Isekai API

The library provides a set of neutral, composable worldgen primitives. Authors combine them in datapack JSON to shape biomes, define new dimensions and control where features and structures are placed, in the way small elements combine on a web page. A new dimension can reuse the vanilla dimension JSON format and ship as a complete world.

Where a feature or structure lands can be controlled through conditions such as the surface it sits on, nearby fluids or blocks, the height band, the terrain slope, the biome and boolean logic combining them. Atmosphere settings such as sky, fog and water colors are set per world. A reference preset, isekai api:hooked, overrides the overworld with one small terrain-shape hook, which replaces the large vanilla noise settings file with a short change. Java is required to run the library, but the worlds built on it need zero Java and zero vanilla edits.

Isekai API is open source under the MIT License, so it can be used, forked and bundled into modpacks freely. Source code and the issue tracker are public. It runs on client and server, so both need it. Four catalog mods, including Sky World, Mountain World, Tropical World and Tundra World, require it, and files exist for 1.21.1 only.