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horizon-airdef/docs/api/mods-reference.md
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Ludovic Marie c9f554d1c4 Mod API 0.3 (experimental): server mods' contract, runtime and SDK 
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>
2026-10-25 15:51:15 +01:00

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<!-- Generated from crates/horizon-mod-sdk/wit/horizon.wit and the manifest's capabilities by crates/horizon-mods/tests/api_reference.rs; a test fails when this file is out of date. Update it with: HORIZON_WRITE_DOCS=1 cargo test -p horizon-mods --features pack --test api_reference -->
# Mod API reference
The contract between Horizon and its mods: the WIT package `horizon:mods@0.1.0`, every interface, type, function and hook in it, and the capabilities a manifest may list. The guide is [mods.md](../mods.md); stability and versions: [stability.md](stability.md). Manifest `api` lines this game serves: `0.1`, `0.2`, `0.3`.
## Capabilities
| In `capabilities` | Grants | The player reads "May …" |
| --- | --- | --- |
| `game-read` | [`game`](#game) | see your position, health and the time |
| `chat` | [`chat`](#chat) | show you chat messages |
| `hud` | [`hud`](#hud) | draw text on your screen |
| `storage` | [`storage`](#storage) | save its own settings to disk |
| `registries` | data files (`assets/`, `data/`), no interface | add blocks and items to your worlds |
| `players` | [`players`](#players) | see who is on the server and message them |
## World `horizon-mod`
A client mod: runs in the player's game.
- Imports (what the game offers): `log`, `game`, `chat`, `hud`, `storage`
- Exports (what the mod provides): `events`
## World `horizon-server-mod`
A server mod (manifest `side = "server"`, `api = "0.3"`, experimental): runs where the world is simulated, on the dedicated server, a LAN host and single player's own server.
- Imports (what the game offers): `log`, `storage`, `players`
- Exports (what the mod provides): `server-events`
## `log`
Access: always granted.
The mod's log, in the game's log under the mod's id. Always granted.
### enum `level`
| Case | |
| --- | --- |
| `debug` | |
| `info` | |
| `warn` | |
| `error` | |
| Function | Signature | What it does |
| --- | --- | --- |
| `write` | `func(level: level, message: string)` | One line in the game's log as `[mod <id>] message`, at that level. |
## `game`
Access: capability `game-read`.
Capability `game-read`: the local player and the world clock.
### record `player`
| Field | Type | |
| --- | --- | --- |
| `x` | `f64` | Feet position, in blocks. |
| `y` | `f64` | |
| `z` | `f64` | |
| `yaw` | `f32` | Degrees, as F3 shows them. |
| `pitch` | `f32` | Degrees: -90 looks straight up, 90 straight down. |
| `health` | `f32` | Hit points as vanilla counts them: 20 is full health without effects. |
| `food` | `u32` | 0 to 20, as the hunger bar. |
| `dimension` | `string` | `minecraft:overworld`, `minecraft:the_nether`, ... |
### record `clock`
| Field | Type | |
| --- | --- | --- |
| `game-time` | `u64` | Ticks since the world was created. |
| `day-time` | `u64` | 24000 ticks per day; 0 is 6:00 on the first day. |
| Function | Signature | What it does |
| --- | --- | --- |
| `get-player` | `func() -> option<player>` | None on the title screen. |
| `get-clock` | `func() -> option<clock>` | None on the title screen. |
## `chat`
Access: capability `chat`.
Capability `chat`: messages only the local player sees.
| Function | Signature | What it does |
| --- | --- | --- |
| `show` | `func(text: string)` | Shown in chat after the mod's name, like a system message. |
## `hud`
Access: capability `hud`.
Capability `hud`: one panel of text lines per mod, drawn by the game every frame until the mod changes or clears it (no call per frame).
### enum `corner`
| Case | |
| --- | --- |
| `top-left` | |
| `top-right` | |
| `bottom-left` | |
| `bottom-right` | |
### record `line`
| Field | Type | |
| --- | --- | --- |
| `text` | `string` | Plain text, up to 256 characters. |
| `color` | `u32` | 0xRRGGBB. |
| Function | Signature | What it does |
| --- | --- | --- |
| `set-panel` | `func(corner: corner, lines: list<line>)` | Replaces the mod's panel: up to 16 lines in that corner of the screen. |
| `clear` | `func()` | Removes the mod's panel. |
## `storage`
Access: capability `storage`.
Capability `storage`: a per-mod folder of small key/value files, quota'd (docs/mods.md). Keys are `[a-z0-9_.-]{1,64}`, one path segment.
| Function | Signature | What it does |
| --- | --- | --- |
| `get` | `func(key: string) -> option<list<u8>>` | None if `key` has never been `set`, or the key is not well-formed. |
| `set` | `func(key: string, value: list<u8>) -> result<_, string>` | Errs on a malformed key or over quota (bytes for the whole mod). |
| `delete` | `func(key: string)` | No error if `key` was already absent. |
## `events`
Access: exported by the mod: the game calls it.
What the game calls. Only the hooks `init` asks for are ever called.
### record `registration`
| Field | Type | |
| --- | --- | --- |
| `commands` | `list<string>` | Chat commands, without the slash. A name vanilla already uses is refused (vanilla's command runs). |
| `tick` | `bool` | Call `tick` every game tick (20 per second while playing). |
| `frame` | `bool` | Call `frame` every rendered frame. Budget: 0.1 ms. |
| `chat` | `bool` | Call `chat` for each chat line the player receives. |
| Function | Signature | What it does |
| --- | --- | --- |
| `init` | `func() -> result<registration, string>` | Once, after loading. An error keeps the mod off and is shown. |
| `tick` | `func(tick: u64)` | Each game tick while in a world, with the game's tick counter. |
| `frame` | `func(seconds: f32)` | Each rendered frame, with the seconds since the last one. |
| `command` | `func(name: string, args: string) -> result<option<string>, string>` | `/name args`: ok(some(text)) is shown as feedback, err(text) in red. |
| `chat` | `func(text: string)` | Each chat line the player receives, as plain text. |
## `players`
Access: capability `players`.
Capability `players`, server mods only (manifest `api = "0.3"`, experimental): the players on the server, and messages to them.
### enum `game-mode`
| Case | |
| --- | --- |
| `survival` | |
| `creative` | |
| `adventure` | |
| `spectator` | |
### record `player`
| Field | Type | |
| --- | --- | --- |
| `id` | `string` | The player's UUID, `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`. |
| `name` | `string` | |
| `x` | `f64` | Feet position, in blocks. |
| `y` | `f64` | |
| `z` | `f64` | |
| `dimension` | `string` | `minecraft:overworld`, `minecraft:the_nether`, ... |
| `game-mode` | `game-mode` | |
| `health` | `f32` | Hit points as vanilla counts them: 20 is full health without effects. |
| Function | Signature | What it does |
| --- | --- | --- |
| `online` | `func() -> list<player>` | The players online, in the order they joined. |
| `tell` | `func(player: string, text: string) -> bool` | A system message to one player, after the mod's name; false when no player with that id is online. Up to 8 messages per call. |
| `broadcast` | `func(text: string)` | A system message to every player and the server's log, after the mod's name. Counts toward the same 8 per call. |
## `server-events`
Access: exported by the mod: the game calls it.
What the server calls in a server mod (manifest `api = "0.3"`, experimental). Only the hooks `init` asks for are ever called. The player in each event is as the server saw them when it happened.
### type `player`
`type player = player`
### record `registration`
| Field | Type | |
| --- | --- | --- |
| `commands` | `list<string>` | Commands players may type, without the slash. A name vanilla already uses is refused (vanilla's command runs). |
| `tick` | `bool` | Call `tick` every server tick (20 per second). |
| `joins` | `bool` | Call `player-joined` and `player-left`. |
| `chat` | `bool` | Call `chat` for each chat message a player sends. |
| `blocks` | `bool` | Call `block-broken` and `block-placed`. |
### record `block-pos`
| Field | Type | |
| --- | --- | --- |
| `x` | `s32` | |
| `y` | `s32` | |
| `z` | `s32` | |
| Function | Signature | What it does |
| --- | --- | --- |
| `init` | `func() -> result<registration, string>` | Once, after loading. An error keeps the mod off and is logged. |
| `tick` | `func(tick: u64)` | Each server tick, with the server's tick counter. |
| `player-joined` | `func(player: player)` | A player joined the server; they are in `players.online` already. |
| `player-left` | `func(player: player)` | A player left the server; they are no longer in `players.online`. |
| `chat` | `func(player: player, text: string)` | A chat message a player sent, as plain text. |
| `command` | `func(player: player, name: string, args: string) -> result<option<string>, string>` | `/name args` typed by `player`: ok(some(text)) is their feedback, err(text) their error in red. |
| `block-broken` | `func(player: player, pos: block-pos, state: string)` | A player broke the block at `pos` in their dimension: `state` is what was there, as `minecraft:oak_log[axis=y]`. |
| `block-placed` | `func(player: player, pos: block-pos, state: string)` | A player placed `state` at `pos` in their dimension. |
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