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A camera shown only as the screen's own view was never prepared: the note required a non-empty picture list. Two runs picking the same local id both claimed the screen's frame, though only the first one's camera was drawn from it. The client now notes whether the frame was a world frame and the (run, camera id) the screen's world was drawn from, so the main-view milestone belongs to the run whose selection the screen used, and a title or loading frame reports the path off without erasing earlier milestones. A world transition clears the submission, the main-view selection and the milestones: another world's cameras are not drawn. A camera given to set-main-view drops its picture target, so its milestone starts over until a frame is drawn from it. prepared stays a host-side encoding milestone, not target registration, not a GPU fence, not a visible pixel. Also gate the client's note on the `mods` feature: without it the default horizon-client build did not compile. Verified: horizon-mods 37 unit, 44 sandbox (1 ignored; real Wasmtime guests, the two-run test shows each id 7 follows only its own pictures and selection) and 4 api_reference; client checks and build with and without `mods`; gfx lib tests. Co-Authored-By: Kimi Code <noreply@moonshot.cn>
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 shows these pages at playhorizon.dev/docs/api, built from this folder, and a summary for players and authors at playhorizon.dev/docs/apis (in the website's repository, horizon-rust/horizon-site).