Isekai API is a toolbox for the shape and rules of a Minecraft world — not a world, but the machine that makes worlds. Like div/span for web pages, it's a small set of neutral, composable primitives you combine to build any world you can imagine, straight from datapack JSON. It ships no biomes, structures, or themes of its own.

What you can control

Every axis of world generation, as composable primitives:

Terrain shape — 16 math/geometry density primitives + 4 worldshape composers (squeeze, y_envelope, blended_noise, band_density). Floating islands, hanging or inverted continents, hollow shells, capped mountains, mirrored/tiled space — any 3D form on any Y band.
Biome placement — the isekai_api:rule biome source places biomes by pure spatial rules: vertical layers, concentric rings, regions, and and/or/not combos (y_below 20 -> deep_dark). No climate-noise tuning.
Surface & fill — re-skin any biome's top block or stone fill from JSON.
New dimensions — combine the above with vanilla dimension JSON to ship a whole new dimension. Zero Java, zero vanilla edits.
Atmosphere — per-world sky, fog, and water colors.
Set-pieces, features & trees — build neutral content from composable parts: drop a hand-authored NBT set-piece only on flat, dry ground (grounded_template), grow connected block clusters and fluid pools, and assemble decay-safe trees from any trunk + foliage placer. No themes baked in — you supply the blocks.
Placement control — decide exactly where features/structures go: relative to surface or fluid, by block context, Y range, slope, proximity to a block or biome, and logical combos.
Re-place existing content — take any vanilla or modded ore, structure, mob, or feature and redistribute it into your world's new shape. A rare ore vanilla buries deep can be remapped to the underside of your floating islands. Strategies: linear, inverted, fixed-range, count-scaling, band-split, pipelines — plus exclude/add.
Boot-time validation — broken or mistyped registry references are caught at server start, before they silently no-op.
Java API — Isekai.query() reads vanilla/modded worldgen rules; Isekai.remap() declares worldshapes in code.

Built to be built on

Isekai isn't tied to any one world or author. However you want to use it, you're welcome — datapack worldshapes, full Java mods, modpack glue, quick experiments, total conversions. MIT licensed: free to use, fork, and bundle in modpacks.

Datapack authors: the Datapack Reference lists every key with its JSON shape; copy a runnable skeleton from examples/ (sky_archipelago, flipped, moon_world).
Java modders: add isekai_api as a compileOnly dependency and use the Isekai facade. Everything outside the api package is internal, so the public surface stays small and stable.
Questions, ideas, bugs: open an issue. Built a world with Isekai? Share it — I'd love to see what people make.

MIT · Source & docs. Unrelated to the "Isekai Adventure" modpack.