Port Flat and Void Age terrain to Minecraft 26.3 beta

docs/MINECRAFT_26_3_PORT.md

@@ -480,6 +480,21 @@ symbol-page data and authoring pipeline, not just drawing more placeholders.
 The local `1.0.1.15-alpha.1` follow-up changes the missing-panel cue to the
 Link Panel's 16×16 item icon while preserving its red pulse and tooltip.
 
+The local `1.0.1.33-beta.1` slice adds Flat and Void terrain to authored Binder
+books. Flat uses vanilla superflat layers and can use one authored Biome; Void
+generates no terrain but can use one authored Biome for environmental behavior.
+Without a Biome page, Void uses `minecraft:the_void`. Books with multiple Biome
+pages and Flat/Void terrain remain unassembled until their custom distribution
+generator is ported. The dedicated-server smoke creates, samples, and reloads
+both terrain families in a disposable world.
+
+The local `1.0.1.34-beta.1` follow-up moves Flat ground to Y=0 and targets its
+surface for travel. `1.0.1.35-beta.1` enables the selected biome's decoration
+on Flat. `1.0.1.36-beta.1` ports a multi-biome Flat/Void chunk generator and
+accepts the existing authored distribution pages for those terrains. New Flat
+chunks can vary vegetation by biome; Void keeps biome climate without adding
+a floor or decorations. These are beta-port slices, not full 1.21.1 parity.
+
 ## Verification of this first slice
 
 The 1.21.1 `compileJava` and `build` tasks passed offline after the backend

ports/mc263/README.md

@@ -3,8 +3,9 @@
 This is an **experimental beta port** for issues #97–#104, not gameplay parity
 with the Minecraft 1.21.1 release. The 26.3 backend persists separate Ages and
 their seeds. Books, the Book Binder, and the Ink Mixer work, and the Binder can
-author Standard, Amplified, or Island terrain with supported biome-distribution
-pages. Other terrain families, feature/material effects, portals, and much of
+author Standard, Amplified, Island, Flat, or Void terrain with supported
+biome-distribution pages. Other terrain families, feature/material effects,
+portals, and much of
 the 1.21.1 gameplay remain unported. Use a test world and keep a backup; do not
 substitute this JAR for the 1.21.1 mod or import 1.21.1 saves.
 
@@ -153,6 +154,29 @@ terrain; and saved terrain/biome plans across reload. A Binder or Ink Mixer
 beside another block no longer hides that block's faces. Unsupported authoring
 still stays in the Binder rather than silently producing an incorrect Age.
 
+The local `1.0.1.33-beta.1` slice adds Flat and Void terrain to Binder
+authoring. Flat uses vanilla's superflat layers; Void has no generated floor
+and uses the selected Biome for its environmental rules. Each accepts zero or
+one authored Biome page. Unsupported multi-biome Flat/Void books remain in the
+Binder until their custom generator is ported. The operator probe can create
+test Ages with `rivencraft_port age create <name> <seed> flat|void`.
+
+The local `1.0.1.34-beta.1` follow-up starts Flat's solid layers at Y=0
+(bedrock at 0, dirt at 1–2, grass at 3) and lands players at the ground surface
+instead of creating an elevated fallback platform. This affects newly
+generated Flat chunks; previously generated Age chunks are not rewritten.
+
+The local `1.0.1.35-beta.1` follow-up enables biome decoration on newly
+generated Flat chunks, allowing the selected biome's trees, vegetation, and
+other placed features to appear on the Flat surface. Void remains undecorated.
+
+The local `1.0.1.36-beta.1` slice accepts multiple Biome pages with Flat and
+Void terrain. It ports the 1.21.1 authored-biome Flat generator to 26.3, so
+Tiled, Grid, and scaled regions affect climate in both terrain types and
+surface decoration in Flat. Void continues to generate no floor or biome
+decoration. The operator-only `age create_multi` probe creates two-biome test
+Ages for the dedicated-server smoke test.
+
 From the repository root:
 
 ```sh

ports/mc263/RELEASE_NOTES_1.0.1.36-beta.1.md

@@ -0,0 +1,12 @@
+# Rivencraft 26.3 beta 1.0.1.36-beta.1
+
+This beta adds Flat and Void terrain to authored Ages on Minecraft 26.3.
+
+- Bind Descriptive Books with Flat or Void terrain, including multiple Biome pages and supported biome-distribution pages.
+- Flat ground starts at Y=0; travel lands on its surface rather than on an elevated fallback platform.
+- Flat Ages generate the selected biomes' surface decorations, including trees and vegetation. Void Ages remain empty while retaining authored biome climate.
+- Expanded dedicated-server checks for Flat and Void terrain, biome distribution, and persistence after restart.
+
+This 26.3 port is still experimental and does not yet include all features of the separate Minecraft 1.21.1 version. Use a test world and keep a backup. Minecraft 1.21.1 saves are not supported by this beta.
+
+Requires Minecraft 26.3, NeoForge 26.3.0.99 or a compatible 26.3 build, and Java 25.

ports/mc263/gradle.properties

@@ -7,4 +7,4 @@ neo_version=26.3.0.99
 mod_id=rivencraft
 mod_name=Rivencraft
 mod_license=LGPL-3.0-or-later
-mod_version=1.0.1.32-beta.1
+mod_version=1.0.1.36-beta.1

ports/mc263/src/main/java/net/dnorberg/rivencraft/Rivencraft.java

@@ -21,6 +21,7 @@ import net.dnorberg.rivencraft.item.PageItem;
 import net.dnorberg.rivencraft.network.PortBookNetworking;
 import net.dnorberg.rivencraft.registry.ModCreativeTabs;
 import net.dnorberg.rivencraft.registry.PortBiomeSources;
+import net.dnorberg.rivencraft.registry.PortChunkGenerators;
 import net.dnorberg.rivencraft.writing.LinkingBookRecipe;
 import net.minecraft.core.component.DataComponentType;
 import net.minecraft.core.registries.Registries;
@@ -152,6 +153,7 @@ public final class Rivencraft {
         RECIPE_SERIALIZERS.register(modEventBus);
         ModCreativeTabs.register(modEventBus);
         PortBiomeSources.register(modEventBus);
+        PortChunkGenerators.register(modEventBus);
         NeoForge.EVENT_BUS.addListener(RuntimeAgeBackend::onLevelLoad);
         NeoForge.EVENT_BUS.addListener(PortAgeCommands::register);
         NeoForge.EVENT_BUS.addListener(AgeCatalogSync::onLogin);

ports/mc263/src/main/java/net/dnorberg/rivencraft/age/PortAgeCommands.java

@@ -3,9 +3,11 @@ package net.dnorberg.rivencraft.age;
 import com.mojang.brigadier.arguments.LongArgumentType;
 import com.mojang.brigadier.arguments.IntegerArgumentType;
 import com.mojang.brigadier.arguments.StringArgumentType;
+import com.mojang.brigadier.context.CommandContext;
 import net.dnorberg.rivencraft.Rivencraft;
 import net.dnorberg.rivencraft.item.LinkBookTarget;
 import net.minecraft.commands.Commands;
+import net.minecraft.commands.CommandSourceStack;
 import net.minecraft.core.registries.Registries;
 import net.minecraft.core.component.DataComponents;
 import net.minecraft.network.chat.Component;
@@ -20,6 +22,7 @@ import net.neoforged.neoforge.event.RegisterCommandsEvent;
 import net.neoforged.fml.ModList;
 import java.util.Map;
 import java.util.TreeMap;
+import java.util.Locale;
 
 /** Operator-only backend and synchronized-travel probes, not normal book behavior. */
 public final class PortAgeCommands {
@@ -66,23 +69,44 @@ public final class PortAgeCommands {
                         .then(Commands.literal("create")
                                 .then(Commands.argument("name", StringArgumentType.word())
                                         .then(Commands.argument("seed", LongArgumentType.longArg())
-                                                .executes(context -> {
-                                                    String name = StringArgumentType.getString(context, "name");
-                                                    Identifier id = Identifier.fromNamespaceAndPath(
-                                                            Rivencraft.MOD_ID, "age/" + name);
-                                                    long seed = LongArgumentType.getLong(context, "seed");
-                                                    ServerLevel level;
-                                                    try {
-                                                        level = RuntimeAgeBackend.create(
-                                                                context.getSource().getServer(), id, seed);
-                                                    } catch (IllegalArgumentException | IllegalStateException exception) {
-                                                        context.getSource().sendFailure(Component.literal(exception.getMessage()));
-                                                        return 0;
-                                                    }
-                                                    context.getSource().sendSuccess(() -> Component.literal(
-                                                            "Created " + id + " with seed " + level.getSeed()), true);
-                                                    return 1;
-                                                }))))
+                                                .executes(context -> createAge(context, PortAgeTerrain.STANDARD))
+                                                .then(Commands.argument("terrain", StringArgumentType.word())
+                                                        .executes(context -> {
+                                                            PortAgeTerrain terrain;
+                                                            try {
+                                                                terrain = PortAgeTerrain.valueOf(StringArgumentType.getString(
+                                                                        context, "terrain").toUpperCase(Locale.ROOT));
+                                                            } catch (IllegalArgumentException exception) {
+                                                                context.getSource().sendFailure(Component.literal(
+                                                                        "Unknown terrain; use standard, amplified, island, flat, or void"));
+                                                                return 0;
+                                                            }
+                                                            return createAge(context, terrain);
+                                                        })))))
+                        .then(Commands.literal("create_multi")
+                                .then(Commands.argument("name", StringArgumentType.word())
+                                        .then(Commands.argument("seed", LongArgumentType.longArg())
+                                                .then(Commands.argument("terrain", StringArgumentType.word())
+                                                        .executes(context -> {
+                                                            PortAgeTerrain terrain;
+                                                            try {
+                                                                terrain = PortAgeTerrain.valueOf(StringArgumentType.getString(
+                                                                        context, "terrain").toUpperCase(Locale.ROOT));
+                                                            } catch (IllegalArgumentException exception) {
+                                                                context.getSource().sendFailure(Component.literal(
+                                                                        "Unknown terrain; use flat or void"));
+                                                                return 0;
+                                                            }
+                                                            if (terrain != PortAgeTerrain.FLAT && terrain != PortAgeTerrain.VOID) {
+                                                                context.getSource().sendFailure(Component.literal(
+                                                                        "Multi-biome probe supports flat or void only"));
+                                                                return 0;
+                                                            }
+                                                            return createAge(context, terrain, new PortBiomePlan(
+                                                                    PortBiomePlan.Distribution.TILED, java.util.List.of(
+                                                                            Identifier.withDefaultNamespace("plains"),
+                                                                            Identifier.withDefaultNamespace("forest"))));
+                                                        })))))
                         .then(Commands.literal("list")
                                 .executes(context -> {
                                     var ages = RuntimeAgeBackend.savedAges(context.getSource().getServer());
@@ -185,7 +209,33 @@ public final class PortAgeCommands {
                                         .executes(context -> sampleAge(context, 0))
                                         .then(Commands.argument("chunkOffset", IntegerArgumentType.integer(0))
                                                 .executes(context -> sampleAge(context,
-                                                        IntegerArgumentType.getInteger(context, "chunkOffset"))))))));
+                                                        IntegerArgumentType.getInteger(context, "chunkOffset"))))))
+                        .then(Commands.literal("biome")
+                                .then(Commands.argument("name", StringArgumentType.word())
+                                        .then(Commands.argument("x", IntegerArgumentType.integer())
+                                                .then(Commands.argument("z", IntegerArgumentType.integer())
+                                                        .executes(PortAgeCommands::sampleBiome)))))));
+    }
+
+    private static int createAge(CommandContext<CommandSourceStack> context, PortAgeTerrain terrain) {
+        return createAge(context, terrain, null);
+    }
+
+    private static int createAge(CommandContext<CommandSourceStack> context,
+                                 PortAgeTerrain terrain, PortBiomePlan biomePlan) {
+        String name = StringArgumentType.getString(context, "name");
+        Identifier id = Identifier.fromNamespaceAndPath(Rivencraft.MOD_ID, "age/" + name);
+        long seed = LongArgumentType.getLong(context, "seed");
+        ServerLevel level;
+        try {
+            level = RuntimeAgeBackend.create(context.getSource().getServer(), id, seed, biomePlan, terrain);
+        } catch (IllegalArgumentException | IllegalStateException exception) {
+            context.getSource().sendFailure(Component.literal(exception.getMessage()));
+            return 0;
+        }
+        context.getSource().sendSuccess(() -> Component.literal(
+                "Created " + id + " with seed " + level.getSeed() + " terrain=" + terrain), true);
+        return 1;
     }
 
     private static int sampleAge(com.mojang.brigadier.context.CommandContext<net.minecraft.commands.CommandSourceStack> context,
@@ -213,6 +263,23 @@ public final class PortAgeCommands {
         return 1;
     }
 
+    private static int sampleBiome(CommandContext<CommandSourceStack> context) {
+        Identifier id = ageId(context);
+        ServerLevel level = context.getSource().getServer().getLevel(ResourceKey.create(Registries.DIMENSION, id));
+        if (level == null) {
+            context.getSource().sendFailure(Component.literal("Age not loaded: " + id));
+            return 0;
+        }
+        int x = IntegerArgumentType.getInteger(context, "x");
+        int z = IntegerArgumentType.getInteger(context, "z");
+        level.getChunk(x >> 4, z >> 4);
+        Identifier biome = level.getBiome(new net.minecraft.core.BlockPos(x, 4, z))
+                .unwrapKey().orElseThrow().identifier();
+        context.getSource().sendSuccess(() -> Component.literal(
+                id + " biome at " + x + "," + z + "=" + biome), false);
+        return 1;
+    }
+
     private static Identifier ageId(com.mojang.brigadier.context.CommandContext<net.minecraft.commands.CommandSourceStack> context) {
         return Identifier.fromNamespaceAndPath(Rivencraft.MOD_ID,
                 "age/" + StringArgumentType.getString(context, "name"));

ports/mc263/src/main/java/net/dnorberg/rivencraft/age/PortAgeTerrain.java

@@ -4,5 +4,7 @@ package net.dnorberg.rivencraft.age;
 public enum PortAgeTerrain {
     STANDARD,
     AMPLIFIED,
-    ISLAND
+    ISLAND,
+    FLAT,
+    VOID
 }

ports/mc263/src/main/java/net/dnorberg/rivencraft/age/PortAgeTravel.java

@@ -11,6 +11,8 @@ import net.minecraft.server.level.ServerLevel;
 import net.minecraft.server.level.ServerPlayer;
 import net.minecraft.world.level.block.Blocks;
 import net.minecraft.world.level.levelgen.Heightmap;
+import net.minecraft.world.level.levelgen.FlatLevelSource;
+import net.minecraft.world.level.levelgen.flat.FlatLevelGeneratorSettings;
 
 /** Shared, server-authoritative landing for the 26.3 book and operator probe. */
 public final class PortAgeTravel {
@@ -31,16 +33,26 @@ public final class PortAgeTravel {
     }
 
     private static BlockPos findLanding(ServerLevel target) {
+        // A Flat Age has a known surface. Use it directly before a heightmap
+        // fallback can place a platform in the air above a low superflat world.
+        FlatLevelGeneratorSettings flatSettings = switch (target.getChunkSource().getGenerator()) {
+            case FlatLevelSource flat -> flat.settings();
+            case PortFlatChunkGenerator flat -> flat.settings();
+            default -> null;
+        };
+        if (flatSettings != null && !flatSettings.getLayersInfo().isEmpty()) {
+            int surfaceY = target.getMinY() + flatSettings.getLayersInfo().stream()
+                    .mapToInt(layer -> layer.getHeight()).sum();
+            BlockPos surface = new BlockPos(0, surfaceY, 0);
+            if (hasFooting(target, surface)) return surface;
+        }
         for (int radius = 0; radius <= 16; radius++) {
             for (int x = -radius; x <= radius; x++) {
                 for (int z = -radius; z <= radius; z++) {
                     if (Math.max(Math.abs(x), Math.abs(z)) != radius) continue;
                     int y = target.getHeight(Heightmap.Types.MOTION_BLOCKING, x, z);
                     BlockPos foot = new BlockPos(x, y, z);
-                    BlockPos floor = foot.below();
-                    if (target.getBlockState(floor).isFaceSturdy(target, floor, Direction.UP)
-                            && target.getBlockState(foot).getCollisionShape(target, foot).isEmpty()
-                            && target.getBlockState(foot.above()).getCollisionShape(target, foot.above()).isEmpty()) {
+                    if (hasFooting(target, foot)) {
                         return foot;
                     }
                 }
@@ -60,4 +72,11 @@ public final class PortAgeTravel {
         target.setBlock(foot.above(), Blocks.AIR.defaultBlockState(), 3);
         return foot;
     }
+
+    private static boolean hasFooting(ServerLevel target, BlockPos foot) {
+        BlockPos floor = foot.below();
+        return target.getBlockState(floor).isFaceSturdy(target, floor, Direction.UP)
+                && target.getBlockState(foot).getCollisionShape(target, foot).isEmpty()
+                && target.getBlockState(foot.above()).getCollisionShape(target, foot.above()).isEmpty();
+    }
 }

ports/mc263/src/main/java/net/dnorberg/rivencraft/age/PortFlatChunkGenerator.java

@@ -0,0 +1,142 @@
+package net.dnorberg.rivencraft.age;
+
+import com.mojang.serialization.MapCodec;
+import com.mojang.serialization.codecs.RecordCodecBuilder;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.stream.Stream;
+import net.minecraft.core.BlockPos;
+import net.minecraft.core.Holder;
+import net.minecraft.core.HolderLookup;
+import net.minecraft.core.HolderSet;
+import net.minecraft.server.level.WorldGenRegion;
+import net.minecraft.world.level.LevelHeightAccessor;
+import net.minecraft.world.level.NoiseColumn;
+import net.minecraft.world.level.StructureManager;
+import net.minecraft.world.level.biome.Biome;
+import net.minecraft.world.level.biome.BiomeGenerationSettings;
+import net.minecraft.world.level.biome.BiomeManager;
+import net.minecraft.world.level.biome.BiomeSource;
+import net.minecraft.world.level.block.Blocks;
+import net.minecraft.world.level.block.state.BlockState;
+import net.minecraft.world.level.chunk.ChunkAccess;
+import net.minecraft.world.level.chunk.ChunkGenerator;
+import net.minecraft.world.level.chunk.ChunkGeneratorStructureState;
+import net.minecraft.world.level.levelgen.GenerationStep;
+import net.minecraft.world.level.levelgen.Heightmap;
+import net.minecraft.world.level.levelgen.RandomState;
+import net.minecraft.world.level.levelgen.blending.Blender;
+import net.minecraft.world.level.levelgen.densityfunction.SamplerContext;
+import net.minecraft.world.level.levelgen.flat.FlatLevelGeneratorSettings;
+import net.minecraft.world.level.levelgen.placement.PlacedFeature;
+import net.minecraft.world.level.levelgen.structure.StructureSet;
+import net.neoforged.neoforge.common.util.MemoizedFunction;
+import org.jspecify.annotations.Nullable;
+
+/** Flat layers with the authored biome distribution used by 1.21.1 Ages. */
+public final class PortFlatChunkGenerator extends ChunkGenerator {
+    public static final MapCodec<PortFlatChunkGenerator> CODEC = RecordCodecBuilder.mapCodec(instance -> instance.group(
+            BiomeSource.CODEC.fieldOf("biome_source").forGetter(generator -> generator.biomeSource),
+            FlatLevelGeneratorSettings.CODEC.fieldOf("settings").forGetter(PortFlatChunkGenerator::settings)
+    ).apply(instance, PortFlatChunkGenerator::new));
+
+    private final FlatLevelGeneratorSettings settings;
+    private final MemoizedFunction<Holder<Biome>, BiomeGenerationSettings> generationSettingsGetter;
+
+    public PortFlatChunkGenerator(BiomeSource biomeSource, FlatLevelGeneratorSettings settings) {
+        super(biomeSource, this.generationSettingsGetter = MemoizedFunction.of(
+                biome -> generationSettings(settings, biome)));
+        this.settings = settings;
+    }
+
+    public FlatLevelGeneratorSettings settings() {
+        return settings;
+    }
+
+    private static BiomeGenerationSettings generationSettings(FlatLevelGeneratorSettings settings, Holder<Biome> biome) {
+        // Void biomes still control climate, but must not place terrain features.
+        if (settings.getLayersInfo().isEmpty()) return BiomeGenerationSettings.EMPTY;
+        if (biome.equals(settings.getBiome())) return settings.adjustGenerationSettings(biome);
+        BiomeGenerationSettings.PlainBuilder builder = new BiomeGenerationSettings.PlainBuilder();
+        List<HolderSet<PlacedFeature>> features = biome.value().getGenerationSettings().features();
+        for (int step = 0; step < features.size(); step++) {
+            if (step == GenerationStep.Decoration.UNDERGROUND_STRUCTURES.ordinal()
+                    || step == GenerationStep.Decoration.SURFACE_STRUCTURES.ordinal()) continue;
+            for (Holder<PlacedFeature> feature : features.get(step)) builder.addFeature(step, feature);
+        }
+        return builder.build();
+    }
+
+    @Override
+    protected MapCodec<? extends ChunkGenerator> codec() {
+        return CODEC;
+    }
+
+    @Override
+    public ChunkGeneratorStructureState createState(HolderLookup<StructureSet> structures,
+                                                    RandomState random, long seed) {
+        Stream<Holder<StructureSet>> entries = settings.structureOverrides()
+                .map(HolderSet::stream)
+                .orElseGet(() -> structures.listElements().map(holder -> (Holder<StructureSet>) holder));
+        return ChunkGeneratorStructureState.createForFlat(random, seed, getOrigin(random), biomeSource, entries);
+    }
+
+    @Override
+    public int getSpawnHeight(LevelHeightAccessor height) {
+        return height.getMinY() + Math.min(height.getHeight(), settings.getLayers().size());
+    }
+
+    @Override
+    public CompletableFuture<ChunkAccess> buildTerrain(ChunkAccess chunk, Blender blender,
+            RandomState random, StructureManager structures, BiomeManager biomes,
+            @Nullable WorldGenRegion carverBiomeRegion, Set<Holder<Biome>> possibleBiomes) {
+        List<BlockState> layers = settings.getLayers();
+        BlockPos.MutableBlockPos pos = new BlockPos.MutableBlockPos();
+        Heightmap floor = chunk.getOrCreateHeightmapUnprimed(Heightmap.Types.OCEAN_FLOOR_WG);
+        Heightmap surface = chunk.getOrCreateHeightmapUnprimed(Heightmap.Types.WORLD_SURFACE_WG);
+        for (int layer = 0; layer < Math.min(chunk.getHeight(), layers.size()); layer++) {
+            BlockState state = layers.get(layer);
+            if (state == null) continue;
+            int y = chunk.getMinY() + layer;
+            for (int x = 0; x < 16; x++) for (int z = 0; z < 16; z++) {
+                chunk.setBlockState(pos.set(x, y, z), state);
+                floor.update(x, y, z, state);
+                surface.update(x, y, z, state);
+            }
+        }
+        return CompletableFuture.completedFuture(chunk);
+    }
+
+    @Override
+    public int getBaseHeight(int x, int z, Heightmap.Types type,
+                             LevelHeightAccessor height, RandomState random) {
+        List<BlockState> layers = settings.getLayers();
+        for (int layer = Math.min(layers.size() - 1, height.getHeight() - 1); layer >= 0; layer--) {
+            BlockState state = layers.get(layer);
+            if (state != null && type.isOpaque().test(state)) return height.getMinY() + layer + 1;
+        }
+        return height.getMinY();
+    }
+
+    @Override
+    public NoiseColumn getBaseColumn(int x, int z, LevelHeightAccessor height, RandomState random) {
+        return new NoiseColumn(height.getMinY(), settings.getLayers().stream().limit(height.getHeight())
+                .map(state -> state == null ? Blocks.AIR.defaultBlockState() : state)
+                .toArray(BlockState[]::new));
+    }
+
+    @Override public void addDebugScreenInfo(List<String> info, RandomState random,
+                                             BlockPos pos, SamplerContext context) { }
+    @Override public void spawnOriginalMobs(WorldGenRegion level) { }
+    @Override public int getMinY() { return 0; }
+    @Override public int getGenDepth() { return 384; }
+    @Override public int getSeaLevel() { return -63; }
+
+    @Override
+    public void refreshFeaturesPerStep() {
+        super.refreshFeaturesPerStep();
+        generationSettingsGetter.clear();
+        settings.updateLayers();
+    }
+}

ports/mc263/src/main/java/net/dnorberg/rivencraft/age/RuntimeAgeBackend.java

@@ -8,6 +8,7 @@ import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.OptionalLong;
 import net.dnorberg.rivencraft.Rivencraft;
 import net.minecraft.core.registries.Registries;
@@ -20,12 +21,18 @@ import net.minecraft.server.level.ServerLevel;
 import net.minecraft.world.level.Level;
 import net.minecraft.world.level.TicketStorage;
 import net.minecraft.world.level.biome.BiomeManager;
+import net.minecraft.world.level.biome.BiomeSource;
 import net.minecraft.world.level.biome.FixedBiomeSource;
 import net.minecraft.world.level.biome.Biome;
+import net.minecraft.world.level.biome.Biomes;
 import net.minecraft.world.level.biome.CheckerboardColumnBiomeSource;
 import net.minecraft.world.level.dimension.LevelStem;
+import net.minecraft.world.level.levelgen.FlatLevelSource;
 import net.minecraft.world.level.levelgen.NoiseBasedChunkGenerator;
 import net.minecraft.world.level.levelgen.NoiseGeneratorSettings;
+import net.minecraft.world.level.levelgen.flat.FlatLevelGeneratorSettings;
+import net.minecraft.world.level.levelgen.flat.FlatLayerInfo;
+import net.minecraft.world.level.block.Blocks;
 import net.minecraft.world.level.storage.DerivedLevelData;
 import net.minecraft.world.level.storage.LevelResource;
 import net.neoforged.neoforge.common.NeoForge;
@@ -254,6 +261,25 @@ public final class RuntimeAgeBackend {
     private record ArchivePaths(Path live, Path archive) {
     }
 
+    private static BiomeSource authoredBiomeSource(MinecraftServer server, PortBiomePlan plan, long seed) {
+        if (plan.distribution() == PortBiomePlan.Distribution.NATIVE)
+            throw new IllegalArgumentException("Native biome distribution has no authored source");
+        var registry = server.registryAccess().lookupOrThrow(Registries.BIOME);
+        List<Holder<Biome>> biomes = plan.biomes().stream().map(id ->
+                (Holder<Biome>) registry.get(ResourceKey.create(Registries.BIOME, id))
+                        .orElseThrow(() -> new IllegalArgumentException("Unknown authored biome: " + id)))
+                .toList();
+        return switch (plan.distribution()) {
+            case NATIVE -> throw new IllegalStateException("Native is not an override");
+            case SINGLE -> new FixedBiomeSource(biomes.getFirst());
+            case TILED, GRID -> new CheckerboardColumnBiomeSource(HolderSet.direct(biomes),
+                    plan.distribution() == PortBiomePlan.Distribution.GRID ? 0 : 4);
+            case TINY, SMALL, MEDIUM, LARGE, HUGE -> new PortScaledBiomeSource(
+                    HolderSet.direct(biomes), seed, plan.distribution().ordinal() -
+                            PortBiomePlan.Distribution.TINY.ordinal());
+        };
+    }
+
     private static ServerLevel load(MinecraftServer server, Identifier id, long seed,
                                     PortBiomePlan biomePlan, PortAgeTerrain terrain) {
         ResourceKey<Level> key = ResourceKey.create(Registries.DIMENSION, id);
@@ -266,26 +292,42 @@ public final class RuntimeAgeBackend {
                 .getValue(LevelStem.OVERWORLD);
         if (overworldStem == null) throw new IllegalStateException("Missing Overworld LevelStem");
         var generator = overworldStem.generator();
-        if ((biomePlan != null && biomePlan.distribution() != PortBiomePlan.Distribution.NATIVE)
+        if (terrain == PortAgeTerrain.FLAT || terrain == PortAgeTerrain.VOID) {
+            var biomes = server.registryAccess().lookupOrThrow(Registries.BIOME);
+            Holder<Biome> biome = biomes.getOrThrow(terrain == PortAgeTerrain.VOID
+                    ? Biomes.THE_VOID : Biomes.PLAINS);
+            if (biomePlan != null && !biomePlan.biomes().isEmpty()) {
+                Identifier biomeId = biomePlan.biomes().getFirst();
+                biome = biomes.get(ResourceKey.create(Registries.BIOME, biomeId))
+                        .orElseThrow(() -> new IllegalArgumentException("Unknown authored biome: " + biomeId));
+            }
+            FlatLevelGeneratorSettings settings;
+            if (terrain == PortAgeTerrain.FLAT) {
+                settings = FlatLevelGeneratorSettings.getDefault(biomes,
+                        server.registryAccess().lookupOrThrow(Registries.STRUCTURE_SET),
+                        server.registryAccess().lookupOrThrow(Registries.PLACED_FEATURE));
+                // Vanilla superflat begins at the modern minimum build height.
+                // Pad the column with air so the first solid layer begins at Y=0.
+                int airBelowZero = Math.max(0, -server.overworld().getMinY());
+                if (airBelowZero > 0)
+                    settings.getLayersInfo().addFirst(new FlatLayerInfo(airBelowZero, Blocks.AIR));
+                settings = settings.withBiomeAndLayers(settings.getLayersInfo(),
+                        settings.structureOverrides(), biome);
+                settings.setDecoration();
+            } else {
+                settings = new FlatLevelGeneratorSettings(Optional.empty(), biome, List.of());
+                settings.updateLayers();
+            }
+            generator = biomePlan != null && biomePlan.biomes().size() > 1
+                    ? new PortFlatChunkGenerator(authoredBiomeSource(server, biomePlan, seed), settings)
+                    : new FlatLevelSource(settings);
+        } else if ((biomePlan != null && biomePlan.distribution() != PortBiomePlan.Distribution.NATIVE)
                 || terrain != PortAgeTerrain.STANDARD) {
             if (!(generator instanceof NoiseBasedChunkGenerator noise))
                 throw new IllegalStateException("Overworld generator cannot accept authored noise settings");
             var source = generator.getBiomeSource();
             if (biomePlan != null && biomePlan.distribution() != PortBiomePlan.Distribution.NATIVE) {
-                var registry = server.registryAccess().lookupOrThrow(Registries.BIOME);
-                List<Holder<Biome>> biomes = biomePlan.biomes().stream().map(biome ->
-                        (Holder<Biome>) registry.get(ResourceKey.create(Registries.BIOME, biome))
-                                .orElseThrow(() -> new IllegalArgumentException("Unknown authored biome: " + biome)))
-                        .toList();
-                source = switch (biomePlan.distribution()) {
-                    case NATIVE -> throw new IllegalStateException("Native is not an override");
-                    case SINGLE -> new FixedBiomeSource(biomes.getFirst());
-                    case TILED, GRID -> new CheckerboardColumnBiomeSource(HolderSet.direct(biomes),
-                            biomePlan.distribution() == PortBiomePlan.Distribution.GRID ? 0 : 4);
-                    case TINY, SMALL, MEDIUM, LARGE, HUGE -> new PortScaledBiomeSource(
-                            HolderSet.direct(biomes), seed, biomePlan.distribution().ordinal() -
-                                    PortBiomePlan.Distribution.TINY.ordinal());
-                };
+                source = authoredBiomeSource(server, biomePlan, seed);
             }
             var settings = switch (terrain) {
                 case STANDARD -> noise.generatorSettings();
@@ -293,6 +335,7 @@ public final class RuntimeAgeBackend {
                         .getOrThrow(NoiseGeneratorSettings.AMPLIFIED);
                 case ISLAND -> server.registryAccess().lookupOrThrow(Registries.NOISE_SETTINGS)
                         .getOrThrow(NoiseGeneratorSettings.FLOATING_ISLANDS);
+                case FLAT, VOID -> throw new IllegalStateException("Flat terrain should use FlatLevelSource");
             };
             generator = new NoiseBasedChunkGenerator(source, settings);
         }

ports/mc263/src/main/java/net/dnorberg/rivencraft/registry/PortChunkGenerators.java

@@ -0,0 +1,25 @@
+package net.dnorberg.rivencraft.registry;
+
+import com.mojang.serialization.MapCodec;
+import net.dnorberg.rivencraft.Rivencraft;
+import net.dnorberg.rivencraft.age.PortFlatChunkGenerator;
+import net.minecraft.core.registries.Registries;
+import net.minecraft.world.level.chunk.ChunkGenerator;
+import net.neoforged.bus.api.IEventBus;
+import net.neoforged.neoforge.registries.DeferredRegister;
+
+/** Codec for the authored Flat/Void biome source and layer combination. */
+public final class PortChunkGenerators {
+    private static final DeferredRegister<MapCodec<? extends ChunkGenerator>> GENERATORS =
+            DeferredRegister.create(Registries.CHUNK_GENERATOR, Rivencraft.MOD_ID);
+
+    static {
+        GENERATORS.register("age_flat", () -> PortFlatChunkGenerator.CODEC);
+    }
+
+    private PortChunkGenerators() {}
+
+    public static void register(IEventBus bus) {
+        GENERATORS.register(bus);
+    }
+}

ports/mc263/src/main/java/net/dnorberg/rivencraft/writing/PortAuthoredWriting.java

@@ -19,6 +19,10 @@ public final class PortAuthoredWriting {
             Rivencraft.MOD_ID, "terrain/amplified");
     private static final Identifier ISLAND_TERRAIN = Identifier.fromNamespaceAndPath(
             Rivencraft.MOD_ID, "terrain/island");
+    private static final Identifier FLAT_TERRAIN = Identifier.fromNamespaceAndPath(
+            Rivencraft.MOD_ID, "terrain/flat");
+    private static final Identifier VOID_TERRAIN = Identifier.fromNamespaceAndPath(
+            Rivencraft.MOD_ID, "terrain/void");
 
     private PortAuthoredWriting() {}
 
@@ -28,8 +32,7 @@ public final class PortAuthoredWriting {
         int biomes = 0;
         int controllers = 0;
         for (Identifier page : pages) {
-            if ((page.equals(STANDARD_TERRAIN) || page.equals(AMPLIFIED_TERRAIN)
-                    || page.equals(ISLAND_TERRAIN)) && !terrain) {
+            if (isTerrain(page) && !terrain) {
                 terrain = true;
             } else if (controller(page) != null && controllers++ == 0) {
                 // An explicit controller needs an authored pool.
@@ -44,15 +47,22 @@ public final class PortAuthoredWriting {
         if (controllers != 0) {
             PortBiomePlan.Distribution distribution = pages.stream().map(PortAuthoredWriting::controller)
                     .filter(value -> value != null).findFirst().orElseThrow();
-            return switch (distribution) {
+            boolean valid = switch (distribution) {
                 case NATIVE -> biomes == 0;
                 case SINGLE -> biomes == 1;
                 default -> biomes >= 2;
             };
+            return valid;
         }
         return true;
     }
 
+    private static boolean isTerrain(Identifier page) {
+        return page.equals(STANDARD_TERRAIN) || page.equals(AMPLIFIED_TERRAIN)
+                || page.equals(ISLAND_TERRAIN) || page.equals(FLAT_TERRAIN)
+                || page.equals(VOID_TERRAIN);
+    }
+
     private static PortBiomePlan.Distribution controller(Identifier page) {
         if (!page.getNamespace().equals(Rivencraft.MOD_ID)) return null;
         return switch (page.getPath()) {
@@ -73,18 +83,19 @@ public final class PortAuthoredWriting {
         if (!supports(pages)) throw new IllegalArgumentException("Unsupported authored Age pages");
         if (pages.contains(AMPLIFIED_TERRAIN)) return PortAgeTerrain.AMPLIFIED;
         if (pages.contains(ISLAND_TERRAIN)) return PortAgeTerrain.ISLAND;
+        if (pages.contains(FLAT_TERRAIN)) return PortAgeTerrain.FLAT;
+        if (pages.contains(VOID_TERRAIN)) return PortAgeTerrain.VOID;
         return PortAgeTerrain.STANDARD;
     }
 
     /** Resolves all authored biomes from the active registry before creating a level. */
     public static PortBiomePlan biomePlan(List<Identifier> pages, HolderLookup.Provider registries) {
         if (!supports(pages)) throw new IllegalArgumentException(
-                "Only Standard/Amplified/Island terrain with supported Biome pages is available in this port slice");
+                "Unsupported terrain or biome-page combination in this port slice");
         List<Identifier> biomes = new ArrayList<>();
         PortBiomePlan.Distribution distribution = null;
         for (Identifier page : pages) {
-            if (page.equals(STANDARD_TERRAIN) || page.equals(AMPLIFIED_TERRAIN)
-                    || page.equals(ISLAND_TERRAIN)) continue;
+            if (isTerrain(page)) continue;
             PortBiomePlan.Distribution selected = controller(page);
             if (selected != null) { distribution = selected; continue; }
             Identifier biome = PortRegistryPages.biomeId(page);

scripts/smoke_server_263.sh

@@ -69,6 +69,10 @@ start_server first
 send_and_wait 'rivencraft_port compat' 'Registered biomes by namespace:'
 send_and_wait 'rivencraft_port age create smoke_alpha 123456789' 'Created rivencraft:age/smoke_alpha with seed 123456789'
 send_and_wait 'rivencraft_port age create smoke_beta 987654321' 'Created rivencraft:age/smoke_beta with seed 987654321'
+send_and_wait 'rivencraft_port age create smoke_flat 24680 flat' 'Created rivencraft:age/smoke_flat with seed 24680 terrain=FLAT'
+send_and_wait 'rivencraft_port age create smoke_void 13579 void' 'Created rivencraft:age/smoke_void with seed 13579 terrain=VOID'
+send_and_wait 'rivencraft_port age create_multi smoke_flat_multi 24681 flat' 'Created rivencraft:age/smoke_flat_multi with seed 24681 terrain=FLAT'
+send_and_wait 'rivencraft_port age create_multi smoke_void_multi 13580 void' 'Created rivencraft:age/smoke_void_multi with seed 13580 terrain=VOID'
 # The index must reach disk while the server is still running. A graceful
 # shutdown would hide the interrupted-save regression this checks.
 index_file="$server_dir/world/dimensions/minecraft/overworld/data/rivencraft/runtime_ages.dat"
@@ -78,26 +82,66 @@ index_file="$server_dir/world/dimensions/minecraft/overworld/data/rivencraft/run
 }
 gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_alpha'
 gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_beta'
+gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_flat'
+gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_void'
+gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_flat_multi'
+gzip -dc "$index_file" | strings | grep -Fq 'rivencraft:age/smoke_void_multi'
 send_and_wait 'rivencraft_port age sample smoke_alpha' 'rivencraft:age/smoke_alpha seed=123456789 chunkOffset=0 terrain='
 send_and_wait 'rivencraft_port age sample smoke_beta' 'rivencraft:age/smoke_beta seed=987654321 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_flat' 'rivencraft:age/smoke_flat seed=24680 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_void' 'rivencraft:age/smoke_void seed=13579 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_flat_multi' 'rivencraft:age/smoke_flat_multi seed=24681 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_void_multi' 'rivencraft:age/smoke_void_multi seed=13580 chunkOffset=0 terrain='
+send_and_wait 'execute in rivencraft:age/smoke_flat run forceload add 0 0' 'Marked chunk [0, 0] in rivencraft:age/smoke_flat to be force loaded'
+send_and_wait 'execute in rivencraft:age/smoke_void run forceload add 0 0' 'Marked chunk [0, 0] in rivencraft:age/smoke_void to be force loaded'
+send_and_wait 'execute in rivencraft:age/smoke_flat_multi run forceload add 0 0' 'Marked chunk [0, 0] in rivencraft:age/smoke_flat_multi to be force loaded'
+send_and_wait 'execute in rivencraft:age/smoke_void_multi run forceload add 0 0' 'Marked chunk [0, 0] in rivencraft:age/smoke_void_multi to be force loaded'
+send_and_wait 'execute in rivencraft:age/smoke_flat if block 0 0 0 minecraft:bedrock run say Flat bedrock starts at Y0' 'Flat bedrock starts at Y0'
+send_and_wait 'execute in rivencraft:age/smoke_flat if block 0 3 0 minecraft:grass_block run say Flat surface is Y3' 'Flat surface is Y3'
+send_and_wait 'execute in rivencraft:age/smoke_void if block 0 0 0 minecraft:air run say Void remains empty' 'Void remains empty'
+send_and_wait 'execute in rivencraft:age/smoke_flat_multi if block 0 0 0 minecraft:bedrock run say Multi Flat bedrock starts at Y0' 'Multi Flat bedrock starts at Y0'
+send_and_wait 'execute in rivencraft:age/smoke_void_multi if block 0 0 0 minecraft:air run say Multi Void remains empty' 'Multi Void remains empty'
+# Check region interiors after loading the sampled chunks.
+send_and_wait 'rivencraft_port age biome smoke_flat_multi 128 128' 'rivencraft:age/smoke_flat_multi biome at 128,128=minecraft:plains'
+send_and_wait 'rivencraft_port age biome smoke_flat_multi 384 128' 'rivencraft:age/smoke_flat_multi biome at 384,128=minecraft:forest'
+send_and_wait 'rivencraft_port age biome smoke_void_multi 384 128' 'rivencraft:age/smoke_void_multi biome at 384,128=minecraft:forest'
 alpha_before=$(grep -F 'rivencraft:age/smoke_alpha seed=123456789 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
 beta_before=$(grep -F 'rivencraft:age/smoke_beta seed=987654321 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
+flat_before=$(grep -F 'rivencraft:age/smoke_flat seed=24680 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
+void_before=$(grep -F 'rivencraft:age/smoke_void seed=13579 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
 if [[ "$alpha_before" == "$beta_before" ]]; then
     printf 'Different Age seeds produced the same terrain probe: %s\n' "$alpha_before" >&2
     exit 1
 fi
+if [[ "$flat_before" == "$void_before" ]]; then
+    printf 'Flat and Void Ages produced the same height probe: %s\n' "$flat_before" >&2
+    exit 1
+fi
 stop_server
 
 start_server restart
 send_and_wait 'rivencraft_port age list' 'smoke_alpha'
 send_and_wait 'rivencraft_port age sample smoke_alpha' 'rivencraft:age/smoke_alpha seed=123456789 chunkOffset=0 terrain='
 send_and_wait 'rivencraft_port age sample smoke_beta' 'rivencraft:age/smoke_beta seed=987654321 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_flat' 'rivencraft:age/smoke_flat seed=24680 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_void' 'rivencraft:age/smoke_void seed=13579 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_flat_multi' 'rivencraft:age/smoke_flat_multi seed=24681 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age sample smoke_void_multi' 'rivencraft:age/smoke_void_multi seed=13580 chunkOffset=0 terrain='
+send_and_wait 'rivencraft_port age biome smoke_flat_multi 384 128' 'rivencraft:age/smoke_flat_multi biome at 384,128=minecraft:forest'
+send_and_wait 'rivencraft_port age biome smoke_void_multi 384 128' 'rivencraft:age/smoke_void_multi biome at 384,128=minecraft:forest'
 alpha_after=$(grep -F 'rivencraft:age/smoke_alpha seed=123456789 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
 beta_after=$(grep -F 'rivencraft:age/smoke_beta seed=987654321 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
+flat_after=$(grep -F 'rivencraft:age/smoke_flat seed=24680 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
+void_after=$(grep -F 'rivencraft:age/smoke_void seed=13579 chunkOffset=0 terrain=' "$log_file" | tail -1 | sed 's/.*terrain=//')
 [[ "$alpha_before" == "$alpha_after" && "$beta_before" == "$beta_after" ]] || {
     printf 'Age terrain changed across restart: %s/%s -> %s/%s\n' \
         "$alpha_before" "$beta_before" "$alpha_after" "$beta_after" >&2
     exit 1
 }
+[[ "$flat_before" == "$flat_after" && "$void_before" == "$void_after" ]] || {
+    printf 'Flat/Void terrain changed across restart: %s/%s -> %s/%s\n' \
+        "$flat_before" "$void_before" "$flat_after" "$void_after" >&2
+    exit 1
+}
 stop_server
-printf '26.3 dedicated-server smoke passed: two distinct Ages persisted across restart.\n'
+printf '26.3 dedicated-server smoke passed: Standard, Flat and Void Ages persisted across restart.\n'