Ludovic Marie
608d1a70a7
Three stages, each a WGSL function with a fixed signature that Horizon calls inside its own pass: sky (fn sky(input: SkyInput) -> vec4<f32>), material (fn material(input: MaterialInput) -> Surface) and post (fn post(input: PostInput) -> vec4<f32>). A stage's WGSL is a generated prelude (the frame uniform, the stage's structs, its resources in bind group 3, the post helpers), the pack's shared files, its own file, then the entry points; each part is located so errors point at the pack's file and line. The frame is the Rust struct the renderer will upload; its WGSL, its offsets and docs/api/shaders-reference.md are generated from it, and tests check that the preludes compile, that naga's layout of the WGSL is the Rust layout, that the sample pack's stages compile, and that the reference is up to date. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Horizon APIs
Horizon has six surfaces that other programs and people build on. Each one has an API (the contract: what you may call, read or write, and what Horizon promises about it) and an SDK (the tools that help you use it). This section has one page per surface, plus the rules every surface follows: stability and versioning.
| Surface | For | API today | Stability | Reference |
|---|---|---|---|---|
| Mods | mod authors | WIT horizon:mods@0.1.0, manifest api 0.1 (code) and 0.2 (content) |
preview | mods-reference.md (generated) |
| Shaders | shader authors | none yet: packs were removed on 2026-10-24 | planned | none |
| Agent control | agents, scripts, tests | horizon-ctl, its MCP server, the game's control socket |
preview | horizon-ctl-commands.md (generated) |
| Servers | hosts, panels, bots | horizon-server commands, server.toml, control API /api/v1 |
stable | server-reference.md (generated) |
| Launcher | players' agents, pack makers | horizon-launcher commands and its MCP server |
preview | launcher-commands.md (generated) |
| Data formats | tools that read Horizon's files | benchmark reports, telemetry, settings files, signed checksums | stable or preview, per format | on that page |
How these APIs are designed
- From the people who use them. Each API starts from what a mod author, a pack author, a server host or an agent needs to do, written down as tasks ("a host backs up the world from a script"), and from how Horizon is built: Rust, wgpu, WebAssembly components and a server thread that runs every world. A function exists because a task needs it.
- Our own design. Horizon's APIs are not modelled on any other project's. Contributors may know other mod
loaders, renderers and server platforms exist, but their source and API references are never read, copied or
paraphrased here, and our names, shapes and docs are not taken from theirs. Vanilla Minecraft 26.3 is the reference for game
concepts: ids, commands,
server.properties, data packs and resource packs. - Safe by default. Mods run in a sandbox and can only use the capabilities their manifest declares; the control sockets listen on this computer only, behind a token; nothing here is on by default where it would change vanilla's look or behaviour.
- Free when unused. An API nobody uses costs nothing per tick or per frame: no engine, no thread, no allocation.
- One contract for every mode. In-frame single player, the server thread, LAN and the dedicated server run the same gameplay code (AGENTS.md), so a server-side API serves all of them.
References are generated
A reference page is written by code from the source it describes, and a test fails when the two differ:
| Page | Generated from | Update it with |
|---|---|---|
| mods-reference.md | crates/horizon-mod-sdk/wit/horizon.wit, the manifest's capabilities |
HORIZON_WRITE_DOCS=1 cargo test -p horizon-mods --features pack --test api_reference |
| server-reference.md | horizon-server's help, config::TEMPLATE, api::ROUTES |
HORIZON_WRITE_DOCS=1 cargo test -p horizon-server reference |
| horizon-ctl-commands.md | the command table in crates/horizon-client/src/bin/horizon-ctl/commands.rs |
horizon-ctl help --markdown > docs/horizon-ctl-commands.md |
| launcher-commands.md | launcher/src/cli.rs |
HORIZON_WRITE_DOCS=1 cargo test --manifest-path launcher/Cargo.toml the_launcher_reference |
So a pull request that changes an API also changes its reference, and the reviewer sees the API's diff in the reference's diff. The guides (overview, how-to, examples) are written by hand.
Each surface's page
Every page has the same parts: Overview (what you can build), Guide (where to start), Reference, Examples, Stability (the level of each part, per stability.md), Changelog and Not there yet (what is planned, so nobody builds on a guess).
The website's summary for players and authors is playhorizon.dev/docs/apis
(website/docs/apis.md).