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.

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`

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: ConfiguredFeature provider

public class ModConfiguredFeatureProvider extends FabricDynamicRegistryProvider {
    // Or extend net.neoforged.neoforge.common.data.DatapackBuiltinEntriesProvider for NeoForge

    @Override
    protected void generate(FabricDataOutput output, RegistryWrapper.WrapperLookup registries) {
        // NeoForge: use RegistrySetBuilder to add entries to worldgen registries
    }
}

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:

# From a datapack or mod resources root:
.agents/skills/minecraft-world-generation/scripts/validate-worldgen-json.sh --root .

# Strict mode treats warnings as failures:
.agents/skills/minecraft-world-generation/scripts/validate-worldgen-json.sh --root . --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/

minecraft-world-generation

Minecraft World Generation Skill

Two Approaches to Custom Worldgen

Routing Boundaries

Directory Layout (Datapack / Mod Resources)

Custom Biome JSON

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

Configured Feature

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

Other feature types

Placed Feature

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

Common placement modifiers

Dimension Type

data/<namespace>/dimension_type/my_type.json

Custom Dimension

data/<namespace>/dimension/my_dimension.json

Multi-biome dimension with minecraft:multi_noise source

NeoForge: Biome Modifier

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

Other NeoForge biome modifier types

Fabric: BiomeModification API (Code)

Mod-Registered Worldgen (NeoForge + Fabric via Datagen)

Register worldgen keys in code

Datagen: ConfiguredFeature provider

Custom Structure

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

Template pool for jigsaw structures

Structure Set

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

Testing Worldgen

Validator Script

References

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

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

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

`data/<namespace>/dimension_type/my_type.json`

`data/<namespace>/dimension/my_dimension.json`

Multi-biome dimension with `minecraft:multi_noise` source

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

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

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