Biome Modifiers are a data-driven system for modifying biomes. They have the ability to modify biomes in several ways:

Adding or removing worldgen features and carvers
Adding or removing mob spawns and mob spawn costs
Modifying the climate of a biome
Modifying a biome's client effects, such as water color

Biome Modifier Types

To define a new type of biome modifier, begin by implementing a new class that extends BiomeModifier. BiomeModifiers can usually be implemented as records, to reduce boilerplate.

public record ExampleBiomeModifier() implements BiomeModifier
{
  public void modify(Holder<Biome> biome, Phase phase, Builder builder)
  {
    // This allows modifications to the given biome via the provided Builder.
  }

  public Codec<? extends BiomeModifier> codec()
  {
    // This must return a registered Codec, see Biome Modifier Serializers below.
  }
}

Beware! The modify method is called once per each phase per biome per biome modifier, so it's important to check which phase you're in and only make your changes in one phase.

public void modify(Holder<Biome> biome, Phase phase, Builder builder)
{
  if (phase == Phase.ADD)
  {
    // add things to biomes
  }
}

Typically we also want to restrict biome modifiers to only apply to certain biomes. We can do that by accepting a HolderSet<Biome> in our constructor:

public record ExampleBiomeModifier(HolderSet<Biome> biomes) implements BiomeModifier
{
  public void modify(Holder<Biome> biome, Phase phase, Builder builder)
  {
    if (phase == Phase.ADD && biomes.contains(biome))
    {
      // add things to biomes
    }
  }

We can also accept Holders or HolderSets for other datapack registry elements, such as PlacedFeatures, which allows our BiomeModifier to refer to those elements, which can then be defined in their own JSON files.

public record ExampleBiomeModifier(HolderSet<Biome> biomes, Holder<PlacedFeature> feature) implements BiomeModifier
{
  public void modify(Holder<Biome> biome, Phase phase, Builder builder)
  {
    // add a feature to all specified biomes
    if (phase == Phase.ADD && biomes.contains(biome))
    {
      builder.getGenerationSettings().addFeature(Decoration.UNDERGROUND_ORES, feature);
    }
  }

Biome Modifier Serializers

Each type of biome modifier must have a Codec registered for it; Deferred Registers are the recommended way to register biome modifier codecs.

public class YourMod
{
  static DeferredRegister<Codec<? extends BiomeModifier>> BIOME_MODIFIER_SERIALIZERS =
    DeferredRegister.create(ForgeRegistries.Keys.BIOME_MODIFIER_SERIALIZERS, "yourmodid");

  static RegistryObject<Codec<ExampleBiomeModifier>> EXAMPLE_CODEC = BIOME_MODIFIER_SERIALIZERS.register("example", () ->
    RecordCodecBuilder.create(builder -> builder.group(
        // declare fields
        Biome.LIST_CODEC.fieldOf("biomes").forGetter(ExampleBiomeModifier::biomes),
        PlacedFeature.CODEC.fieldOf("feature").forGetter(ExampleBiomeModifier::feature)
      // declare constructor
      ).apply(builder, ExampleBiomeModifier::new)));
}

We can now implement the codec() method in our biome modifier class:

public record ExampleBiomeModifier(HolderSet<Biome> biomes, Holder<PlacedFeature> feature) implements BiomeModifier
{
  public void modify(Holder<Biome> biome, Phase phase, Builder builder)
  {
    // add a feature to all specified biomes
    if (phase == Phase.ADD && biomes.contains(biome))
    {
      builder.getGenerationSettings().addFeature(Decoration.UNDERGROUND_ORES, feature);
    }
  }

  public Codec<? extends BiomeModifier> codec()
  {
    return YourMod.EXAMPLE_CODEC.get();
  }
}

Biome Modifier JSONs

Once we've registered a codec for our biome modifier type, we can define jsons for it. Biome modifier jsons must be defined in the directory data/modid/forge/biome_modifier/; a biome modifier at data/modid/forge/biome_modifier/your_biome_modifier.json has the namespaced id modid:your_biome_modifier.

Our codec above defines the json format:

"biomes" is a special HolderSet field that accepts a single biome id, [list of biome ids], or #biome_tag.
"feature" accepts a single placed feature id. These can be defined in jsons as well.
We must also specify "type": "yourmodid:example" to tell the data loader to use our registered codec to read our json.

A json instance of our biome modifier that adds some feature to badlands biomes might look like this:

{
  "type": "yourmodid:example",
  "biomes": "#minecraft:is_badlands",
  "feature": "yourmod:some_feature"
}

Side Note on HolderSets

HolderSets are a vanilla feature that, when defined in JSON, can be specified as a single id, list of ids, or tag. Our example above uses a holderset of biomes, so all three of these are valid biome fields:

{
  "biomes": "forest",
  "biomes": ["forest", "birch_forest"],
  "biomes": "#is_forest"
}

Builtin Biome Modifier Types

None

Forge provides a no-op biome modifier type, whose jsons have the following format:

{
  "type": "forge:none"
}

This allows pack devs or server operators to disable mods' biome modifiers by overriding their biome modifier jsons with the above.

Best Practices

Avoid using biome modifiers to add vanilla placed features to biomes, as this may cause a feature cycle violation (the game will crash if two biomes have the same two features in their feature lists but in different orders). Placed features can be referenced in biome jsons or added via biome modifiers, but should not be used in both.