Isekai API

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.

A floating-island world in two files

Reference the shipped isekai_api:hooked_overworld preset from your dimension, then override one small terrain-shape hook file. No Java, and no copy of vanilla's 2500-line noise_settings — change the hook and the whole world changes shape. Runnable example: examples/1_shape/floating_island/.

What you can control

Every axis of world generation, as composable primitives:

• Terrain shape — 17 math/geometry density primitives + 5 worldshape composers (squeeze, y_envelope, blended_noise, band_density, sloped_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. isekai_api:vanilla_overworld_surface hands you the whole vanilla overworld surface rule as one line, so overworld replacements skip the 30 KB copy.
• 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.
• Terrain-relative re-placement — isekai_api:column_local resolves an ore's depth against each column's own surface and underside instead of one absolute Y band. Floating islands, orbiting planets and sky continents get the same internal ore layout whatever altitude they sit at.
• 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/ — shape, placement, and adaptation each have their own folder.
• Java modders: add isekai_api as a compileOnly Gradle dependency (Cursemaven, or the repo's own maven branch — see COMPATIBILITY) and use the Isekai facade. Everything outside the api package is internal, so the public surface stays small and stable. Sources and javadoc jars ship with every release.
• Extend the library itself: the five dispatch interfaces — spatial predicates, remap strategies, biome zones, surface anchors, transition rules — are registry-backed extension points. Register your own variant from your own mod id and use it in JSON alongside the built-ins. No fork, no PR.
• 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.