Minecraft World Generation Skill

Two Approaches to Custom Worldgen

Approach	Best When	Platform
Datapack JSON	Overriding/extending vanilla worldgen	Vanilla, any server
Mod + Datagen	Registering new biomes/dimensions, code-driven	NeoForge / Fabric
Biome Modifier (NeoForge)	Adding features/spawns to existing biomes	NeoForge
BiomeModification API (Fabric)	Adding features/spawns to existing biomes	Fabric

Routing Boundaries

• Use when: the task is biome/dimension/feature/structure worldgen design or registration.
• Do not use when: the task is general non-worldgen datapack work (recipes, advancements, predicates, function orchestration) (minecraft-datapack).
• Do not use when: the task is non-worldgen mod systems (items, entities, GUI, gameplay logic) (minecraft-modding).

---

Directory Layout (Datapack / Mod Resources)

data/<namespace>/
├── worldgen/
│   ├── biome/
│   │   └── my_biome.json
│   ├── configured_feature/
│   │   └── my_ore.json
│   ├── placed_feature/
│   │   └── my_ore_placed.json
│   ├── noise_settings/
│   │   └── my_dimension_noise.json
│   ├── density_function/
│   │   └── my_density.json    (advanced)
│   ├── structure/
│   │   └── my_structure.json
│   ├── structure_set/
│   │   └── my_structures.json
│   ├── processor_list/
│   │   └── my_processors.json
│   ├── template_pool/
│   │   └── my_pool.json
│   └── carver/
│       └── my_carver.json
├── dimension/
│   └── my_dimension.json
├── dimension_type/
│   └── my_type.json
├── tags/
│   └── worldgen/
│       └── biome/
│           └── is_forest.json
└── neoforge/
    └── biome_modifier/      (NeoForge mod only)
        └── add_ores.json

---

Custom Biome JSON

data/<namespace>/worldgen/biome/my_biome.json

{
  "has_precipitation": true,
  "temperature": 0.7,
  "temperature_modifier": "none",
  "downfall": 0.8,
  "effects": {
    "sky_color": 7907327,
    "fog_color": 12638463,
    "water_color": 4159204,
    "water_fog_color": 329011,
    "grass_color_modifier": "none",
    "ambient_sound": "minecraft:ambient.cave",
    "mood_sound": {
      "sound": "minecraft:ambient.cave",
      "tick_delay": 6000,
      "block_search_extent": 8,
      "offset": 2.0
    }
  },
  "spawners": {
    "monster": [
      { "type": "minecraft:zombie", "weight": 95, "minCount": 4, "maxCount": 4 },
      { "type": "minecraft:skeleton", "weight": 100, "minCount": 4, "maxCount": 4 }
    ],
    "creature": [
      { "type": "minecraft:sheep", "weight": 12, "minCount": 4, "maxCount": 4 }
    ],
    "ambient": [],
    "axolotls": [],
    "underground_water_creature": [],
    "water_creature": [],
    "water_ambient": [],
    "misc": []
  },
  "spawn_costs": {},
  "carvers": {
    "air": ["minecraft:cave", "minecraft:cave_extra_underground", "minecraft:canyon"]
  },
  "features": [
    [],
    [],
    ["minecraft:lake_lava_underground", "minecraft:lake_lava_surface"],
    ["minecraft:amethyst_geode", "minecraft:monster_room"],
    [],
    [],
    [
      "minecraft:ore_dirt", "minecraft:ore_gravel", "minecraft:ore_granite_upper",
      "minecraft:ore_coal_upper", "minecraft:ore_coal_lower",
      "<namespace>:my_ore_placed"
    ],
    [],
    ["minecraft:spring_lava"],
    [],
    ["minecraft:freeze_top_layer"]
  ]
}

The features array has exactly 11 slots (indices 0–10), one per GenerationStep.Decoration:

Index	Step	Put here
0	RAW_GENERATION	(rarely used)
1	LAKES	Surface water/lava lakes
2	LOCAL_MODIFICATIONS	Underground lava lakes, geodes
3	UNDERGROUND_STRUCTURES	Amethyst geodes, dungeons
4	SURFACE_STRUCTURES	Glaciers, blue ice patches
5	STRONGHOLDS	(unused in biome JSON)
6	UNDERGROUND_ORES	All ores go here
7	UNDERGROUND_DECORATION	Fossils, infested stone
8	FLUID_SPRINGS	spring_water, spring_lava
9	VEGETAL_DECORATION	Trees, grass, flowers
10	TOP_LAYER_MODIFICATION	freeze_top_layer

Custom ores added via placed features must be placed at index 6.

---

Configured Feature

data/<namespace>/worldgen/configured_feature/my_ore.json

{
  "type": "minecraft:ore",
  "config": {
    "targets": [
      {
        "target": {
          "predicate_type": "minecraft:tag_match",
          "tag": "minecraft:stone_ore_replaceables"
        },
        "state": {
          "Name": "minecraft:emerald_ore"
        }
      }
    ],
    "size": 4,
    "discard_chance_on_air_exposure": 0.0
  }
}

Other feature types

Type	Use
minecraft:ore	Ore veins
minecraft:tree	Tree placement
minecraft:random_patch	Grass, flowers, mushrooms
minecraft:block_pile	Hay bales, pumpkins
minecraft:lake	Water/lava lakes
minecraft:disk	Sand/gravel/clay disks
minecraft:no_bonemeal_flower	Wither roses, etc.
minecraft:simple_block	Single block placement
minecraft:fill_layer	Fill an entire layer
minecraft:geode	Amethyst geodes
minecraft:decorated	Wraps another feature with placement

---

Placed Feature

data/<namespace>/worldgen/placed_feature/my_ore_placed.json

{
  "feature": "<namespace>:my_ore",
  "placement": [
    {
      "type": "minecraft:count",
      "count": 8
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:height_range",
      "height": {
        "type": "minecraft:trapezoid",
        "min_inclusive": { "above_bottom": 0 },
        "max_inclusive": { "absolute": 64 }
      }
    },
    {
      "type": "minecraft:biome"
    }
  ]
}

Common placement modifiers

Type	Effect
minecraft:count	Number of attempts
minecraft:count_on_every_layer	Per layer
minecraft:in_square	Randomize X/Z within chunk
minecraft:biome	Only place if biome has this feature
minecraft:height_range	Y-level range
minecraft:surface_relative_threshold_filter	Filter by surface depth
minecraft:noise_based_count	Count varies with noise
minecraft:rarity_filter	1-in-N chance
minecraft:environment_scan	Scans up/down for a condition

---

Dimension Type

data/<namespace>/dimension_type/my_type.json

{
  "ultrawarm": false,
  "natural": true,
  "coordinate_scale": 1.0,
  "has_skylight": true,
  "has_ceiling": false,
  "ambient_light": 0.0,
  "fixed_time": false,
  "monster_spawn_light_level": {
    "type": "minecraft:uniform",
    "min_inclusive": 0,
    "max_inclusive": 7
  },
  "monster_spawn_block_light_limit": 0,
  "piglin_safe": false,
  "bed_works": true,
  "respawn_anchor_works": false,
  "has_raids": true,
  "logical_height": 384,
  "height": 384,
  "min_y": -64,
  "infiniburn": "#minecraft:infiniburn_overworld",
  "effects": "minecraft:overworld"
}

---

Custom Dimension

data/<namespace>/dimension/my_dimension.json

{
  "type": "<namespace>:my_type",
  "generator": {
    "type": "minecraft:noise",
    "biome_source": {
      "type": "minecraft:fixed",
      "biome": "<namespace>:my_biome"
    },
    "settings": "minecraft:overworld"
  }
}

Multi-biome dimension with minecraft:multi_noise source

{
  "type": "<namespace>:my_type",
  "generator": {
    "type": "minecraft:noise",
    "biome_source": {
      "type": "minecraft:multi_noise",
      "biomes": [
        {
          "parameters": {
            "temperature": [ -1.0, -0.45 ],
            "humidity":    [ -1.0, -0.35 ],
            "continentalness": [ -1.2, -1.05 ],
            "erosion":     [ -0.78, 0.0 ],
            "weirdness":   [ 0.0, 0.0 ],
            "depth":       [ 0.0, 0.0 ],
            "offset":      0.0
          },
          "biome": "<namespace>:my_biome"
        }
      ]
    },
    "settings": "minecraft:overworld"
  }
}

---

NeoForge: Biome Modifier

Biome Modifiers let you add features, spawns, or carvers to existing biomes without replacing the biome JSON.

JSON biome modifier (data/<namespace>/neoforge/biome_modifier/add_ores.json)

{
  "type": "neoforge:add_features",
  "biomes": "#minecraft:is_overworld",
  "features": "<namespace>:my_ore_placed",
  "step": "underground_ores"
}

Other NeoForge biome modifier types

{ "type": "neoforge:add_spawns", "biomes": "#minecraft:is_forest",
  "spawners": [{ "type": "minecraft:wolf", "weight": 5, "minCount": 2, "maxCount": 4 }] }

{ "type": "neoforge:remove_features", "biomes": "#minecraft:is_plains",
  "features": "minecraft:ore_coal_upper", "steps": ["underground_ores"] }

{ "type": "neoforge:remove_spawns", "biomes": "#minecraft:is_ocean",
  "entity_types": "#minecraft:skeletons" }

---

Fabric: BiomeModification API (Code)

import net.fabricmc.fabric.api.biome.v1.BiomeModifications;
import net.fabricmc.fabric.api.biome.v1.BiomeSelectors;
import net.minecraft.world.level.levelgen.GenerationStep;

public class MyModWorldgen {
    public static void init() {
        // Add a placed feature to all overworld biomes
        BiomeModifications.addFeature(
            BiomeSelectors.foundInOverworld(),
            GenerationStep.Decoration.UNDERGROUND_ORES,
            ResourceKey.create(
                Registries.PLACED_FEATURE,
                ResourceLocation.fromNamespaceAndPath(MyMod.MOD_ID, "my_ore_placed")
            )
        );

        // Add mob spawns
        BiomeModifications.addSpawn(
            BiomeSelectors.tag(BiomeTags.IS_FOREST),
            MobCategory.CREATURE,
            EntityType.WOLF,
            5, 2, 4
        );
    }
}

---

Mod-Registered Worldgen (NeoForge + Fabric via Datagen)

Register worldgen keys in code

// In a dedicated worldgen registry class
public class ModWorldgen {
    public static final ResourceKey<Biome> MY_BIOME = ResourceKey.create(
        Registries.BIOME,
        ResourceLocation.fromNamespaceAndPath(MyMod.MOD_ID, "my_biome")
    );

    public static final ResourceKey<PlacedFeature> MY_ORE_PLACED = ResourceKey.create(
        Registries.PLACED_FEATURE,
        ResourceLocation.fromNamespaceAndPath(MyMod.MOD_ID, "my_ore_placed")
    );
}

Datagen: NeoForge (DatapackBuiltinEntriesProvider)

public class ModWorldgenProvider extends DatapackBuiltinEntriesProvider {

    private static final RegistrySetBuilder BUILDER = new RegistrySetBuilder()
        .add(Registries.CONFIGURED_FEATURE, ModWorldgenProvider::bootstrapConfigured)
        .add(Registries.PLACED_FEATURE, ModWorldgenProvider::bootstrapPlaced);

    public ModWorldgenProvider(PackOutput output, CompletableFuture<HolderLookup.Provider> registries) {
        super(output, registries, BUILDER, Set.of(MyMod.MOD_ID));
    }

    private static void bootstrapConfigured(BootstrapContext<ConfiguredFeature<?, ?>> ctx) {
        ctx.register(
            ModWorldgen.MY_ORE_CONFIGURED,
            new ConfiguredFeature<>(Feature.ORE, new OreConfiguration(
                OreConfiguration.target(
                    new TagMatchTest(BlockTags.STONE_ORE_REPLACEABLES),
                    ModBlocks.MY_ORE.get().defaultBlockState()
                ),
                9  // vein size
            ))
        );
    }

    private static void bootstrapPlaced(BootstrapContext<PlacedFeature> ctx) {
        HolderGetter<ConfiguredFeature<?, ?>> configured =
            ctx.lookup(Registries.CONFIGURED_FEATURE);
        ctx.register(
            ModWorldgen.MY_ORE_PLACED,
            new PlacedFeature(
                configured.getOrThrow(ModWorldgen.MY_ORE_CONFIGURED),
                List.of(
                    HeightRangePlacement.triangle(
                        VerticalAnchor.absolute(-64),
                        VerticalAnchor.absolute(32)
                    ),
                    CountPlacement.of(8),
                    InSquarePlacement.spread(),
                    BiomeFilter.biome()
                )
            )
        );
    }
}

Register in your GatherDataEvent handler:

@SubscribeEvent
public static void onGatherData(GatherDataEvent event) {
    DataGenerator gen = event.getGenerator();
    PackOutput output = gen.getPackOutput();
    gen.addProvider(event.includeServer(),
        new ModWorldgenProvider(output, event.getLookupProvider()));
}

Datagen: Fabric (FabricDynamicRegistryProvider)

public class ModWorldgenProvider extends FabricDynamicRegistryProvider {

    public ModWorldgenProvider(FabricDataOutput output, CompletableFuture<HolderLookup.Provider> registries) {
        super(output, registries);
    }

    @Override
    protected void configure(HolderLookup.Provider registries, Entries entries) {
        entries.addAll(registries.lookupOrThrow(Registries.CONFIGURED_FEATURE));
        entries.addAll(registries.lookupOrThrow(Registries.PLACED_FEATURE));
    }

    @Override
    public String getName() {
        return "Worldgen";
    }
}

---

Custom Structure

data/<namespace>/worldgen/structure/my_structure.json

{
  "type": "minecraft:jigsaw",
  "biomes": "#<namespace>:my_biome_tag",
  "step": "surface_structures",
  "terrain_adaptation": "beard_thin",
  "start_pool": "<namespace>:my_pool/start",
  "size": 6,
  "max_distance_from_center": 80,
  "use_expansion_hack": false,
  "spawn_overrides": {}
}

Template pool for jigsaw structures

{
  "fallback": "minecraft:empty",
  "elements": [
    {
      "weight": 1,
      "element": {
        "element_type": "minecraft:single_pool_element",
        "location": "<namespace>:my_structure/start",
        "projection": "rigid",
        "processors": "minecraft:empty"
      }
    }
  ]
}

---

Structure Set

data/<namespace>/worldgen/structure_set/my_structures.json

{
  "structures": [
    {
      "structure": "<namespace>:my_structure",
      "weight": 1
    }
  ],
  "placement": {
    "type": "minecraft:random_spread",
    "spacing": 32,
    "separation": 8,
    "salt": 12345678
  }
}

---

Testing Worldgen

# Create a new world with datapack loaded
# In-game with a mod:
/locate structure <namespace>:my_structure
/locate biome <namespace>:my_biome
/placefeature <namespace>:my_ore_placed

# Reload worldgen data (does not affect already-generated chunks)
/reload

# For dimension testing — must use /execute in <namespace>:my_dimension
execute in <namespace>:my_dimension run tp @s 0 100 0

Validator Script

Use the bundled validator before shipping worldgen JSON changes:

# Run from the installed skill directory (for example `.agents/skills/minecraft-world-generation`):
./scripts/validate-worldgen-json.sh --root /path/to/datapack-or-mod-resources

# Strict mode treats warnings as failures:
./scripts/validate-worldgen-json.sh --root /path/to/datapack-or-mod-resources --strict

What it checks:

• JSON validity for worldgen/** and neoforge/biome_modifier/**
• Cross-reference integrity for placed_feature -> configured_feature
• Cross-reference integrity for structure_set -> structure and biome/biome_modifier feature targets

---

References

• Minecraft Wiki — World generation: https://minecraft.wiki/w/Custom_world_generation
• Minecraft Wiki — Biome: https://minecraft.wiki/w/Biome/JSON_format
• Minecraft Wiki — Features: https://minecraft.wiki/w/World_generation/Configured_feature
• NeoForge Biome Modifiers: https://docs.neoforged.net/docs/worldgen/biomemodifier/
• Fabric BiomeModifications: https://wiki.fabricmc.net/tutorial:biomemodification
• misode's data pack generator (worldgen UI): https://misode.github.io/worldgen/