Spawn Bias

Spawn bias answers: how likely is this root to appear here?

spawn_bias can be authored on ancestry, lineage, and root files. Ancestry and lineage provide reusable place weights, and the root can provide identity-specific weights for the same places.

Field

{
  "spawn_bias": {
    "default": 0.2,
    "biomes": {
      "minecraft:dark_forest": 3.0
    },
    "biome_tags": {
      "#minecraft:is_forest": 1.5
    },
    "dimensions": {
      "minecraft:the_nether": 0.0
    }
  }
}

Field	Type	Required	Notes
default	number	no	General weight when no more specific entry applies.
biomes	object	no	Biome id to weight.
biome_tags	object	no	Biome tag id to weight. The leading # is optional.
dimensions	object	no	Dimension id to weight.

No individual field inside spawn_bias is required. Use only the weights that matter for that ancestry, lineage, or root override.

Resolution

At a spawn point, Townstead resolves the effective weight in this order:

Match	Behaviour
Exact biome	Wins over every other match.
Biome tag	Uses the highest matching tag weight.
Dimension	Used when no biome or tag matches.
default	Used when no specific entry matches.
Missing weight	Treated as 1.0.

Spawn bias from supporting files is resolved for the selected root. If the same place is weighted in more than one supporting file, the more identity-specific value is used.

Example

{
  "spawn_bias": {
    "default": 0.05,
    "biomes": {
      "minecraft:deep_dark": 2.4,
      "minecraft:dripstone_caves": 1.6,
      "minecraft:lush_caves": 1.6
    },
    "biome_tags": {
      "c:is_underground": 1.2
    }
  }
}

This is the sort of weighting a Moon Elf lineage might use. The lineage still belongs to the Elf ancestry; mixed ancestry labels belong in Heritage.