Minecraft Custom Biome Data Pack: Make One in 26.3

Written for Minecraft 26.3.

A minecraft custom biome data pack is one JSON file in data/<namespace>/worldgen/biome/. The file sets the biome's temperature, rainfall, water colour and what generates in it. You can then paint the biome into your world with a command.

Before you start

• Minecraft Java Edition 26.3 (data pack format 121.0).

• A text editor. Any editor that saves plain JSON works.

• A new world or a copy of your world. Read Pack basics first if you have never made a pack.

A data pack is a folder in your world's datapacks folder. It holds a pack.mcmeta file and a data folder. A namespace is just your own folder name inside data, such as mypack.

Create the data pack

1. Make the folder <world>/datapacks/mybiomes. Inside it create pack.mcmeta with the content below.

2. Create data/mypack/worldgen/biome/ inside the pack, then a file named glow_meadow.json.

3. Paste the biome JSON below into that file and save it.

4. Restart the server. Worldgen files load at start-up, so a restart is the safe way to pick them up.

{
  "pack": {
    "description": "My custom biomes",
    "min_format": [121, 0],
    "max_format": [121, 0]
  }
}

Write the biome file

This is the smallest biome that passes the format. Every key shown is required:

{
  "has_precipitation": true,
  "temperature": 0.8,
  "downfall": 0.4,
  "effects": {
    "water_color": 4159204
  },
  "carvers": [],
  "features": []
}

• has_precipitation: true for rain or snow, false for none. Temperature decides which.

• temperature and downfall: floats. They tint grass and foliage.

• water_color: a colour as one decimal number. 4159204 is the normal value. Use our colour converter to turn a hex code into a number.

• carvers and features: lists of caves and generated things (trees, ores, flowers). Both may be empty. Add the ids of vanilla features to copy a vanilla look.

• grass_color, foliage_color and grass_color_modifier (none, dark_forest or swamp) are optional extras inside effects.

Features are ordered by generation step: raw generation, lakes, local modifications, underground structures, surface structures, strongholds, underground ores, underground decoration, fluid springs, vegetal decoration and top layer modification. Vanilla lists one array per step, 11 in total, and an empty list is allowed.

Mob spawns and sky/fog colours moved. Since 25w42a the sky, fog and sound keys are no longer in effects, and in 26.3 the mob spawn lists were moved to environment attributes. We did not verify the attribute format, so check the Minecraft Wiki biome definition page before setting spawns or fog.

Put the biome in your world

The simplest route is to paint it onto existing terrain. Stand in the area and run, with your own coordinates:

/fillbiome ~-10 ~-10 ~-10 ~10 ~10 ~10 mypack:glow_meadow

It needs permission level 2 and only changes biome data, not blocks. The area must be loaded and its volume cannot exceed the commandModificationBlockLimit game rule. In 26.3 the biome changes in 4x4x4 cells, not per block. Add replace <filter> to change only chosen biomes.

To make the biome generate in new chunks, it has to be listed in a dimension's biome source, which is more advanced and not covered here (we did not verify that format for 26.3).

Check it worked

1. Run the /fillbiome command. Success is a count above 0.

2. Look at grass and water colour in the painted area. Press F3 and read the Biome line.

3. Run /locate biome mypack:glow_meadow. It finds the biome only where it exists in the world.

Common problems

The command says the biome is unknown

The pack did not load. Check the folder path, the JSON for stray commas, and that you restarted.

The server will not start

A required key is missing or has the wrong type. Compare against the example above and read the console error.

The colours changed but old chunks look the same

Biome colour blends across borders, and /fillbiome works in 4x4x4 cells. Paint a bigger area.

Nothing generates in the biome

The empty features list is why. Add feature ids to the step arrays.

Related: custom structures to give your biome landmarks.