Ludovic Marie
c9f554d1c4
A server mod runs where the world is simulated. Its manifest says side = "server" and api = "0.3" (only a server mod may), and its component targets the new WIT world horizon-server-mod: - imports log and storage as client mods do, and the new capability players: online() (id, name, position, dimension, game mode, health), tell(id, text), broadcast(text), 8 lines per call shared with chat; - exports server-events: init, tick, player-joined, player-left, chat, command (with the player who typed it; its feedback goes back to that player), block-broken and block-placed (with the block state as text). horizon-mods: a second bindgen world sharing log and storage with the first; a running mod holds either kind of hooks; Mods::load_for(Runs) starts only the mods a game runs (the window client mods, the world's server server mods, in-frame single player both) and marks the others Elsewhere. Client and server commands are owned separately. The game passes Runs::Client for now, so nothing changes in game until its server code calls these hooks. The SDK gains horizon_mod_sdk::server (ServerMod, Registration, export_server_mod!), the example server mod welcome (greets, announces newcomers kept in storage, /online, /mined), and cargo horizon-mod check's trial run plays a server mod with a player joining, chatting, typing its commands and leaving. The manifest refuses client capabilities on a server mod and players on a client mod. The generated reference, the guide, the portal and the stability table describe it. 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).