = List.copyOf(policy.compatBiomes(policy.getShoreBiomes(), data, "shoreBiomes"));
             }
             if (policy.getBankBiomes() != null) {
-                bankBiomes = List.copyOf(policy.getBankBiomes());
+                bankBiomes = List.copyOf(policy.compatBiomes(policy.getBankBiomes(), data, "bankBiomes"));
             }
             if (policy.getFloodedCaveBiomes() != null) {
-                floodedCaveBiomes = List.copyOf(policy.getFloodedCaveBiomes());
+                floodedCaveBiomes = List.copyOf(policy.compatBiomes(policy.getFloodedCaveBiomes(), data, "floodedCaveBiomes"));
             }
             if (policy.getSurfacePools() != null) {
                 surfacePools = List.copyOf(policy.getSurfacePools());

core/src/main/java/art/arcane/iris/engine/mantle/components/MantleFloatingObjectComponent.java

@@ -171,6 +171,10 @@ public class MantleFloatingObjectComponent extends IrisMantleComponent {
         if (placement == null || columns == null || columns.isEmpty()) {
             return;
         }
+
+        if (placement.isCompatExcluded(data)) {
+            return;
+        }
         int density = placement.getDensity(rng, minX, minZ, data);
         double perAttempt = placement.getChance();
         for (int i = 0; i < density; i++) {
@@ -217,6 +221,10 @@ public class MantleFloatingObjectComponent extends IrisMantleComponent {
         if (placement == null || columns.isEmpty()) {
             return;
         }
+
+        if (placement.isCompatExcluded(data)) {
+            return;
+        }
         int density = placement.getDensity(rng, minX, minZ, data);
         double perAttempt = placement.getChance();
 
@@ -302,6 +310,10 @@ public class MantleFloatingObjectComponent extends IrisMantleComponent {
         if (placement == null || columns.isEmpty()) {
             return;
         }
+
+        if (placement.isCompatExcluded(data)) {
+            return;
+        }
         int density = placement.getDensity(rng, minX, minZ, data);
         double perAttempt = placement.getChance();
 

core/src/main/java/art/arcane/iris/engine/mantle/components/MantleObjectComponent.java

@@ -417,7 +417,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
 
         for (IrisObjectPlacement i : surfaceBiome.getSurfaceObjects()) {
             biomeSurfaceChecked++;
-            boolean chance = rng.chance(i.getChance() + rng.d(-0.005, 0.005));
+            boolean chance = rng.chance(i.getChance() + rng.d(-0.005, 0.005)) && !i.isCompatExcluded(getData());
             if (traceRegen) {
                 IrisLogging.debug("Regen object placer chance: chunk=" + x + "," + z
                         + " scope=biome-surface"
@@ -450,7 +450,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
                 continue;
             }
             biomeCaveChecked++;
-            boolean chance = rng.chance(i.getChance());
+            boolean chance = rng.chance(i.getChance()) && !i.isCompatExcluded(getData());
             if (traceRegen) {
                 IrisLogging.debug("Regen object placer chance: chunk=" + x + "," + z
                         + " scope=biome-cave"
@@ -480,7 +480,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
 
         for (IrisObjectPlacement i : region.getSurfaceObjects()) {
             regionSurfaceChecked++;
-            boolean chance = rng.chance(i.getChance() + rng.d(-0.005, 0.005));
+            boolean chance = rng.chance(i.getChance() + rng.d(-0.005, 0.005)) && !i.isCompatExcluded(getData());
             if (traceRegen) {
                 IrisLogging.debug("Regen object placer chance: chunk=" + x + "," + z
                         + " scope=region-surface"
@@ -513,7 +513,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
                 continue;
             }
             regionCaveChecked++;
-            boolean chance = rng.chance(i.getChance());
+            boolean chance = rng.chance(i.getChance()) && !i.isCompatExcluded(getData());
             if (traceRegen) {
                 IrisLogging.debug("Regen object placer chance: chunk=" + x + "," + z
                         + " scope=region-cave"
@@ -1182,7 +1182,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
         boolean forcePlace = dimension.isUpperObjectsForcePlace();
         if (upperBiome != null) {
             for (IrisObjectPlacement i : upperBiome.getSurfaceObjects()) {
-                if (!rng.chance(i.getChance() + rng.d(-0.005, 0.005))) {
+                if (!rng.chance(i.getChance() + rng.d(-0.005, 0.005)) || i.isCompatExcluded(getData())) {
                     continue;
                 }
                 try {
@@ -1197,7 +1197,7 @@ public class MantleObjectComponent extends IrisMantleComponent {
 
         if (upperRegion != null) {
             for (IrisObjectPlacement i : upperRegion.getSurfaceObjects()) {
-                if (!rng.chance(i.getChance() + rng.d(-0.005, 0.005))) {
+                if (!rng.chance(i.getChance() + rng.d(-0.005, 0.005)) || i.isCompatExcluded(getData())) {
                     continue;
                 }
                 try {
@@ -1635,13 +1635,13 @@ public class MantleObjectComponent extends IrisMantleComponent {
         IrisRegion region = getEngineMantle().getEngine().getRegion((x << 4) + 8, (z << 4) + 8);
         Set<String> v = new KSet<>();
         for (IrisObjectPlacement i : biome.getSurfaceObjects()) {
-            if (rng.chance(i.getChance() + rng.d(-0.005, 0.005))) {
+            if (rng.chance(i.getChance() + rng.d(-0.005, 0.005)) && !i.isCompatExcluded(getData())) {
                 v.addAll(guessPlacedKeys(rng, x, z, i));
             }
         }
 
         for (IrisObjectPlacement i : region.getSurfaceObjects()) {
-            if (rng.chance(i.getChance() + rng.d(-0.005, 0.005))) {
+            if (rng.chance(i.getChance() + rng.d(-0.005, 0.005)) && !i.isCompatExcluded(getData())) {
                 v.addAll(guessPlacedKeys(rng, x, z, i));
             }
         }

core/src/main/java/art/arcane/iris/engine/object/CompatPools.java

@@ -0,0 +1,219 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.IrisRegistrant;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.volmlib.util.collection.KList;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Registrant reference pools filtered by the version-content gate. A reference to a registrant the gate EXCLUDED is
+ * dropped from every pool that could pick it and reported once against the containing registrant; a container whose
+ * required pool ends up empty cascades to EXCLUDED through {@link #cascade}. Cascade and drop findings reuse the
+ * registry and key of the cause so the console groups everything under the one missing content key.
+ * <p>
+ * Surviving entries keep their declaration order, so only the removed entries change what a pool picks.
+ */
+final class CompatPools {
+    private CompatPools() {
+    }
+
+    static boolean excluded(IrisRegistrant registrant) {
+        return registrant != null && registrant.isCompatExcluded();
+    }
+
+    /**
+     * Loads a registrant reference pool, skipping entries that fail to load and entries the gate excluded. Each
+     * excluded reference produces one DROPPED finding in {@code sink} (when given) and in the pack report.
+     *
+     * @param field the JSON field the keys came from, used for the finding detail ({@code landBiomes[2] key})
+     */
+    static <T extends IrisRegistrant> KList<T> load(ResourceLoader<T> loader,
+                                                    List<String> keys,
+                                                    IrisData data,
+                                                    String subjectType,
+                                                    String subjectKey,
+                                                    String field,
+                                                    List<CompatFinding> sink) {
+        KList<T> kept = new KList<>();
+
+        if (loader == null || keys == null) {
+            return kept;
+        }
+
+        for (int index = 0; index < keys.size(); index++) {
+            String key = keys.get(index);
+            T loaded = loader.load(key);
+
+            if (loaded == null) {
+                continue;
+            }
+
+            if (loaded.isCompatExcluded()) {
+                drop(data, loaded, subjectType, subjectKey, field + "[" + index + "] " + key, sink);
+                continue;
+            }
+
+            kept.add(loaded);
+        }
+
+        return kept;
+    }
+
+    /** The surviving keys of a reference pool, in declaration order. */
+    static <T extends IrisRegistrant> KList<String> surviving(ResourceLoader<T> loader,
+                                                              List<String> keys,
+                                                              IrisData data,
+                                                              String subjectType,
+                                                              String subjectKey,
+                                                              String field) {
+        KList<String> kept = new KList<>();
+
+        if (keys == null) {
+            return kept;
+        }
+
+        if (loader == null) {
+            kept.addAll(keys);
+            return kept;
+        }
+
+        for (int index = 0; index < keys.size(); index++) {
+            String key = keys.get(index);
+            T loaded = loader.load(key);
+
+            if (loaded != null && loaded.isCompatExcluded()) {
+                drop(data, loaded, subjectType, subjectKey, field + "[" + index + "] " + key, null);
+                continue;
+            }
+
+            kept.add(key);
+        }
+
+        return kept;
+    }
+
+    /**
+     * Records one DROPPED finding for a reference to an excluded registrant, reusing the registry and key of the
+     * child's exclusion cause. Returns null when the child carries no cause to attribute the drop to.
+     */
+    static CompatFinding drop(IrisData data,
+                              IrisRegistrant child,
+                              String subjectType,
+                              String subjectKey,
+                              String detail,
+                              List<CompatFinding> sink) {
+        CompatFinding cause = causeOf(child == null ? null : child.getCompat());
+
+        if (cause == null) {
+            return null;
+        }
+
+        CompatFinding finding = new CompatFinding(cause.registry(), cause.key(), CompatAction.DROPPED,
+                subjectType, subjectKey, detail);
+        record(data, finding);
+
+        if (sink != null) {
+            sink.add(finding);
+        }
+
+        return finding;
+    }
+
+    /**
+     * The EXCLUDED status for a container whose required pool lost every entry. Returns {@code base} untouched when no
+     * cause was collected, so a container that never declared the pool is not excluded for being empty.
+     */
+    static CompatStatus cascade(IrisData data,
+                                CompatStatus base,
+                                List<CompatFinding> causes,
+                                String subjectType,
+                                String subjectKey,
+                                String detail) {
+        if (causes == null || causes.isEmpty()) {
+            return base;
+        }
+
+        CompatFinding cause = causes.getFirst();
+        CompatFinding finding = new CompatFinding(cause.registry(), cause.key(), CompatAction.EXCLUDED,
+                subjectType, subjectKey, detail);
+        record(data, finding);
+
+        List<CompatFinding> reasons = new ArrayList<>(base == null ? List.of() : base.reasons());
+        reasons.add(finding);
+        return CompatStatus.excludedBy(reasons);
+    }
+
+    /** The findings on a status that dropped an entry, used as the cause set of a cascade. */
+    static List<CompatFinding> droppedReasons(CompatStatus status) {
+        List<CompatFinding> dropped = new ArrayList<>();
+
+        if (status == null) {
+            return dropped;
+        }
+
+        for (CompatFinding reason : status.reasons()) {
+            if (reason.action() == CompatAction.DROPPED) {
+                dropped.add(reason);
+            }
+        }
+
+        return dropped;
+    }
+
+    static void record(IrisData data, CompatFinding finding) {
+        if (data == null || finding == null) {
+            return;
+        }
+
+        try {
+            data.getCompatReport().record(finding);
+        } catch (Throwable ignored) {
+            // A report is best effort: never let reporting break a pool build.
+        }
+    }
+
+    /** The finding that explains an exclusion: the EXCLUDED reason if there is one, otherwise the first reason. */
+    private static CompatFinding causeOf(CompatStatus status) {
+        if (status == null) {
+            return null;
+        }
+
+        CompatFinding fallback = null;
+
+        for (CompatFinding reason : status.reasons()) {
+            if (reason.action() == CompatAction.EXCLUDED) {
+                return reason;
+            }
+
+            if (fallback == null) {
+                fallback = reason;
+            }
+        }
+
+        return fallback;
+    }
+}

core/src/main/java/art/arcane/iris/engine/object/IrisBiome.java

@@ -559,13 +559,9 @@ public class IrisBiome extends IrisRegistrant implements IRare {
     public KList<IrisBiome> getRealChildren(DataProvider g) {
         return realChildren.aquire(() ->
         {
-            KList<IrisBiome> realChildren = new KList<>();
-
-            for (String i : getChildren()) {
-                realChildren.add(g.getData().getBiomeLoader().load(i));
-            }
-
-            return realChildren;
+            IrisData data = g == null ? null : g.getData();
+            return CompatPools.load(data == null ? null : data.getBiomeLoader(), getChildren(), data,
+                    "biome", getLoadKey(), "children", null);
         });
     }
 

core/src/main/java/art/arcane/iris/engine/object/IrisBiomeCustom.java

@@ -18,6 +18,8 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.KeyStatus;
 import art.arcane.iris.core.nms.datapack.IDataFixer;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.DependsOn;
@@ -132,6 +134,16 @@ public class IrisBiomeCustom {
     }
 
     public String generateJson(IDataFixer fixer) {
+        return generateJson(fixer, null);
+    }
+
+    /**
+     * @param gate version-content gate used to keep spawns whose entity type is missing on this server out of the
+     *             generated datapack (the game rejects a datapack that names an unknown entity). Null consults the
+     *             live platform registries; an unreadable registry keeps every spawn.
+     */
+    public String generateJson(IDataFixer fixer, ContentGate gate) {
+        ContentGate contentGate = gate == null ? ContentGate.forData(null) : gate;
         JSONObject effects = new JSONObject();
         effects.put("sky_color", parseColor(getSkyColor()));
         effects.put("fog_color", parseColor(getFogColor()));
@@ -196,6 +208,10 @@ public class IrisBiomeCustom {
                     IrisLogging.warn("Skipping custom biome spawn with null entity type in biome " + getId());
                     continue;
                 }
+                if (contentGate.entity(key) == KeyStatus.MISSING) {
+                    IrisLogging.debug("Dropping custom biome spawn " + key + " in biome " + getId() + ": entity missing on this server");
+                    continue;
+                }
                 IrisBiomeCustomSpawnType group = i.getGroup() == null ? IrisBiomeCustomSpawnType.MISC : i.getGroup();
                 JSONArray g = groups.computeIfAbsent(group, (k) -> new JSONArray());
                 JSONObject o = new JSONObject();

core/src/main/java/art/arcane/iris/engine/object/IrisDimensionCarvingEntry.java

@@ -63,7 +63,14 @@ public class IrisDimensionCarvingEntry {
                 return null;
             }
 
-            return data.getBiomeLoader().load(biomeKey.trim());
+            IrisBiome loaded = data.getBiomeLoader().load(biomeKey.trim());
+
+            if (loaded != null && loaded.isCompatExcluded()) {
+                CompatPools.drop(data, loaded, "carving entry", getId(), "biome " + biomeKey.trim(), null);
+                return null;
+            }
+
+            return loaded;
         });
     }
 

core/src/main/java/art/arcane/iris/engine/object/IrisEntitySpawn.java

@@ -18,6 +18,7 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.platform.bukkit.BukkitWorldBinding;
 import art.arcane.iris.platform.bukkit.BukkitPlatform;
 import art.arcane.iris.engine.data.cache.AtomicCache;
@@ -147,7 +148,17 @@ public class IrisEntitySpawn implements IRare {
     }
 
     public IrisEntity getRealEntity(Engine g) {
-        return ent.aquire(() -> g.getData().getEntityLoader().load(getEntity()));
+        return ent.aquire(() -> resolveEntity(g.getData()));
+    }
+
+    /** The referenced entity, or null when it does not load or the version-content gate excluded it. */
+    IrisEntity resolveEntity(IrisData data) {
+        if (data == null || data.getEntityLoader() == null) {
+            return null;
+        }
+
+        IrisEntity entity = data.getEntityLoader().load(getEntity());
+        return entity == null || entity.isCompatExcluded() ? null : entity;
     }
 
     public Entity spawn(Engine g, Location at) {

core/src/main/java/art/arcane/iris/engine/object/IrisFloatingChildBiomes.java

@@ -358,6 +358,11 @@ public class IrisFloatingChildBiomes implements IRare {
                 return parent;
             }
 
+            if (loaded.isCompatExcluded()) {
+                CompatPools.drop(data, loaded, "floating child biome", parent.getLoadKey(), "biome " + biome, null);
+                return parent;
+            }
+
             return loaded;
         });
     }

core/src/main/java/art/arcane/iris/engine/object/IrisJigsawPiece.java

@@ -18,6 +18,12 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
@@ -81,6 +87,42 @@ public class IrisJigsawPiece extends IrisRegistrant {
         return rules == null ? new IrisJigsawPieceRules() : rules;
     }
 
+    /**
+     * A piece is one object stamped verbatim - nothing rewrites its blocks - so a palette key the server does not
+     * have (after the pack's declared fallbacks) takes the whole piece out of every pool that lists it.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+
+        if (base.excluded() || gate == null || !gate.ready()) {
+            return base;
+        }
+
+        IrisData data = getLoader();
+
+        if (data == null || object == null || object.isBlank()) {
+            return base;
+        }
+
+        for (String paletteKey : IrisObjectIO.readPaletteKeysCached(data.getObjectLoader().findFile(object.trim()))) {
+            String key = ContentGate.baseKey(ContentGate.normalizeState(paletteKey));
+
+            if (key == null || gate.resolveBlock(key) != null) {
+                continue;
+            }
+
+            CompatFinding finding = new CompatFinding(CompatRegistry.BLOCK, key, CompatAction.EXCLUDED,
+                    "jigsaw piece", getLoadKey(), "object " + object.trim());
+            gate.report().record(finding);
+            KList<CompatFinding> reasons = new KList<>(base.reasons());
+            reasons.add(finding);
+            return CompatStatus.excludedBy(reasons);
+        }
+
+        return base;
+    }
+
     @Override
     public String getFolderName() {
         return "jigsaw-pieces";

core/src/main/java/art/arcane/iris/engine/object/IrisJigsawPool.java

@@ -18,6 +18,12 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
@@ -51,6 +57,75 @@ public class IrisJigsawPool extends IrisRegistrant {
         return structureRequiresCaps || mandatoryFallback;
     }
 
+    /**
+     * Excluded pieces drop out of the pool; a pool with nothing left to place is excluded in turn so every connector
+     * that targets it terminates instead of failing assembly.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+
+        if (base.excluded() || gate == null || !gate.ready()) {
+            return base;
+        }
+
+        IrisData data = getLoader();
+
+        if (data == null || pieces == null || pieces.isEmpty()) {
+            return base;
+        }
+
+        KList<CompatFinding> reasons = new KList<>(base.reasons());
+        CompatFinding cause = null;
+        int usable = 0;
+
+        for (int i = 0; i < pieces.size(); i++) {
+            IrisJigsawPieceEntry entry = pieces.get(i);
+
+            if (entry == null) {
+                continue;
+            }
+
+            String key = entry.getPiece() == null ? "" : entry.getPiece().trim();
+
+            // An empty membership terminates a branch on purpose, and a piece key that does not resolve at all is a
+            // pack error the loader already reports - neither is a version-content problem.
+            if (entry.isEmpty() || key.isEmpty()) {
+                usable++;
+                continue;
+            }
+
+            IrisJigsawPiece piece = data.load(IrisJigsawPiece.class, key, false);
+
+            if (piece == null || !piece.isCompatExcluded()) {
+                usable++;
+                continue;
+            }
+
+            CompatFinding reason = piece.getCompat().reasons().isEmpty() ? null : piece.getCompat().reasons().getFirst();
+            CompatFinding dropped = new CompatFinding(
+                    reason == null ? CompatRegistry.BLOCK : reason.registry(),
+                    reason == null ? key : reason.key(),
+                    CompatAction.DROPPED, "jigsaw pool", getLoadKey(), "pieces[" + i + "] " + key);
+            gate.report().record(dropped);
+            reasons.add(dropped);
+
+            if (cause == null) {
+                cause = dropped;
+            }
+        }
+
+        if (usable > 0 || cause == null) {
+            return new CompatStatus(false, reasons);
+        }
+
+        CompatFinding excluded = new CompatFinding(cause.registry(), cause.key(), CompatAction.EXCLUDED,
+                "jigsaw pool", getLoadKey(), "no pieces remain");
+        gate.report().record(excluded);
+        reasons.add(excluded);
+        return CompatStatus.excludedBy(reasons);
+    }
+
     @Override
     public String getFolderName() {
         return "jigsaw-pools";

core/src/main/java/art/arcane/iris/engine/object/IrisLootReference.java

@@ -18,6 +18,7 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.data.cache.AtomicCache;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
@@ -57,9 +58,25 @@ public class IrisLootReference {
         return tt.aquire(() ->
         {
             KList<IrisLootTable> t = new KList<>();
+            IrisData data = g == null ? null : g.getData();
+
+            if (data == null || data.getLootLoader() == null) {
+                return t;
+            }
 
             for (String i : tables) {
-                t.add(g.getData().getLootLoader().load(i));
+                IrisLootTable table = data.getLootLoader().load(i);
+
+                if (table == null) {
+                    continue;
+                }
+
+                if (table.isCompatExcluded()) {
+                    CompatPools.drop(data, table, "loot reference", table.getLoadKey(), "tables " + i, null);
+                    continue;
+                }
+
+                t.add(table);
             }
 
             return t;

core/src/main/java/art/arcane/iris/engine/object/IrisLootTable.java

@@ -18,6 +18,8 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.framework.LootResolver;
 import art.arcane.iris.engine.object.annotations.ArrayType;
@@ -103,6 +105,25 @@ public class IrisLootTable extends IrisRegistrant {
         return lootf;
     }
 
+    /**
+     * Cascade: the gate walker drops loot entries whose item is missing on this server. A table with nothing left to
+     * roll leaves every reference pool that could pick it.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        return cascadeEmptyLoot(super.evaluateCompat(gate));
+    }
+
+    /** EXCLUDED when the walker dropped every loot entry; the walker's verdict is passed in as {@code base}. */
+    CompatStatus cascadeEmptyLoot(CompatStatus base) {
+        if (base.excluded() || !loot.isEmpty()) {
+            return base;
+        }
+
+        return CompatPools.cascade(getLoader(), base, CompatPools.droppedReasons(base),
+                "loot", getLoadKey(), "no loot entries remain");
+    }
+
     @Override
     public String getFolderName() {
         return "loot";

core/src/main/java/art/arcane/iris/engine/object/IrisMod.java

@@ -18,6 +18,9 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
@@ -104,6 +107,49 @@ public class IrisMod extends IrisRegistrant {
     @Desc("Replace noise styles with other styles")
     private KList<IrisModNoiseStyleReplacer> styleReplacers = new KList<>();
 
+    /**
+     * A pack modification only references other registrants, so it never cascades: excluded biome and region
+     * references are dropped from its lists and reported, and the modification keeps applying whatever is left.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+        IrisData data = getLoader();
+
+        if (data == null) {
+            return base;
+        }
+
+        removeBiomes = biomes(data, removeBiomes, "removeBiomes");
+        removeRegions = regions(data, removeRegions, "removeRegions");
+        injectRegions = regions(data, injectRegions, "injectRegions");
+
+        for (int index = 0; index < biomeInjectors.size(); index++) {
+            IrisModBiomeInjector injector = biomeInjectors.get(index);
+            injector.setInject(biomes(data, injector.getInject(), "biomeInjectors[" + index + "].inject"));
+        }
+
+        for (int index = 0; index < biomeReplacers.size(); index++) {
+            IrisModBiomeReplacer replacer = biomeReplacers.get(index);
+            replacer.setFind(biomes(data, replacer.getFind(), "biomeReplacers[" + index + "].find"));
+        }
+
+        for (int index = 0; index < regionReplacers.size(); index++) {
+            IrisModRegionReplacer replacer = regionReplacers.get(index);
+            replacer.setFind(regions(data, replacer.getFind(), "regionReplacers[" + index + "].find"));
+        }
+
+        return base;
+    }
+
+    private KList<String> biomes(IrisData data, KList<String> keys, String field) {
+        return CompatPools.surviving(data.getBiomeLoader(), keys, data, "mod", getLoadKey(), field);
+    }
+
+    private KList<String> regions(IrisData data, KList<String> keys, String field) {
+        return CompatPools.surviving(data.getRegionLoader(), keys, data, "mod", getLoadKey(), field);
+    }
+
     @Override
     public String getFolderName() {
         return "mods";

core/src/main/java/art/arcane/iris/engine/object/IrisObjectMarker.java

@@ -63,7 +63,7 @@ public class IrisObjectMarker {
             KList<PlatformBlockState> b = new KList<>();
 
             for (IrisBlockData i : mark) {
-                PlatformBlockState bx = i.getBlockData(rdata);
+                PlatformBlockState bx = i.getBlockDataOrPlaceholder(rdata);
 
                 if (bx != null) {
                     b.add(bx);

core/src/main/java/art/arcane/iris/engine/object/IrisObjectPlacement.java

@@ -20,6 +20,12 @@ package art.arcane.iris.engine.object;
 
 import art.arcane.iris.platform.bukkit.BukkitBlockResolution;
 
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.data.cache.AtomicCache;
 import art.arcane.iris.engine.data.cache.LazyBoundedCache;
@@ -181,6 +187,274 @@ public class IrisObjectPlacement {
     @SerializedName(value = "forcePlace", alternate = {"force"})
     private boolean forcePlace = false;
     private transient AtomicCache<TableCache> cache = new AtomicCache<>();
+    private transient AtomicCache<CompatStatus> compatCache = new AtomicCache<>();
+    @Getter(AccessLevel.NONE)
+    @Setter(AccessLevel.NONE)
+    private transient volatile KList<String> compatPlaceCache;
+
+    /**
+     * Version-content gate verdict for this placement on the running server. The placement's own generated blocks
+     * (every {@code edit[].replace} palette option) exclude the whole placement when one of them is unavailable; each
+     * object in {@code place} is then dropped when its .iob palette carries a key that neither resolves nor is
+     * rewritten by a matching {@code edit} rule with chance 1. Match sides ({@code edit[].find}, {@code markers[].mark})
+     * never count. Cached per placement.
+     */
+    public CompatStatus evaluateCompat(IrisData data) {
+        return compatCache.aquire(() -> computeCompat(data));
+    }
+
+    /** True when nothing in this placement can generate on the running server. */
+    public boolean isCompatExcluded(IrisData data) {
+        return evaluateCompat(data).excluded();
+    }
+
+    /** The {@code place} pool with objects that cannot generate on the running server removed. */
+    public KList<String> compatPlace(IrisData data) {
+        evaluateCompat(data);
+        KList<String> survivors = compatPlaceCache;
+        return survivors == null ? place : survivors;
+    }
+
+    private CompatStatus computeCompat(IrisData data) {
+        if (data == null) {
+            compatPlaceCache = place;
+            return CompatStatus.OK;
+        }
+
+        ContentGate gate = data.getContentGate();
+        PackCompatReport report = data.getCompatReport();
+
+        if (gate == null || !gate.ready()) {
+            // Registry not bound or not ready: UNKNOWN is never MISSING, so nothing is gated.
+            report.markIncomplete("registry not ready while evaluating object placements");
+            compatPlaceCache = place;
+            return CompatStatus.OK;
+        }
+
+        String subject = compatSubject();
+        KList<CompatFinding> reasons = new KList<>();
+        CompatFinding blocked = evaluatePlacementBlocks(data, gate, subject, reasons);
+        record(report, reasons);
+
+        if (blocked != null) {
+            compatPlaceCache = new KList<>();
+            return CompatStatus.excludedBy(reasons);
+        }
+
+        KList<String> survivors = new KList<>(place.size());
+        CompatFinding firstDrop = null;
+
+        for (int i = 0; i < place.size(); i++) {
+            String objectKey = place.get(i);
+            KList<CompatFinding> objectReasons = new KList<>();
+            KList<String> missing = unplaceableKeys(objectKey, data, gate, subject + " place[" + i + "]", objectReasons);
+            record(report, objectReasons);
+            reasons.addAll(objectReasons);
+
+            if (missing.isEmpty()) {
+                survivors.add(objectKey);
+                continue;
+            }
+
+            // One finding per missing key, so the operator sees every fallback the object needs at once.
+            for (String key : missing) {
+                CompatFinding dropped = new CompatFinding(CompatRegistry.BLOCK, key, CompatAction.DROPPED,
+                        "object", objectKey, subject + " place[" + i + "]");
+                report.record(dropped);
+                reasons.add(dropped);
+
+                if (firstDrop == null) {
+                    firstDrop = dropped;
+                }
+            }
+        }
+
+        if (firstDrop != null && survivors.isEmpty()) {
+            CompatFinding excluded = new CompatFinding(firstDrop.registry(), firstDrop.key(), CompatAction.EXCLUDED,
+                    "placement", subject, "no objects remain");
+            report.record(excluded);
+            reasons.add(excluded);
+            compatPlaceCache = new KList<>();
+            return CompatStatus.excludedBy(reasons);
+        }
+
+        compatPlaceCache = survivors.size() == place.size() ? place : survivors;
+        return new CompatStatus(false, reasons);
+    }
+
+    /** The subject a finding names: the single object, or the whole place list when there is more than one. */
+    private String compatSubject() {
+        if (place.isEmpty()) {
+            return "";
+        }
+        return place.size() == 1 ? place.getFirst() : place.toString(", ");
+    }
+
+    /**
+     * The placement's own composition blocks: the {@code edit[].replace} palettes. {@code edit[].find} and
+     * {@code markers[].mark} are match sides and never count - matching a block the server does not have is harmless.
+     *
+     * @return the finding that excludes this placement, or null when every declared block resolves
+     */
+    private CompatFinding evaluatePlacementBlocks(IrisData data, ContentGate gate, String subject, KList<CompatFinding> reasons) {
+        CompatFinding blocked = null;
+
+        for (int i = 0; i < edit.size(); i++) {
+            IrisObjectReplace rule = edit.get(i);
+
+            if (rule == null || rule.getReplace() == null) {
+                continue;
+            }
+
+            KList<IrisBlockData> palette = rule.getReplace().getPalette();
+
+            for (int j = 0; j < palette.size(); j++) {
+                CompatFinding finding = evaluateBlockEntry(palette.get(j), data, gate, subject,
+                        "edit[" + i + "].replace.palette[" + j + "]", reasons);
+
+                if (finding != null && blocked == null) {
+                    blocked = finding;
+                }
+            }
+        }
+
+        return blocked;
+    }
+
+    /** Records what happened to one declared block entry; returns the finding when it excludes the placement. */
+    private CompatFinding evaluateBlockEntry(IrisBlockData block, IrisData data, ContentGate gate, String subject,
+                                             String detail, KList<CompatFinding> reasons) {
+        if (block == null) {
+            return null;
+        }
+
+        BlockCompat resolution = resolveBlockEntry(block, data, gate, new KList<>());
+
+        if (resolution.missing()) {
+            CompatFinding finding = new CompatFinding(CompatRegistry.BLOCK, resolution.key(), CompatAction.EXCLUDED,
+                    "placement", subject, detail);
+            reasons.add(finding);
+            return finding;
+        }
+
+        if (resolution.substituted()) {
+            reasons.add(new CompatFinding(CompatRegistry.BLOCK, resolution.key(), CompatAction.SUBSTITUTED,
+                    "placement", subject, detail + " (using " + resolution.replacement() + ")"));
+        }
+
+        return null;
+    }
+
+    /**
+     * Every distinct palette key of {@code objectKey} that cannot reach the world, in palette order; empty when the
+     * object still places. A key survives when the gate resolves it (dimension fallbacks count) or when an
+     * {@code edit} rule with chance 1 matches it - that rule's replace palette was already verified by
+     * {@link #evaluatePlacementBlocks}. A rule below chance 1 does not cover the key: the original block can still be
+     * rolled.
+     */
+    private KList<String> unplaceableKeys(String objectKey, IrisData data, ContentGate gate, String detail, KList<CompatFinding> reasons) {
+        KList<String> missing = new KList<>();
+
+        // Palette header only - no IrisObject is built and no state is resolved. A legacy (V1) object reads as an
+        // empty palette and is therefore never dropped; V1 has no palette block to scan without a full load.
+        for (String paletteKey : IrisObjectIO.readPaletteKeysCached(data.getObjectLoader().findFile(objectKey))) {
+            String base = ContentGate.baseKey(ContentGate.normalizeState(paletteKey));
+
+            if (base == null) {
+                continue;
+            }
+
+            // Existence is a registry question about the block, never about its properties: a property that was
+            // renamed on a block the server still has is out of scope for the gate.
+            ContentGate.BlockResolution resolution = gate.resolveBlock(base);
+
+            if (resolution != null) {
+                if (resolution.substituted()) {
+                    reasons.add(new CompatFinding(CompatRegistry.BLOCK, base, CompatAction.SUBSTITUTED,
+                            "object", objectKey, detail + " (using " + resolution.resolvedKey() + ")"));
+                }
+
+                continue;
+            }
+
+            if (coveredByEdit(paletteKey)) {
+                continue;
+            }
+
+            missing.addIfMissing(base);
+        }
+
+        return missing;
+    }
+
+    /** True when an {@code edit} rule certainly rewrites this palette state before it is written. */
+    private boolean coveredByEdit(String paletteKey) {
+        for (IrisObjectReplace rule : edit) {
+            if (rule == null || rule.getChance() < 1F) {
+                continue;
+            }
+
+            if (rule.matchesState(paletteKey)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * Mirrors {@code IrisBlockData.getBlockData} against the gate: pack custom block first, then the registry with
+     * dimension fallbacks, then the entry's own {@code backup}.
+     */
+    private static BlockCompat resolveBlockEntry(IrisBlockData block, IrisData data, ContentGate gate, KList<IrisBlockData> seen) {
+        for (IrisBlockData visited : seen) {
+            if (visited == block) {
+                return new BlockCompat(String.valueOf(block.getBlock()), null, true, false);
+            }
+        }
+
+        seen.add(block);
+        String declared = block.getBlock();
+        IrisBlockData custom = data.getBlockLoader() == null ? null : data.getBlockLoader().load(declared, false);
+
+        if (custom != null && custom != block) {
+            return resolveBlockEntry(custom, data, gate, seen);
+        }
+
+        String base = ContentGate.baseKey(ContentGate.normalizeState(declared));
+
+        if (base == null) {
+            return new BlockCompat(String.valueOf(declared), null, true, false);
+        }
+
+        ContentGate.BlockResolution resolution = gate.resolveBlock(base);
+
+        if (resolution != null) {
+            return new BlockCompat(base, resolution.resolvedKey(), false, resolution.substituted());
+        }
+
+        IrisBlockData backup = block.getBackup();
+
+        if (backup != null) {
+            BlockCompat viaBackup = resolveBlockEntry(backup, data, gate, seen);
+
+            if (!viaBackup.missing()) {
+                return new BlockCompat(base, viaBackup.replacement(), false, true);
+            }
+        }
+
+        return new BlockCompat(base, null, true, false);
+    }
+
+    private static void record(PackCompatReport report, KList<CompatFinding> findings) {
+        for (CompatFinding finding : findings) {
+            report.record(finding);
+        }
+    }
+
+    /** One block entry's verdict: the key asked for, what will actually generate, and how it got there. */
+    private record BlockCompat(String key, String replacement, boolean missing, boolean substituted) {
+    }
 
     public IrisObjectPlacement toPlacement(String... place) {
         IrisObjectPlacement p = new IrisObjectPlacement();
@@ -244,11 +518,14 @@ public class IrisObjectPlacement {
     }
 
     public IrisObject getObject(DataProvider g, RNG random) {
-        if (place.isEmpty()) {
+        IrisData data = g.getData();
+        KList<String> pool = compatPlace(data);
+
+        if (pool.isEmpty()) {
             return null;
         }
 
-        return g.getData().getObjectLoader().load(place.get(random.nextInt(place.size())));
+        return data.getObjectLoader().load(pool.get(random.nextInt(pool.size())));
     }
 
     public boolean matches(IrisTreeSize size, TreeType type) {
@@ -298,6 +575,9 @@ public class IrisObjectPlacement {
                 IrisLogging.warn("Couldn't find loot table " + loot.getName());
                 continue;
             }
+            if (table.isCompatExcluded()) {
+                continue;
+            }
 
             if (loot.getFilter().isEmpty()) //Table applies to all containers
             {

core/src/main/java/art/arcane/iris/engine/object/IrisObjectPlacementRunner.java

@@ -424,7 +424,7 @@ final class IrisObjectPlacementRunner {
                     IrisMarker marker = self.getLoader().getMarkerLoader().load(j.getMarker());
                     int markerSalt = markerIndex++;
 
-                    if (marker == null) {
+                    if (marker == null || marker.isCompatExcluded()) {
                         continue;
                     }
 

core/src/main/java/art/arcane/iris/engine/object/IrisObjectReplace.java

@@ -18,6 +18,7 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.ContentGate;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.data.cache.AtomicCache;
 import art.arcane.iris.engine.object.annotations.ArrayType;
@@ -35,6 +36,9 @@ import lombok.NoArgsConstructor;
 import art.arcane.iris.spi.PlatformBlockState;
 import lombok.experimental.Accessors;
 
+import java.util.LinkedHashMap;
+import java.util.Map;
+
 @Snippet("object-block-replacer")
 @Accessors(chain = true)
 @NoArgsConstructor
@@ -65,7 +69,7 @@ public class IrisObjectReplace {
             KList<PlatformBlockState> b = new KList<>();
 
             for (IrisBlockData i : find) {
-                PlatformBlockState bx = i.getBlockData(rdata);
+                PlatformBlockState bx = i.getBlockDataOrPlaceholder(rdata);
 
                 if (bx != null) {
                     b.add(bx);
@@ -79,4 +83,89 @@ public class IrisObjectReplace {
     public PlatformBlockState getReplace(RNG seed, double x, double y, double z, IrisData rdata) {
         return getReplace().get(seed, x, y, z, rdata);
     }
+
+    /**
+     * Key-level mirror of the runtime find match, for the version-content gate: it has to decide whether a palette
+     * key the server does not have is rewritten by this rule, and a missing key has no resolved state to compare.
+     * Non-exact compares the base block; exact requires every property named by the find entry to match, the same
+     * partial-match contract {@code PlatformBlockState.matches} uses at runtime.
+     */
+    public boolean matchesState(String stateKey) {
+        String state = ContentGate.normalizeState(stateKey);
+
+        if (state == null) {
+            return false;
+        }
+
+        String stateBase = ContentGate.baseKey(state);
+
+        for (IrisBlockData f : find) {
+            if (f == null) {
+                continue;
+            }
+
+            String findState = ContentGate.normalizeState(declaredState(f));
+
+            if (findState == null || !ContentGate.baseKey(findState).equals(stateBase)) {
+                continue;
+            }
+
+            if (!exact || propertiesMatch(findState, state)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /** The find entry as one state string; {@code block} already carrying properties wins over the data map. */
+    private static String declaredState(IrisBlockData block) {
+        String declared = block.getBlock();
+
+        if (declared == null) {
+            return null;
+        }
+
+        return declared.indexOf('[') < 0 ? declared + block.computeProperties() : declared;
+    }
+
+    /** Every property the find side names must be present with the same value on the candidate state. */
+    private static boolean propertiesMatch(String findState, String candidateState) {
+        Map<String, String> required = properties(findState);
+
+        if (required.isEmpty()) {
+            return true;
+        }
+
+        Map<String, String> candidate = properties(candidateState);
+
+        for (Map.Entry<String, String> entry : required.entrySet()) {
+            if (!entry.getValue().equals(candidate.get(entry.getKey()))) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static Map<String, String> properties(String state) {
+        int open = state.indexOf('[');
+        int close = state.lastIndexOf(']');
+
+        if (open < 0 || close < open) {
+            return Map.of();
+        }
+
+        Map<String, String> properties = new LinkedHashMap<>();
+
+        for (String part : state.substring(open + 1, close).split(",")) {
+            int split = part.indexOf('=');
+
+            if (split > 0) {
+                properties.put(part.substring(0, split).trim(), part.substring(split + 1).trim());
+            }
+        }
+
+        return properties;
+    }
 }

core/src/main/java/art/arcane/iris/engine/object/IrisRegion.java

@@ -19,6 +19,9 @@
 package art.arcane.iris.engine.object;
 
 import art.arcane.iris.engine.framework.render.RenderType;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
 import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.spi.IrisLogging;
 import art.arcane.iris.spi.PlatformBlockState;
@@ -44,6 +47,8 @@ import lombok.NoArgsConstructor;
 import lombok.experimental.Accessors;
 
 import java.awt.Color;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Random;
 
 
@@ -321,6 +326,11 @@ public class IrisRegion extends IrisRegistrant implements IRare {
                     continue;
                 }
 
+                if (biome.isCompatExcluded()) {
+                    CompatPools.drop(g.getData(), biome, "region", getLoadKey(), "biomes " + i, null);
+                    continue;
+                }
+
                 names.add(biome.getCarvingBiome());
                 b.put(biome.getLoadKey(), biome);
                 names.addAll(biome.getChildren());
@@ -345,55 +355,26 @@ public class IrisRegion extends IrisRegistrant implements IRare {
     }
 
     public KList<IrisBiome> getRealCaveBiomes(DataProvider g) {
-        return realCaveBiomes.aquire(() ->
-        {
-            KList<IrisBiome> realCaveBiomes = new KList<>();
-
-            for (String i : getCaveBiomes()) {
-                realCaveBiomes.add(g.getData().getBiomeLoader().load(i));
-            }
-
-            return realCaveBiomes;
-        });
+        return realCaveBiomes.aquire(() -> resolvePool(g, getCaveBiomes(), "caveBiomes", null));
     }
 
     public KList<IrisBiome> getRealShoreBiomes(DataProvider g) {
-        return realShoreBiomes.aquire(() ->
-        {
-            KList<IrisBiome> realShoreBiomes = new KList<>();
-
-            for (String i : getShoreBiomes()) {
-                realShoreBiomes.add(g.getData().getBiomeLoader().load(i));
-            }
-
-            return realShoreBiomes;
-        });
+        return realShoreBiomes.aquire(() -> resolvePool(g, getShoreBiomes(), "shoreBiomes", null));
     }
 
     public KList<IrisBiome> getRealSeaBiomes(DataProvider g) {
-        return realSeaBiomes.aquire(() ->
-        {
-            KList<IrisBiome> realSeaBiomes = new KList<>();
-
-            for (String i : getSeaBiomes()) {
-                realSeaBiomes.add(g.getData().getBiomeLoader().load(i));
-            }
-
-            return realSeaBiomes;
-        });
+        return realSeaBiomes.aquire(() -> resolvePool(g, getSeaBiomes(), "seaBiomes", null));
     }
 
     public KList<IrisBiome> getRealLandBiomes(DataProvider g) {
-        return realLandBiomes.aquire(() ->
-        {
-            KList<IrisBiome> realLandBiomes = new KList<>();
+        return realLandBiomes.aquire(() -> resolvePool(g, getLandBiomes(), "landBiomes", null));
+    }
 
-            for (String i : getLandBiomes()) {
-                realLandBiomes.add(g.getData().getBiomeLoader().load(i));
-            }
-
-            return realLandBiomes;
-        });
+    /** A biome reference pool with unloadable and gate-excluded biomes removed; drops are reported once. */
+    private KList<IrisBiome> resolvePool(DataProvider g, KList<String> keys, String field, List<CompatFinding> sink) {
+        IrisData data = g == null ? null : g.getData();
+        return CompatPools.load(data == null ? null : data.getBiomeLoader(), keys, data,
+                "region", getLoadKey(), field, sink);
     }
 
     public KList<IrisBiome> getAllAnyBiomes() {
@@ -468,6 +449,42 @@ public class IrisRegion extends IrisRegistrant implements IRare {
 
     }
 
+    /**
+     * Cascade: a region that lost every land biome to the gate cannot generate, so it leaves the dimension's region
+     * pool too. Loading biomes here is safe - biomes never load regions.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+
+        if (base.excluded()) {
+            return base;
+        }
+
+        IrisData data = getLoader();
+
+        if (data == null) {
+            return base;
+        }
+
+        for (int index = 0; index < structures.size(); index++) {
+            structures.get(index).reportExcludedCaveBiomes(data, "region", getLoadKey(),
+                    "structures[" + index + "].caveBiomes");
+        }
+
+        if (getLandBiomes().isEmpty()) {
+            return base;
+        }
+
+        List<CompatFinding> drops = new ArrayList<>();
+
+        if (!resolvePool(() -> data, getLandBiomes(), "landBiomes", drops).isEmpty()) {
+            return base;
+        }
+
+        return CompatPools.cascade(data, base, drops, "region", getLoadKey(), "no land biomes remain");
+    }
+
     @Override
     public String getFolderName() {
         return "regions";

core/src/main/java/art/arcane/iris/engine/object/IrisRiverPolicy.java

@@ -4,6 +4,7 @@ import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
 import art.arcane.iris.engine.object.annotations.MaxNumber;
 import art.arcane.iris.engine.object.annotations.MinNumber;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.object.annotations.RegistryListResource;
 import art.arcane.volmlib.util.collection.KList;
 import art.arcane.volmlib.util.collection.KSet;
@@ -83,6 +84,19 @@ public class IrisRiverPolicy {
     @Desc("Multiplier applied to the eroded bank width outside the shore band.")
     private Double bankMultiplier = null;
 
+    /**
+     * A declared river biome selection with biomes the version-content gate excluded removed. A declared-but-now-empty
+     * selection keeps its "explicitly clears the inherited selection" meaning; null (undeclared) stays null.
+     */
+    public KList<String> compatBiomes(KList<String> declared, IrisData data, String field) {
+        if (declared == null) {
+            return null;
+        }
+
+        return CompatPools.surviving(data == null ? null : data.getBiomeLoader(), declared, data,
+                "river policy", "", field);
+    }
+
     public KSet<String> getAllBiomeIds() {
         KSet<String> biomeIds = new KSet<>();
         addAll(biomeIds, surfaceBiomes);

core/src/main/java/art/arcane/iris/engine/object/IrisSpawner.java

@@ -18,6 +18,10 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.framework.Engine;
 import art.arcane.iris.engine.object.annotations.ArrayType;
@@ -30,6 +34,9 @@ import lombok.EqualsAndHashCode;
 import lombok.NoArgsConstructor;
 import lombok.experimental.Accessors;
 
+import java.util.ArrayList;
+import java.util.List;
+
 @EqualsAndHashCode(callSuper = true)
 @Accessors(chain = true)
 @NoArgsConstructor
@@ -123,6 +130,64 @@ public class IrisSpawner extends IrisRegistrant {
         engine.getEngineData().getChunk(x, z).getCooldown(this).spawn(engine);
     }
 
+    /**
+     * Cascade: a spawn whose entity the gate excluded can never spawn, so it leaves this spawner's lists; a spawner
+     * with nothing left to spawn leaves every pool that references it. Loading entities here is safe - entities never
+     * load spawners.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+
+        if (base.excluded()) {
+            return base;
+        }
+
+        IrisData data = getLoader();
+
+        if (data == null || data.getEntityLoader() == null) {
+            return base;
+        }
+
+        boolean declared = !spawns.isEmpty() || !initialSpawns.isEmpty();
+        List<CompatFinding> drops = new ArrayList<>();
+        spawns = dropExcludedSpawns(data, spawns, "spawns", drops);
+        initialSpawns = dropExcludedSpawns(data, initialSpawns, "initialSpawns", drops);
+
+        if (!declared || !spawns.isEmpty() || !initialSpawns.isEmpty()) {
+            return base;
+        }
+
+        return CompatPools.cascade(data, base, drops, "spawner", getLoadKey(), "no entity spawns remain");
+    }
+
+    private KList<IrisEntitySpawn> dropExcludedSpawns(IrisData data,
+                                                      KList<IrisEntitySpawn> declared,
+                                                      String field,
+                                                      List<CompatFinding> drops) {
+        KList<IrisEntitySpawn> kept = new KList<>();
+
+        for (int index = 0; index < declared.size(); index++) {
+            IrisEntitySpawn spawn = declared.get(index);
+
+            if (spawn == null) {
+                continue;
+            }
+
+            IrisEntity entity = data.getEntityLoader().load(spawn.getEntity());
+
+            if (entity != null && entity.isCompatExcluded()) {
+                CompatPools.drop(data, entity, "spawner", getLoadKey(),
+                        field + "[" + index + "] " + spawn.getEntity(), drops);
+                continue;
+            }
+
+            kept.add(spawn);
+        }
+
+        return kept;
+    }
+
     @Override
     public String getFolderName() {
         return "spawners";

core/src/main/java/art/arcane/iris/engine/object/IrisStructure.java

@@ -18,6 +18,12 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.core.loader.IrisRegistrant;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
@@ -126,6 +132,42 @@ public class IrisStructure extends IrisRegistrant {
         return placement;
     }
 
+    /**
+     * The start pool is the only mandatory entry point into an assembly: when it has nothing left to place, the
+     * structure cannot generate on this server and every placement that lists it skips it.
+     */
+    @Override
+    public CompatStatus evaluateCompat(ContentGate gate) {
+        CompatStatus base = super.evaluateCompat(gate);
+
+        if (base.excluded() || gate == null || !gate.ready()) {
+            return base;
+        }
+
+        IrisData data = getLoader();
+
+        if (data == null || startPool == null || startPool.isBlank()) {
+            return base;
+        }
+
+        IrisJigsawPool pool = data.load(IrisJigsawPool.class, startPool.trim(), false);
+
+        if (pool == null || !pool.isCompatExcluded()) {
+            return base;
+        }
+
+        CompatFinding reason = pool.getCompat().reasons().isEmpty() ? null : pool.getCompat().reasons().getLast();
+        CompatFinding excluded = new CompatFinding(
+                reason == null ? CompatRegistry.BLOCK : reason.registry(),
+                reason == null ? startPool.trim() : reason.key(),
+                CompatAction.EXCLUDED, "structure", getLoadKey(),
+                "start pool " + startPool.trim() + " is unavailable");
+        gate.report().record(excluded);
+        KList<CompatFinding> reasons = new KList<>(base.reasons());
+        reasons.add(excluded);
+        return CompatStatus.excludedBy(reasons);
+    }
+
     @Override
     public String getFolderName() {
         return "structures";

core/src/main/java/art/arcane/iris/engine/object/IrisStructurePlacement.java

@@ -18,6 +18,7 @@
 
 package art.arcane.iris.engine.object;
 
+import art.arcane.iris.core.loader.IrisData;
 import art.arcane.iris.engine.object.annotations.ArrayType;
 import art.arcane.iris.engine.object.annotations.Desc;
 import art.arcane.iris.engine.object.annotations.MaxNumber;
@@ -147,4 +148,24 @@ public class IrisStructurePlacement {
         IrisStructureAnchorMode activeAnchor = resolvedAnchor();
         return activeAnchor == IrisStructureAnchorMode.HEIGHT_BAND || activeAnchor.isCave();
     }
+
+    /**
+     * Reports cave biome references the version-content gate excluded. The list is deliberately not filtered: it is an
+     * allowlist whose emptiness means "any cave biome", and it feeds the placement determinism signature. An excluded
+     * biome never generates, so the entry is inert - this only makes it visible in the pack compat report.
+     */
+    void reportExcludedCaveBiomes(IrisData data, String subjectType, String subjectKey, String field) {
+        if (data == null || data.getBiomeLoader() == null || caveBiomes == null) {
+            return;
+        }
+
+        for (int index = 0; index < caveBiomes.size(); index++) {
+            String key = caveBiomes.get(index);
+            IrisBiome biome = data.getBiomeLoader().load(key);
+
+            if (biome != null && biome.isCompatExcluded()) {
+                CompatPools.drop(data, biome, subjectType, subjectKey, field + "[" + index + "] " + key, null);
+            }
+        }
+    }
 }

core/src/test/java/art/arcane/iris/engine/framework/StructureAssemblerCompatTest.java

@@ -0,0 +1,145 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.framework;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.engine.framework.structure.StructureAssemblyResult;
+import art.arcane.iris.engine.framework.structure.StructureAssemblyStatus;
+import art.arcane.iris.engine.object.IrisDirection;
+import art.arcane.iris.engine.object.IrisJigsawConnector;
+import art.arcane.iris.engine.object.IrisJigsawPiece;
+import art.arcane.iris.engine.object.IrisJigsawPieceEntry;
+import art.arcane.iris.engine.object.IrisJigsawPool;
+import art.arcane.iris.engine.object.IrisObject;
+import art.arcane.iris.engine.object.IrisPosition;
+import art.arcane.iris.engine.object.IrisStructure;
+import art.arcane.iris.engine.object.JigsawJoint;
+import art.arcane.iris.spi.IrisPlatform;
+import art.arcane.iris.spi.IrisPlatforms;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.spi.PlatformRegistries;
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class StructureAssemblerCompatTest {
+    @BeforeClass
+    public static void bindPlatform() {
+        IrisPlatforms.unbind();
+        PlatformBlockState block = mock(PlatformBlockState.class);
+        PlatformRegistries registries = mock(PlatformRegistries.class);
+        when(registries.block(anyString())).thenReturn(block);
+        IrisPlatform platform = mock(IrisPlatform.class);
+        when(platform.registries()).thenReturn(registries);
+        IrisPlatforms.bind(platform);
+    }
+
+    @AfterClass
+    public static void unbindPlatform() {
+        IrisPlatforms.unbind();
+    }
+
+    @Test
+    public void anExcludedPieceIsNeverDrawnFromItsPool() {
+        IrisData data = mock(IrisData.class);
+        IrisJigsawPiece good = piece("good-object");
+        IrisJigsawPiece bad = excluded(piece("bad-object"));
+        when(data.load(IrisJigsawPool.class, "start", false)).thenReturn(pool("good-piece", "bad-piece"));
+        when(data.load(IrisJigsawPool.class, "terminal", false)).thenReturn(new IrisJigsawPool());
+        when(data.load(IrisJigsawPiece.class, "good-piece", false)).thenReturn(good);
+        when(data.load(IrisJigsawPiece.class, "bad-piece", false)).thenReturn(bad);
+        when(data.load(IrisObject.class, "good-object", false)).thenReturn(new IrisObject(1, 1, 1));
+        when(data.load(IrisObject.class, "bad-object", false)).thenReturn(new IrisObject(1, 1, 1));
+
+        for (int seed = 0; seed < 32; seed++) {
+            StructureAssemblyResult result = StructureAssembler
+                    .forData(data, structure("start"), new IrisPosition(0, 0, 0))
+                    .assemble(new RNG(seed));
+
+            assertTrue(result.detail(), result.hasOutput());
+            assertTrue(result.pieces().toString(), result.pieces().stream()
+                    .allMatch(placed -> "good-object".equals(placed.getPiece().getObject())));
+        }
+    }
+
+    @Test
+    public void aPoolWithOnlyExcludedPiecesStopsTheAssemblyWithoutThrowing() {
+        IrisData data = mock(IrisData.class);
+        IrisJigsawPool startPool = pool("bad-piece");
+        startPool.setCompat(CompatStatus.excludedBy(List.of(finding(CompatAction.EXCLUDED))));
+        when(data.load(IrisJigsawPool.class, "start", false)).thenReturn(startPool);
+        when(data.load(IrisJigsawPool.class, "terminal", false)).thenReturn(new IrisJigsawPool());
+        when(data.load(IrisJigsawPiece.class, "bad-piece", false)).thenReturn(excluded(piece("bad-object")));
+        when(data.load(IrisObject.class, "bad-object", false)).thenReturn(new IrisObject(1, 1, 1));
+
+        StructureAssemblyResult result = StructureAssembler
+                .forData(data, structure("start"), new IrisPosition(0, 0, 0))
+                .assemble(new RNG(3));
+
+        assertFalse(result.hasOutput());
+        assertEquals(StructureAssemblyStatus.FAILED_RULES, result.status());
+    }
+
+    private static IrisJigsawPiece excluded(IrisJigsawPiece piece) {
+        piece.setCompat(CompatStatus.excludedBy(List.of(finding(CompatAction.EXCLUDED))));
+        return piece;
+    }
+
+    private static CompatFinding finding(CompatAction action) {
+        return new CompatFinding(CompatRegistry.BLOCK, "minecraft:sulfur", action, "jigsaw piece", "bad-piece", "object bad-object");
+    }
+
+    private static IrisJigsawPiece piece(String object) {
+        IrisJigsawPiece piece = new IrisJigsawPiece().setObject(object).setRotatable(false);
+        piece.getConnectors().add(new IrisJigsawConnector()
+                .setPosition(new IrisPosition())
+                .setDirection(IrisDirection.EAST_POSITIVE_X)
+                .setPool("terminal")
+                .setName("start")
+                .setTargetName("door")
+                .setJoint(JigsawJoint.ROLLABLE));
+        return piece;
+    }
+
+    private static IrisJigsawPool pool(String... pieces) {
+        IrisJigsawPool pool = new IrisJigsawPool();
+        for (String piece : pieces) {
+            pool.getPieces().add(new IrisJigsawPieceEntry().setPiece(piece).setWeight(1));
+        }
+        return pool;
+    }
+
+    private static IrisStructure structure(String startPool) {
+        return new IrisStructure().setStartPool(startPool).setMaxDepth(1).setMaxSizeChunks(8);
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/CompatFixtures.java

@@ -0,0 +1,84 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.IrisRegistrant;
+
+import java.util.List;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Shared fixtures for the version-content pool cascade tests. The gate walker (lane L1) is not exercised here: the
+ * tests stamp {@code compat} on a registrant directly, which is exactly what the walker stores.
+ */
+final class CompatFixtures {
+    static final String MISSING_BLOCK = "minecraft:sulfur";
+    static final String MISSING_ENTITY = "minecraft:camel";
+
+    private CompatFixtures() {
+    }
+
+    static IrisData data(PackCompatReport report) {
+        IrisData data = mock(IrisData.class);
+        when(data.getCompatReport()).thenReturn(report);
+        return data;
+    }
+
+    /** Marks a registrant excluded the way the gate walker would for a missing composed block. */
+    static <T extends IrisRegistrant> T excludeBlock(T registrant) {
+        return exclude(registrant, CompatRegistry.BLOCK, MISSING_BLOCK);
+    }
+
+    /** Marks a registrant excluded the way the gate walker would for a missing entity type. */
+    static <T extends IrisRegistrant> T excludeEntity(T registrant) {
+        return exclude(registrant, CompatRegistry.ENTITY, MISSING_ENTITY);
+    }
+
+    static <T extends IrisRegistrant> T exclude(T registrant, CompatRegistry registry, String key) {
+        registrant.setCompat(CompatStatus.excludedBy(List.of(new CompatFinding(registry, key,
+                CompatAction.EXCLUDED, registrant.getTypeName().toLowerCase(java.util.Locale.ROOT),
+                registrant.getLoadKey(), "composition"))));
+        return registrant;
+    }
+
+    static IrisBiome biome(String key) {
+        IrisBiome biome = new IrisBiome();
+        biome.setLoadKey(key);
+        return biome;
+    }
+
+    static IrisRegion region(String key) {
+        IrisRegion region = new IrisRegion();
+        region.setLoadKey(key);
+        return region;
+    }
+
+    static IrisEntity entity(String key) {
+        IrisEntity entity = new IrisEntity();
+        entity.setLoadKey(key);
+        return entity;
+    }
+
+    static IrisLootTable lootTable(String key) {
+        IrisLootTable table = new IrisLootTable();
+        table.setLoadKey(key);
+        return table;
+    }
+
+    static CompatFinding find(PackCompatReport report, CompatAction action, String subjectType, String subjectKey) {
+        for (CompatFinding finding : report.findings()) {
+            if (finding.action() == action
+                    && finding.subjectType().equals(subjectType)
+                    && finding.subjectKey().equals(subjectKey)) {
+                return finding;
+            }
+        }
+        return null;
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/CompatTestPlatform.java

@@ -0,0 +1,254 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.engine.framework.MeteredCache;
+import art.arcane.iris.engine.framework.PreservationRegistry;
+import art.arcane.iris.spi.IrisPlatform;
+import art.arcane.iris.spi.IrisPlatforms;
+import art.arcane.iris.spi.IrisServices;
+import art.arcane.iris.spi.PlatformBiome;
+import art.arcane.iris.spi.PlatformBlockProperty;
+import art.arcane.iris.spi.PlatformBlockState;
+import art.arcane.iris.spi.PlatformEntityType;
+import art.arcane.iris.spi.PlatformItem;
+import art.arcane.iris.spi.PlatformRegistries;
+import org.mockito.Answers;
+
+import java.io.DataOutputStream;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.concurrent.ExecutorService;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Binds a platform whose block registry holds exactly the keys a compat test declares, so the gate can be exercised
+ * without a server. Also writes the .iob and JSON fixtures those tests need.
+ */
+final class CompatTestPlatform {
+    private final IrisPlatform previous;
+
+    private CompatTestPlatform(IrisPlatform previous) {
+        this.previous = previous;
+    }
+
+    static CompatTestPlatform bind(File dataFolder, List<String> blockKeys) {
+        IrisPlatform previous = IrisPlatforms.isBound() ? IrisPlatforms.get() : null;
+        IrisPlatforms.unbind();
+        IrisPlatform platform = mock(IrisPlatform.class, Answers.CALLS_REAL_METHODS);
+        when(platform.dataFolder()).thenReturn(dataFolder);
+        when(platform.registries()).thenReturn(new FixedRegistries(blockKeys));
+        IrisPlatforms.bind(platform);
+        IrisServices.register(PreservationRegistry.class, new NoOpPreservationRegistry());
+        return new CompatTestPlatform(previous);
+    }
+
+    void unbind() {
+        IrisServices.clear();
+        IrisPlatforms.unbind();
+        if (previous != null) {
+            IrisPlatforms.bind(previous);
+        }
+    }
+
+    /** A minimal V2 .iob under {@code <pack>/objects/<key>.iob} carrying only the given palette. */
+    static File writeObject(File pack, String key, String... palette) throws IOException {
+        File file = new File(pack, "objects/" + key + ".iob");
+        file.getParentFile().mkdirs();
+        try (DataOutputStream out = new DataOutputStream(new FileOutputStream(file))) {
+            out.writeInt(1);
+            out.writeInt(1);
+            out.writeInt(1);
+            out.writeUTF("Iris V2 IOB;");
+            out.writeShort(palette.length);
+            for (String entry : palette) {
+                out.writeUTF(entry);
+            }
+            out.writeInt(0);
+            out.writeInt(0);
+        }
+        return file;
+    }
+
+    static File write(File pack, String path, String content) throws IOException {
+        File file = new File(pack, path);
+        file.getParentFile().mkdirs();
+        Files.write(file.toPath(), content.getBytes(StandardCharsets.UTF_8));
+        return file;
+    }
+
+    private record FixedRegistries(List<String> blocks) implements PlatformRegistries {
+        private static String base(String key) {
+            if (key == null) {
+                return null;
+            }
+            String trimmed = key.trim().toLowerCase(Locale.ROOT);
+            int props = trimmed.indexOf('[');
+            String head = props < 0 ? trimmed : trimmed.substring(0, props);
+            return head.indexOf(':') < 0 ? "minecraft:" + head : head;
+        }
+
+        private boolean has(String key) {
+            String normalized = base(key);
+            if (normalized == null) {
+                return false;
+            }
+            for (String block : blocks) {
+                if (base(block).equals(normalized)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
+        @Override
+        public PlatformBlockState block(String key) {
+            PlatformBlockState state = blockOrNull(key, false);
+            return state == null ? air() : state;
+        }
+
+        @Override
+        public PlatformBlockState blockOrNull(String key) {
+            return blockOrNull(key, false);
+        }
+
+        @Override
+        public PlatformBlockState blockOrNull(String key, boolean warn) {
+            return has(key) ? StateCache.of(base(key)) : null;
+        }
+
+        @Override
+        public PlatformBlockState air() {
+            return StateCache.of("minecraft:air");
+        }
+
+        @Override
+        public PlatformBlockState deepSlateOre(PlatformBlockState block, PlatformBlockState ore) {
+            return ore;
+        }
+
+        @Override
+        public PlatformBiome biome(String key) {
+            return null;
+        }
+
+        @Override
+        public PlatformItem item(String key) {
+            return null;
+        }
+
+        @Override
+        public PlatformEntityType entity(String key) {
+            return null;
+        }
+
+        @Override
+        public List<String> blockKeys() {
+            return blocks;
+        }
+
+        @Override
+        public List<String> biomeKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> structureKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> itemKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> entityKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> blockTypeKeys() {
+            return blocks;
+        }
+
+        @Override
+        public List<String> enchantmentKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> potionEffectKeys() {
+            return List.of();
+        }
+
+        @Override
+        public List<String> lootTableKeys() {
+            return List.of();
+        }
+
+        @Override
+        public Map<String, List<PlatformBlockProperty>> blockStateProperties() {
+            return Map.of();
+        }
+    }
+
+    /** One interned mock per key so {@code matches} and identity behave like a real adapter. */
+    private static final class StateCache {
+        private static final Map<String, PlatformBlockState> STATES = new LinkedHashMap<>();
+
+        private StateCache() {
+        }
+
+        static synchronized PlatformBlockState of(String key) {
+            return STATES.computeIfAbsent(key, k -> {
+                PlatformBlockState state = mock(PlatformBlockState.class);
+                when(state.key()).thenReturn(k);
+                return state;
+            });
+        }
+    }
+
+    private static final class NoOpPreservationRegistry implements PreservationRegistry {
+        @Override
+        public void register(Thread thread) {
+        }
+
+        @Override
+        public void register(ExecutorService service) {
+        }
+
+        @Override
+        public void registerCache(MeteredCache cache) {
+        }
+
+        @Override
+        public void dereference() {
+        }
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisBiomeCompatChildrenTest.java

@@ -0,0 +1,95 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.volmlib.util.collection.KList;
+import org.junit.Test;
+
+import static art.arcane.iris.engine.object.CompatFixtures.MISSING_BLOCK;
+import static art.arcane.iris.engine.object.CompatFixtures.biome;
+import static art.arcane.iris.engine.object.CompatFixtures.excludeBlock;
+import static art.arcane.iris.engine.object.CompatFixtures.find;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisBiomeCompatChildrenTest {
+    @SuppressWarnings("unchecked")
+    private static IrisData dataWith(PackCompatReport report, IrisBiome... biomes) {
+        IrisData data = CompatFixtures.data(report);
+        ResourceLoader<IrisBiome> biomeLoader = mock(ResourceLoader.class);
+        when(data.getBiomeLoader()).thenReturn(biomeLoader);
+        for (IrisBiome biome : biomes) {
+            when(biomeLoader.load(biome.getLoadKey())).thenReturn(biome);
+        }
+        return data;
+    }
+
+    @Test
+    public void realChildrenDropsExcludedChildAndRecordsFinding() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome parent = biome("land/plains");
+        IrisBiome hills = biome("land/hills");
+        IrisBiome sulfur = excludeBlock(biome("land/sulfur-flats"));
+        parent.setChildren(new KList<>("land/hills", "land/sulfur-flats"));
+        IrisData data = dataWith(report, parent, hills, sulfur);
+        parent.setLoader(data);
+
+        KList<IrisBiome> children = parent.getRealChildren(() -> data);
+
+        assertEquals(1, children.size());
+        assertEquals("land/hills", children.get(0).getLoadKey());
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "biome", "land/plains");
+        assertNotNull(dropped);
+        assertEquals(MISSING_BLOCK, dropped.key());
+        assertEquals("children[1] land/sulfur-flats", dropped.detail());
+    }
+
+    @Test
+    public void realChildrenSkipsUnloadableChild() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome parent = biome("land/plains");
+        parent.setChildren(new KList<>("land/gone"));
+        IrisData data = dataWith(report, parent);
+        parent.setLoader(data);
+
+        assertTrue(parent.getRealChildren(() -> data).isEmpty());
+    }
+
+    @Test
+    public void floatingChildBiomeFallsBackToParentWhenTargetExcluded() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome parent = biome("land/plains");
+        IrisBiome sulfur = excludeBlock(biome("sky/sulfur-isles"));
+        IrisData data = dataWith(report, parent, sulfur);
+        parent.setLoader(data);
+        IrisFloatingChildBiomes floating = new IrisFloatingChildBiomes().setBiome("sky/sulfur-isles");
+
+        assertSame(parent, floating.getRealBiome(parent, data));
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "floating child biome", "land/plains");
+        assertNotNull(dropped);
+        assertEquals(MISSING_BLOCK, dropped.key());
+    }
+
+    @Test
+    public void carvingEntryResolvesNullWhenBiomeExcluded() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome sulfur = excludeBlock(biome("cave/sulfur-grotto"));
+        IrisData data = dataWith(report, sulfur);
+        IrisDimensionCarvingEntry entry = new IrisDimensionCarvingEntry()
+                .setId("deep-band")
+                .setBiome("cave/sulfur-grotto");
+
+        assertNull(entry.getRealBiome(data));
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "carving entry", "deep-band");
+        assertNotNull(dropped);
+        assertEquals(MISSING_BLOCK, dropped.key());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisBiomeCustomCompatTest.java

@@ -0,0 +1,91 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.nms.datapack.IDataFixer;
+import art.arcane.iris.spi.PlatformRegistries;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.json.JSONArray;
+import art.arcane.volmlib.util.json.JSONObject;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+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.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisBiomeCustomCompatTest {
+    private static final IDataFixer PASSTHROUGH = new IDataFixer() {
+        @Override
+        public JSONObject resolve(IDataFixer.Dimension dimension, art.arcane.iris.engine.object.IrisDimensionTypeOptions options) {
+            return new JSONObject();
+        }
+
+        @Override
+        public void fixDimension(IDataFixer.Dimension dimension, JSONObject json) {
+        }
+    };
+
+    private static ContentGate gateWithEntities(String... present) {
+        List<String> entities = new ArrayList<>();
+        entities.add("minecraft:pig");
+        entities.addAll(List.of(present));
+        PlatformRegistries registries = mock(PlatformRegistries.class);
+        when(registries.blockKeys()).thenReturn(List.of("minecraft:stone"));
+        when(registries.entityKeys()).thenReturn(entities);
+        return new ContentGate(registries, Map.of(), new PackCompatReport());
+    }
+
+    private static IrisBiomeCustom customBiome() {
+        IrisBiomeCustom biome = new IrisBiomeCustom();
+        biome.setId("sulfur_grotto");
+        biome.setSpawnRarity(1);
+        biome.setSpawns(new KList<>(
+                new IrisBiomeCustomSpawn().setType("minecraft:zombie"),
+                new IrisBiomeCustomSpawn().setType("minecraft:camel")));
+        return biome;
+    }
+
+    @Test
+    public void generateJsonOmitsSpawnWhoseEntityTypeIsMissing() {
+        JSONObject json = new JSONObject(customBiome().generateJson(PASSTHROUGH, gateWithEntities("minecraft:zombie")));
+        JSONArray monsters = json.getJSONObject("spawners").getJSONArray("misc");
+
+        assertEquals(1, monsters.length());
+        assertEquals("minecraft:zombie", monsters.getJSONObject(0).getString("type"));
+        assertFalse(json.getJSONObject("spawners").toString().contains("minecraft:camel"));
+    }
+
+    @Test
+    public void generateJsonKeepsEverySpawnWhenAllEntitiesArePresent() {
+        JSONObject json = new JSONObject(customBiome()
+                .generateJson(PASSTHROUGH, gateWithEntities("minecraft:zombie", "minecraft:camel")));
+
+        assertEquals(2, json.getJSONObject("spawners").getJSONArray("misc").length());
+    }
+
+    @Test
+    public void generateJsonKeepsEverySpawnWhenRegistryCannotBeConsulted() {
+        // Registry not ready (modded early boot): UNKNOWN must never behave like MISSING.
+        PlatformRegistries registries = mock(PlatformRegistries.class);
+        when(registries.blockKeys()).thenReturn(List.of());
+        when(registries.entityKeys()).thenReturn(List.of());
+        ContentGate gate = new ContentGate(registries, Map.of(), new PackCompatReport());
+
+        JSONObject json = new JSONObject(customBiome().generateJson(PASSTHROUGH, gate));
+
+        assertEquals(2, json.getJSONObject("spawners").getJSONArray("misc").length());
+    }
+
+    @Test
+    public void generateJsonWritesNoSpawnersWhenEveryEntityIsMissing() {
+        JSONObject json = new JSONObject(customBiome().generateJson(PASSTHROUGH, gateWithEntities()));
+
+        assertEquals(0, json.getJSONObject("spawners").length());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisBiomeDerivativeCompatTest.java

@@ -0,0 +1,34 @@
+package art.arcane.iris.engine.object;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * The gate walker drops a {@code vanillaDerivative} whose biome key is missing (policy table: entry DROPPED, falls back
+ * to derivative). This pins the fallback the drop relies on; the drop itself is simulated by clearing the field, which
+ * is exactly what the walker does.
+ */
+public class IrisBiomeDerivativeCompatTest {
+    @Test
+    public void droppedVanillaDerivativeFallsBackToDerivative() {
+        IrisBiome biome = CompatFixtures.biome("cave/sulfur-grotto");
+        biome.setDerivative("minecraft:dripstone_caves");
+        biome.setVanillaDerivative("minecraft:sulfur_caves");
+
+        assertEquals("minecraft:sulfur_caves", biome.getVanillaDerivativeKey());
+
+        biome.setVanillaDerivative(null);
+
+        assertEquals("minecraft:dripstone_caves", biome.getVanillaDerivativeKey());
+    }
+
+    @Test
+    public void blankVanillaDerivativeFallsBackToDerivative() {
+        IrisBiome biome = CompatFixtures.biome("cave/sulfur-grotto");
+        biome.setDerivative("minecraft:dripstone_caves");
+        biome.setVanillaDerivative("");
+
+        assertEquals("minecraft:dripstone_caves", biome.getVanillaDerivativeKey());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisDimensionCompatRegionTest.java

@@ -0,0 +1,92 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.volmlib.util.collection.KList;
+import org.junit.Test;
+
+import java.util.Map;
+
+import static art.arcane.iris.engine.object.CompatFixtures.MISSING_BLOCK;
+import static art.arcane.iris.engine.object.CompatFixtures.excludeBlock;
+import static art.arcane.iris.engine.object.CompatFixtures.find;
+import static art.arcane.iris.engine.object.CompatFixtures.region;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisDimensionCompatRegionTest {
+    @SuppressWarnings("unchecked")
+    private static IrisData dataWith(PackCompatReport report, IrisRegion... regions) {
+        IrisData data = CompatFixtures.data(report);
+        ResourceLoader<IrisRegion> regionLoader = mock(ResourceLoader.class);
+        when(data.getRegionLoader()).thenReturn(regionLoader);
+        for (IrisRegion region : regions) {
+            when(regionLoader.load(region.getLoadKey())).thenReturn(region);
+        }
+        return data;
+    }
+
+    private static ContentGate gate(PackCompatReport report) {
+        return new ContentGate(null, Map.of(), report);
+    }
+
+    @Test
+    public void allRegionsDropsExcludedRegionAndRecordsFinding() {
+        PackCompatReport report = new PackCompatReport();
+        IrisRegion plains = region("overworld");
+        IrisRegion sulfur = excludeBlock(region("sulfur-lands"));
+        IrisData data = dataWith(report, plains, sulfur);
+        IrisDimension dimension = new IrisDimension().setRegions(new KList<>("overworld", "sulfur-lands"));
+        dimension.setLoadKey("overworld");
+        dimension.setLoader(data);
+
+        KList<IrisRegion> pool = dimension.getAllRegions(() -> data);
+
+        assertEquals(1, pool.size());
+        assertEquals("overworld", pool.get(0).getLoadKey());
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "dimension", "overworld");
+        assertNotNull(dropped);
+        assertEquals(MISSING_BLOCK, dropped.key());
+        assertEquals("regions[1] sulfur-lands", dropped.detail());
+    }
+
+    @Test
+    public void evaluateCompatExcludesDimensionWithNoRegionsLeft() {
+        PackCompatReport report = new PackCompatReport();
+        IrisRegion sulfur = excludeBlock(region("sulfur-lands"));
+        IrisData data = dataWith(report, sulfur);
+        IrisDimension dimension = new IrisDimension().setRegions(new KList<>("sulfur-lands"));
+        dimension.setLoadKey("sulfurworld");
+        dimension.setLoader(data);
+
+        CompatStatus status = dimension.evaluateCompat(gate(report));
+
+        assertTrue(status.excluded());
+        CompatFinding cascade = find(report, CompatAction.EXCLUDED, "dimension", "sulfurworld");
+        assertNotNull(cascade);
+        assertEquals(MISSING_BLOCK, cascade.key());
+        assertEquals("no regions remain", cascade.detail());
+    }
+
+    @Test
+    public void evaluateCompatKeepsDimensionWithSurvivingRegion() {
+        PackCompatReport report = new PackCompatReport();
+        IrisRegion plains = region("overworld");
+        IrisRegion sulfur = excludeBlock(region("sulfur-lands"));
+        IrisData data = dataWith(report, plains, sulfur);
+        IrisDimension dimension = new IrisDimension().setRegions(new KList<>("overworld", "sulfur-lands"));
+        dimension.setLoadKey("overworld");
+        dimension.setLoader(data);
+
+        assertFalse(dimension.evaluateCompat(gate(report)).excluded());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisJigsawCompatTest.java

@@ -0,0 +1,158 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.IrisSettings;
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.loader.IrisData;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.io.File;
+import java.util.List;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+public class IrisJigsawCompatTest {
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    private CompatTestPlatform platform;
+    private IrisSettings previousSettings;
+    private IrisData data;
+
+    @Before
+    public void bindPlatform() {
+        previousSettings = IrisSettings.settings;
+        IrisSettings.settings = new IrisSettings();
+        platform = CompatTestPlatform.bind(temporaryFolder.getRoot(),
+                List.of("minecraft:stone", "minecraft:cobblestone", "minecraft:air"));
+    }
+
+    @After
+    public void restorePlatform() {
+        if (data != null) {
+            data.close();
+            data = null;
+        }
+        if (platform != null) {
+            platform.unbind();
+            platform = null;
+        }
+        IrisSettings.settings = previousSettings;
+    }
+
+    @Test
+    public void aPieceWhoseObjectNeedsAMissingBlockIsExcluded() throws Exception {
+        File pack = pack("piece");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/bad.json", "{\"object\":\"bad\"}");
+        data = IrisData.get(pack);
+
+        IrisJigsawPiece piece = data.load(IrisJigsawPiece.class, "bad", false);
+
+        assertNotNull(piece);
+        assertTrue(piece.isCompatExcluded());
+        assertTrue(data.getCompatReport().findings().stream()
+                .anyMatch(finding -> "minecraft:sulfur".equals(finding.key())
+                        && finding.action() == CompatAction.EXCLUDED));
+    }
+
+    @Test
+    public void aPieceWhoseObjectResolvesIsKept() throws Exception {
+        File pack = pack("piece-ok");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/good.json", "{\"object\":\"good\"}");
+        data = IrisData.get(pack);
+
+        IrisJigsawPiece piece = data.load(IrisJigsawPiece.class, "good", false);
+
+        assertNotNull(piece);
+        assertFalse(piece.isCompatExcluded());
+    }
+
+    @Test
+    public void aPoolKeepsGeneratingWhileOnePieceSurvives() throws Exception {
+        File pack = pack("pool-partial");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/good.json", "{\"object\":\"good\"}");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/bad.json", "{\"object\":\"bad\"}");
+        CompatTestPlatform.write(pack, "jigsaw-pools/pool.json",
+                "{\"pieces\":[{\"piece\":\"good\",\"weight\":1},{\"piece\":\"bad\",\"weight\":1}]}");
+        data = IrisData.get(pack);
+
+        IrisJigsawPool pool = data.load(IrisJigsawPool.class, "pool", false);
+
+        assertNotNull(pool);
+        assertFalse(pool.isCompatExcluded());
+        assertTrue(data.getCompatReport().findings().stream()
+                .anyMatch(finding -> finding.action() == CompatAction.DROPPED
+                        && "jigsaw pool".equals(finding.subjectType())));
+    }
+
+    @Test
+    public void pieceExclusionCascadesToThePoolAndTheStructure() throws Exception {
+        File pack = pack("cascade");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/bad.json", "{\"object\":\"bad\"}");
+        CompatTestPlatform.write(pack, "jigsaw-pools/pool.json",
+                "{\"pieces\":[{\"piece\":\"bad\",\"weight\":1}]}");
+        CompatTestPlatform.write(pack, "structures/keep.json", "{\"startPool\":\"pool\"}");
+        data = IrisData.get(pack);
+
+        IrisJigsawPool pool = data.load(IrisJigsawPool.class, "pool", false);
+        IrisStructure structure = data.load(IrisStructure.class, "keep", false);
+
+        assertNotNull(pool);
+        assertNotNull(structure);
+        assertTrue(pool.isCompatExcluded());
+        assertTrue(structure.isCompatExcluded());
+        assertTrue(data.getCompatReport().findings().stream()
+                .anyMatch(finding -> "structure".equals(finding.subjectType())
+                        && finding.action() == CompatAction.EXCLUDED
+                        && "minecraft:sulfur".equals(finding.key())));
+    }
+
+    @Test
+    public void aStructureOverAHealthyPoolIsKept() throws Exception {
+        File pack = pack("structure-ok");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        CompatTestPlatform.write(pack, "jigsaw-pieces/good.json", "{\"object\":\"good\"}");
+        CompatTestPlatform.write(pack, "jigsaw-pools/pool.json",
+                "{\"pieces\":[{\"piece\":\"good\",\"weight\":1}]}");
+        CompatTestPlatform.write(pack, "structures/keep.json", "{\"startPool\":\"pool\"}");
+        data = IrisData.get(pack);
+
+        IrisStructure structure = data.load(IrisStructure.class, "keep", false);
+
+        assertNotNull(structure);
+        assertFalse(structure.isCompatExcluded());
+        assertTrue(data.getCompatReport().isEmpty());
+    }
+
+    private File pack(String name) throws Exception {
+        return temporaryFolder.newFolder(name);
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisLootTableCompatTest.java

@@ -0,0 +1,101 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.volmlib.util.collection.KList;
+import org.junit.Test;
+
+import java.util.List;
+import java.util.Map;
+
+import static art.arcane.iris.engine.object.CompatFixtures.find;
+import static art.arcane.iris.engine.object.CompatFixtures.lootTable;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisLootTableCompatTest {
+    @SuppressWarnings("unchecked")
+    private static IrisData dataWith(PackCompatReport report, IrisLootTable... tables) {
+        IrisData data = CompatFixtures.data(report);
+        ResourceLoader<IrisLootTable> lootLoader = mock(ResourceLoader.class);
+        when(data.getLootLoader()).thenReturn(lootLoader);
+        for (IrisLootTable table : tables) {
+            when(lootLoader.load(table.getLoadKey())).thenReturn(table);
+        }
+        return data;
+    }
+
+    private static ContentGate gate(PackCompatReport report) {
+        return new ContentGate(null, Map.of(), report);
+    }
+
+    @Test
+    public void evaluateCompatExcludesTableWhoseEntriesWereAllDropped() {
+        PackCompatReport report = new PackCompatReport();
+        IrisLootTable table = lootTable("chest/sulfur-cache");
+        table.setLoader(CompatFixtures.data(report));
+        // The gate walker drops every entry with a missing item and hands the cascade its reasons.
+        CompatStatus walked = new CompatStatus(false, List.of(new CompatFinding(CompatRegistry.ITEM,
+                "minecraft:sulfur_dust", CompatAction.DROPPED, "loot", "chest/sulfur-cache", "loot[0]")));
+
+        CompatStatus status = table.cascadeEmptyLoot(walked);
+
+        assertTrue(status.excluded());
+        CompatFinding cascade = find(report, CompatAction.EXCLUDED, "loot", "chest/sulfur-cache");
+        assertNotNull(cascade);
+        assertEquals("minecraft:sulfur_dust", cascade.key());
+        assertEquals(CompatRegistry.ITEM, cascade.registry());
+        assertEquals("no loot entries remain", cascade.detail());
+    }
+
+    @Test
+    public void evaluateCompatKeepsTableWithSurvivingEntries() {
+        PackCompatReport report = new PackCompatReport();
+        IrisLootTable table = lootTable("chest/mixed");
+        table.setLoader(CompatFixtures.data(report));
+        table.setLoot(new KList<>(new IrisLoot()));
+        CompatStatus walked = new CompatStatus(false, List.of(new CompatFinding(CompatRegistry.ITEM,
+                "minecraft:sulfur_dust", CompatAction.DROPPED, "loot", "chest/mixed", "loot[1]")));
+
+        assertFalse(table.cascadeEmptyLoot(walked).excluded());
+    }
+
+    @Test
+    public void evaluateCompatKeepsTableThatNeverDeclaredLoot() {
+        PackCompatReport report = new PackCompatReport();
+        IrisLootTable table = lootTable("chest/empty");
+        table.setLoader(CompatFixtures.data(report));
+
+        assertFalse(table.evaluateCompat(gate(report)).excluded());
+        assertTrue(report.isEmpty());
+    }
+
+    @Test
+    public void lootReferenceSkipsExcludedAndUnloadableTables() {
+        PackCompatReport report = new PackCompatReport();
+        IrisLootTable kept = lootTable("chest/common");
+        IrisLootTable excluded = CompatFixtures.exclude(lootTable("chest/sulfur-cache"),
+                CompatRegistry.ITEM, "minecraft:sulfur_dust");
+        IrisData data = dataWith(report, kept, excluded);
+        IrisLootReference reference = new IrisLootReference()
+                .setTables(new KList<>("chest/common", "chest/sulfur-cache", "chest/gone"));
+
+        KList<IrisLootTable> tables = reference.getLootTables(() -> data);
+
+        assertEquals(1, tables.size());
+        assertEquals("chest/common", tables.get(0).getLoadKey());
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "loot reference", "chest/sulfur-cache");
+        assertNotNull(dropped);
+        assertEquals("minecraft:sulfur_dust", dropped.key());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisObjectPlacementCompatTest.java

@@ -0,0 +1,282 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2026 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.IrisSettings;
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.util.common.data.DataProvider;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.io.File;
+import java.util.List;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class IrisObjectPlacementCompatTest {
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    private CompatTestPlatform platform;
+    private IrisSettings previousSettings;
+    private IrisData data;
+
+    @Before
+    public void bindPlatform() {
+        previousSettings = IrisSettings.settings;
+        IrisSettings.settings = new IrisSettings();
+        platform = CompatTestPlatform.bind(temporaryFolder.getRoot(),
+                List.of("minecraft:stone", "minecraft:cobblestone", "minecraft:air", "minecraft:oak_log"));
+    }
+
+    @After
+    public void restorePlatform() {
+        if (data != null) {
+            data.close();
+            data = null;
+        }
+        if (platform != null) {
+            platform.unbind();
+            platform = null;
+        }
+        IrisSettings.settings = previousSettings;
+    }
+
+    @Test
+    public void missingPaletteKeyDropsTheObjectAndRecordsAFinding() throws Exception {
+        File pack = pack("drop");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:stone", "minecraft:sulfur");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("good", "bad");
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertEquals(new KList<String>().qadd("good"), placement.compatPlace(data));
+
+        List<CompatFinding> findings = data.getCompatReport().findings();
+        assertEquals(findings.toString(), 1, findings.size());
+        assertEquals("minecraft:sulfur", findings.getFirst().key());
+        assertEquals(CompatAction.DROPPED, findings.getFirst().action());
+        assertEquals("object", findings.getFirst().subjectType());
+        assertEquals("bad", findings.getFirst().subjectKey());
+    }
+
+    @Test
+    public void editRuleWithFullChanceKeepsTheObject() throws Exception {
+        File pack = pack("edit-keeps");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("bad");
+        placement.getEdit().add(replace(1F, false, "minecraft:sulfur", "minecraft:stone"));
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertEquals(new KList<String>().qadd("bad"), placement.compatPlace(data));
+        assertTrue(data.getCompatReport().findings().toString(), data.getCompatReport().isEmpty());
+    }
+
+    @Test
+    public void editRuleBelowFullChanceDoesNotCoverTheMissingKey() throws Exception {
+        File pack = pack("edit-chance");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("bad");
+        placement.getEdit().add(replace(0.5F, false, "minecraft:sulfur", "minecraft:stone"));
+
+        assertTrue(placement.isCompatExcluded(data));
+        assertTrue(placement.compatPlace(data).isEmpty());
+    }
+
+    @Test
+    public void exactEditRuleOnlyCoversTheMatchingState() throws Exception {
+        File pack = pack("edit-exact");
+        CompatTestPlatform.writeObject(pack, "axisY", "minecraft:sulfur[axis=y]");
+        CompatTestPlatform.writeObject(pack, "axisX", "minecraft:sulfur[axis=x]");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("axisY", "axisX");
+        IrisObjectReplace rule = replace(1F, true, "minecraft:sulfur", "minecraft:stone");
+        rule.getFind().getFirst().getData().put("axis", "y");
+        placement.getEdit().add(rule);
+
+        assertEquals(new KList<String>().qadd("axisY"), placement.compatPlace(data));
+    }
+
+    @Test
+    public void nonExactEditRuleCoversEveryStateOfTheSameBlock() throws Exception {
+        File pack = pack("edit-non-exact");
+        CompatTestPlatform.writeObject(pack, "axisX", "minecraft:sulfur[axis=x]");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("axisX");
+        placement.getEdit().add(replace(1F, false, "minecraft:sulfur", "minecraft:stone"));
+
+        assertFalse(placement.isCompatExcluded(data));
+    }
+
+    @Test
+    public void missingReplacePaletteOptionExcludesTheWholePlacement() throws Exception {
+        File pack = pack("replace-missing");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("good");
+        placement.getEdit().add(replace(1F, false, "minecraft:stone", "minecraft:sulfur"));
+
+        assertTrue(placement.isCompatExcluded(data));
+        assertTrue(placement.compatPlace(data).isEmpty());
+
+        List<CompatFinding> findings = data.getCompatReport().findings();
+        assertEquals(findings.toString(), 1, findings.size());
+        assertEquals(CompatAction.EXCLUDED, findings.getFirst().action());
+        assertEquals("placement", findings.getFirst().subjectType());
+        assertTrue(findings.getFirst().detail(), findings.getFirst().detail().contains("edit[0].replace.palette[0]"));
+    }
+
+    @Test
+    public void missingFindKeyOnTheMatchSideIsIgnored() throws Exception {
+        File pack = pack("find-missing");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("good");
+        placement.getEdit().add(replace(1F, false, "minecraft:sulfur", "minecraft:cobblestone"));
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertTrue(data.getCompatReport().findings().toString(), data.getCompatReport().isEmpty());
+    }
+
+    @Test
+    public void missingMarkerBlockIsMatchOnly() throws Exception {
+        File pack = pack("marker-missing");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("good");
+        IrisObjectMarker marker = new IrisObjectMarker();
+        marker.setMarker("thing");
+        marker.getMark().add(new IrisBlockData("minecraft:sulfur"));
+        placement.getMarkers().add(marker);
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertEquals(new KList<String>().qadd("good"), placement.compatPlace(data));
+        assertTrue(data.getCompatReport().isEmpty());
+    }
+
+    @Test
+    public void dimensionFallbackRescuesTheObjectAndRecordsASubstitution() throws Exception {
+        File pack = pack("fallback");
+        CompatTestPlatform.write(pack, "dimensions/main.json",
+                "{\"blockFallbacks\":{\"minecraft:sulfur\":\"minecraft:cobblestone\"}}");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("bad");
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertEquals(new KList<String>().qadd("bad"), placement.compatPlace(data));
+
+        List<CompatFinding> findings = data.getCompatReport().findings();
+        assertEquals(findings.toString(), 1, findings.size());
+        assertEquals(CompatAction.SUBSTITUTED, findings.getFirst().action());
+    }
+
+    @Test
+    public void blockBackupRescuesAReplacePaletteEntry() throws Exception {
+        File pack = pack("backup");
+        CompatTestPlatform.writeObject(pack, "good", "minecraft:stone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("good");
+        IrisObjectReplace rule = replace(1F, false, "minecraft:stone", "minecraft:sulfur");
+        rule.getReplace().getPalette().getFirst().setBackup(new IrisBlockData("minecraft:cobblestone"));
+        placement.getEdit().add(rule);
+
+        assertFalse(placement.isCompatExcluded(data));
+        assertEquals(CompatAction.SUBSTITUTED, data.getCompatReport().findings().getFirst().action());
+    }
+
+    @Test
+    public void everyObjectDroppedExcludesThePlacement() throws Exception {
+        File pack = pack("all-dropped");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("bad");
+
+        assertTrue(placement.isCompatExcluded(data));
+        assertTrue(placement.compatPlace(data).isEmpty());
+        assertTrue(data.getCompatReport().findings().stream()
+                .anyMatch(finding -> finding.action() == CompatAction.EXCLUDED
+                        && "placement".equals(finding.subjectType())));
+    }
+
+    @Test
+    public void survivingPoolKeepsAuthoredOrderAndGetObjectOnlyDrawsSurvivors() throws Exception {
+        File pack = pack("draw");
+        CompatTestPlatform.writeObject(pack, "a", "minecraft:stone");
+        CompatTestPlatform.writeObject(pack, "bad", "minecraft:sulfur");
+        CompatTestPlatform.writeObject(pack, "b", "minecraft:cobblestone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("a", "bad", "b");
+
+        assertEquals(new KList<String>().qadd("a").qadd("b"), placement.compatPlace(data));
+
+        IrisData loaded = data;
+        DataProvider provider = () -> loaded;
+        for (int seed = 0; seed < 64; seed++) {
+            IrisObject drawn = placement.getObject(provider, new RNG(seed));
+            assertTrue("Draw must never return the dropped object",
+                    drawn != null && !"bad".equals(drawn.getLoadKey()));
+        }
+    }
+
+    @Test
+    public void aCompletePackLeavesThePoolIdentical() throws Exception {
+        File pack = pack("complete");
+        CompatTestPlatform.writeObject(pack, "a", "minecraft:stone");
+        CompatTestPlatform.writeObject(pack, "b", "minecraft:cobblestone");
+        data = IrisData.get(pack);
+        IrisObjectPlacement placement = placement("a", "b");
+
+        assertTrue("A complete pack must hand back the authored pool untouched",
+                placement.getPlace() == placement.compatPlace(data));
+        assertTrue(data.getCompatReport().isEmpty());
+    }
+
+    private File pack(String name) throws Exception {
+        return temporaryFolder.newFolder(name);
+    }
+
+    private static IrisObjectPlacement placement(String... place) {
+        IrisObjectPlacement placement = new IrisObjectPlacement();
+        placement.setPlace(new KList<>(place));
+        return placement;
+    }
+
+    private static IrisObjectReplace replace(float chance, boolean exact, String find, String to) {
+        IrisObjectReplace rule = new IrisObjectReplace();
+        rule.setChance(chance);
+        rule.setExact(exact);
+        rule.getFind().add(new IrisBlockData(find));
+        rule.getReplace().qclear();
+        rule.getReplace().add(to);
+        return rule;
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisRegionCompatPoolTest.java

@@ -0,0 +1,142 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.volmlib.util.collection.KList;
+import org.junit.Test;
+
+import java.util.Map;
+
+import static art.arcane.iris.engine.object.CompatFixtures.MISSING_BLOCK;
+import static art.arcane.iris.engine.object.CompatFixtures.biome;
+import static art.arcane.iris.engine.object.CompatFixtures.excludeBlock;
+import static art.arcane.iris.engine.object.CompatFixtures.find;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisRegionCompatPoolTest {
+    @SuppressWarnings("unchecked")
+    private static IrisData dataWith(PackCompatReport report, IrisBiome... biomes) {
+        IrisData data = CompatFixtures.data(report);
+        ResourceLoader<IrisBiome> biomeLoader = mock(ResourceLoader.class);
+        when(data.getBiomeLoader()).thenReturn(biomeLoader);
+        for (IrisBiome biome : biomes) {
+            when(biomeLoader.load(biome.getLoadKey())).thenReturn(biome);
+        }
+        return data;
+    }
+
+    private static ContentGate gate(PackCompatReport report) {
+        return new ContentGate(null, Map.of(), report);
+    }
+
+    @Test
+    public void realLandBiomesDropsExcludedReferenceAndRecordsFinding() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome plains = biome("land/plains");
+        IrisBiome sulfur = excludeBlock(biome("cave/sulfur-grotto"));
+        IrisData data = dataWith(report, plains, sulfur);
+        IrisRegion region = CompatFixtures.region("overworld");
+        region.setLoader(data);
+        region.setLandBiomes(new KList<>("land/plains", "cave/sulfur-grotto"));
+
+        KList<IrisBiome> pool = region.getRealLandBiomes(() -> data);
+
+        assertEquals(1, pool.size());
+        assertEquals("land/plains", pool.get(0).getLoadKey());
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "region", "overworld");
+        assertNotNull(dropped);
+        assertEquals(CompatRegistry.BLOCK, dropped.registry());
+        assertEquals(MISSING_BLOCK, dropped.key());
+        assertEquals("landBiomes[1] cave/sulfur-grotto", dropped.detail());
+    }
+
+    @Test
+    public void realPoolsSkipUnloadableReferences() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome shore = biome("shore/sand");
+        IrisData data = dataWith(report, shore);
+        IrisRegion region = CompatFixtures.region("overworld");
+        region.setLoader(data);
+        region.setSeaBiomes(new KList<>("sea/gone"));
+        region.setShoreBiomes(new KList<>("shore/sand"));
+        region.setCaveBiomes(new KList<>("cave/gone"));
+
+        assertTrue(region.getRealSeaBiomes(() -> data).isEmpty());
+        assertEquals(1, region.getRealShoreBiomes(() -> data).size());
+        assertTrue(region.getRealCaveBiomes(() -> data).isEmpty());
+        assertTrue(report.isEmpty());
+    }
+
+    @Test
+    public void naturalBiomesSkipExcludedBiomesAndTheirChildren() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome plains = biome("land/plains");
+        IrisBiome sulfur = excludeBlock(biome("cave/sulfur-grotto"));
+        sulfur.setChildren(new KList<>("cave/sulfur-deep"));
+        IrisBiome sulfurDeep = biome("cave/sulfur-deep");
+        IrisData data = dataWith(report, plains, sulfur, sulfurDeep);
+        IrisRegion region = CompatFixtures.region("overworld");
+        region.setLoader(data);
+        region.setLandBiomes(new KList<>("land/plains"));
+        region.setCaveBiomes(new KList<>("cave/sulfur-grotto"));
+
+        KList<IrisBiome> natural = region.getNaturalBiomes(() -> data);
+
+        assertEquals(1, natural.size());
+        assertEquals("land/plains", natural.get(0).getLoadKey());
+    }
+
+    @Test
+    public void evaluateCompatExcludesRegionWithNoLandBiomesLeft() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome sulfur = excludeBlock(biome("cave/sulfur-flats"));
+        IrisData data = dataWith(report, sulfur);
+        IrisRegion region = CompatFixtures.region("sulfur-lands");
+        region.setLoader(data);
+        region.setLandBiomes(new KList<>("cave/sulfur-flats"));
+
+        CompatStatus status = region.evaluateCompat(gate(report));
+
+        assertTrue(status.excluded());
+        CompatFinding cascade = find(report, CompatAction.EXCLUDED, "region", "sulfur-lands");
+        assertNotNull(cascade);
+        assertEquals(MISSING_BLOCK, cascade.key());
+        assertEquals(CompatRegistry.BLOCK, cascade.registry());
+        assertEquals("no land biomes remain", cascade.detail());
+    }
+
+    @Test
+    public void evaluateCompatKeepsRegionWithSurvivingLandBiome() {
+        PackCompatReport report = new PackCompatReport();
+        IrisBiome plains = biome("land/plains");
+        IrisBiome sulfur = excludeBlock(biome("cave/sulfur-flats"));
+        IrisData data = dataWith(report, plains, sulfur);
+        IrisRegion region = CompatFixtures.region("overworld");
+        region.setLoader(data);
+        region.setLandBiomes(new KList<>("land/plains", "cave/sulfur-flats"));
+
+        assertFalse(region.evaluateCompat(gate(report)).excluded());
+    }
+
+    @Test
+    public void evaluateCompatDoesNotExcludeRegionThatNeverDeclaredLandBiomes() {
+        PackCompatReport report = new PackCompatReport();
+        IrisData data = dataWith(report);
+        IrisRegion region = CompatFixtures.region("empty");
+        region.setLoader(data);
+
+        assertFalse(region.evaluateCompat(gate(report)).excluded());
+        assertTrue(report.isEmpty());
+    }
+}

core/src/test/java/art/arcane/iris/engine/object/IrisSpawnerCompatTest.java

@@ -0,0 +1,126 @@
+package art.arcane.iris.engine.object;
+
+import art.arcane.iris.core.compat.CompatAction;
+import art.arcane.iris.core.compat.CompatFinding;
+import art.arcane.iris.core.compat.CompatRegistry;
+import art.arcane.iris.core.compat.CompatStatus;
+import art.arcane.iris.core.compat.ContentGate;
+import art.arcane.iris.core.compat.PackCompatReport;
+import art.arcane.iris.core.loader.IrisData;
+import art.arcane.iris.core.loader.ResourceLoader;
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.volmlib.util.collection.KList;
+import org.junit.Test;
+
+import java.util.Map;
+
+import static art.arcane.iris.engine.object.CompatFixtures.MISSING_ENTITY;
+import static art.arcane.iris.engine.object.CompatFixtures.entity;
+import static art.arcane.iris.engine.object.CompatFixtures.excludeEntity;
+import static art.arcane.iris.engine.object.CompatFixtures.find;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class IrisSpawnerCompatTest {
+    @SuppressWarnings("unchecked")
+    private static IrisData dataWith(PackCompatReport report, IrisEntity... entities) {
+        IrisData data = CompatFixtures.data(report);
+        ResourceLoader<IrisEntity> entityLoader = mock(ResourceLoader.class);
+        when(data.getEntityLoader()).thenReturn(entityLoader);
+        for (IrisEntity entity : entities) {
+            when(entityLoader.load(entity.getLoadKey())).thenReturn(entity);
+        }
+        return data;
+    }
+
+    private static ContentGate gate(PackCompatReport report) {
+        return new ContentGate(null, Map.of(), report);
+    }
+
+    @Test
+    public void evaluateCompatDropsSpawnsWhoseEntityIsExcluded() {
+        PackCompatReport report = new PackCompatReport();
+        IrisEntity zombie = entity("zombie");
+        IrisEntity camel = excludeEntity(entity("camel"));
+        IrisData data = dataWith(report, zombie, camel);
+        IrisSpawner spawner = new IrisSpawner();
+        spawner.setLoadKey("desert");
+        spawner.setLoader(data);
+        spawner.setSpawns(new KList<>(
+                new IrisEntitySpawn().setEntity("zombie"),
+                new IrisEntitySpawn().setEntity("camel")));
+
+        CompatStatus status = spawner.evaluateCompat(gate(report));
+
+        assertFalse(status.excluded());
+        assertEquals(1, spawner.getSpawns().size());
+        assertEquals("zombie", spawner.getSpawns().get(0).getEntity());
+        CompatFinding dropped = find(report, CompatAction.DROPPED, "spawner", "desert");
+        assertNotNull(dropped);
+        assertEquals(CompatRegistry.ENTITY, dropped.registry());
+        assertEquals(MISSING_ENTITY, dropped.key());
+        assertEquals("spawns[1] camel", dropped.detail());
+    }
+
+    @Test
+    public void evaluateCompatExcludesSpawnerWithNoSpawnsLeft() {
+        PackCompatReport report = new PackCompatReport();
+        IrisEntity camel = excludeEntity(entity("camel"));
+        IrisData data = dataWith(report, camel);
+        IrisSpawner spawner = new IrisSpawner();
+        spawner.setLoadKey("desert");
+        spawner.setLoader(data);
+        spawner.setSpawns(new KList<>(new IrisEntitySpawn().setEntity("camel")));
+        spawner.setInitialSpawns(new KList<>(new IrisEntitySpawn().setEntity("camel")));
+
+        CompatStatus status = spawner.evaluateCompat(gate(report));
+
+        assertTrue(status.excluded());
+        assertTrue(spawner.getSpawns().isEmpty());
+        assertTrue(spawner.getInitialSpawns().isEmpty());
+        CompatFinding cascade = find(report, CompatAction.EXCLUDED, "spawner", "desert");
+        assertNotNull(cascade);
+        assertEquals(MISSING_ENTITY, cascade.key());
+        assertEquals("no entity spawns remain", cascade.detail());
+    }
+
+    @Test
+    public void evaluateCompatKeepsSpawnerThatNeverDeclaredSpawns() {
+        PackCompatReport report = new PackCompatReport();
+        IrisData data = dataWith(report);
+        IrisSpawner spawner = new IrisSpawner();
+        spawner.setLoadKey("empty");
+        spawner.setLoader(data);
+
+        assertFalse(spawner.evaluateCompat(gate(report)).excluded());
+        assertTrue(report.isEmpty());
+    }
+
+    @Test
+    public void entitySpawnResolvesNoEntityWhenExcluded() {
+        PackCompatReport report = new PackCompatReport();
+        IrisEntity camel = excludeEntity(entity("camel"));
+        IrisData data = dataWith(report, camel);
+        Engine engine = mock(Engine.class);
+        when(engine.getData()).thenReturn(data);
+
+        assertNull(new IrisEntitySpawn().setEntity("camel").getRealEntity(engine));
+    }
+
+    @Test
+    public void entitySpawnResolvesPresentEntity() {
+        PackCompatReport report = new PackCompatReport();
+        IrisEntity zombie = entity("zombie");
+        IrisData data = dataWith(report, zombie);
+        Engine engine = mock(Engine.class);
+        when(engine.getData()).thenReturn(data);
+
+        assertSame(zombie, new IrisEntitySpawn().setEntity("zombie").getRealEntity(engine));
+    }
+}