horizon-airdef/docs/api/mods.md

Mod API

Mods are Rust crates compiled to sandboxed WebAssembly components. They run in the player's game and, once server mods land, wherever a world is simulated. Design and the runtime: mods.md.

Overview

A mod today can read the local player and the world clock, show chat lines only that player sees, draw a text panel on the HUD, keep small files of its own, register chat commands, and run on every tick, frame or chat line it asks for. A content mod adds blocks and items from JSON in its .hmod, copying vanilla templates, plus tags, recipes, loot, models, textures and structures in its own namespaces. Every capability is declared in the manifest, shown to the player before the mod is turned on, and checked against the component's real imports.

	Count
Interfaces the game offers	6: log, game, chat, hud, storage; players for server mods
Functions in them	12
Hooks a mod exports	5: init, tick, frame, command, chat; a server mod 8: init, tick, player-joined, player-left, chat, command, block-broken, block-placed
Capabilities	6: game-read, chat, hud, storage, registries, players
Manifest api lines served	3: 0.1 (code), 0.2 (content), 0.3 (server mods, experimental)
SDK	the Mod trait (5 methods), Registration (4 builders), export_mod!, log helpers; server::ServerMod (8 methods), server::Registration (5 builders), export_server_mod!
cargo horizon-mod	new, build, run, test, check
Example mods	coords-hud (code), ruby-content (content), welcome (server)

Guide

1. rustup target add wasm32-unknown-unknown, cargo install --git https://github.com/ludovic111/horizon cargo-horizon-mod, then cargo horizon-mod new my-mod (Making a mod).
2. Implement Mod for your type, list the capabilities you import in horizon-mod.toml (The manifest).
3. cargo horizon-mod run builds the .hmod and starts the game with it as a dev mod; after an edit, cargo horizon-mod build and the running game reloads it.
4. cargo horizon-mod test runs the mod's tests/*.toml scripts in the sandbox, with no game: ticks, commands, chat lines and the player's position in, chat, feedback and the HUD panel checked (Testing a mod).
5. Content mods: Content mods (API 0.2).

Reference

mods-reference.md, generated from crates/horizon-mod-sdk/wit/horizon.wit: every capability, interface, type, function and hook, with the limits the guide lists (16 HUD lines of 256 characters, 8 chat lines per call, 4 MiB of storage, fuel per call).

Examples

• crates/horizon-mod-sdk/examples/coords-hud: coordinates, facing and time on the HUD; /where, /coords; storage.
• crates/horizon-mod-sdk/examples/ruby-content: no code; a gem and a block of ruby from vanilla templates, with their recipes, loot, tags, models, textures and names.
• crates/horizon-mod-sdk/examples/welcome: a server mod; greets players who join, announces newcomers (kept in storage), /online and /mined.
• crates/horizon-mods/tests/guest: a mod that misbehaves on command (spins, panics, grows its memory), for the sandbox tests.

Ways in

One API (the WIT world) and one sandbox (WebAssembly), whatever the mod is written in. Every way in produces a component for the same world, checked against the same capabilities and given the same limits; there is no second scripting runtime in the game.

Way in	For	Status	Template and example
Rust	most mods; the fastest	first-class: the SDK, cargo horizon-mod	cargo horizon-mod new, coords-hud
Lua scripts	people who don't want a compiler	planned: a script mod whose code is a .lua file, run by Horizon's own interpreter mod	planned, with the same example as Rust
C, C++, Zig	authors of those languages	to evaluate: bindings generated from the WIT, built without WASI	one each if it passes
Go (TinyGo), AssemblyScript, JavaScript / TypeScript	authors of those languages	to evaluate: each needs its toolchain to emit a component without WASI imports, or an interpreter mod as for Lua	one each if it passes

• Scripts run inside the sandbox. The Lua interpreter is itself a WebAssembly mod, built in Rust with the SDK and shipped by Horizon. A script mod's manifest names the interpreter and its own capabilities; the game links the interpreter with only those, so a script can do exactly what a compiled mod with the same manifest can.
• Hot reload. The game keeps the interpreter compiled and loads the script into a fresh instance when the file changes, without restarting the game; what it saved with storage survives, as for dev mods.
• Measured, not guessed. Each way in is listed with its measured cost on the same tick-heavy example (time and fuel per tick, memory, load time) against Rust, and a language is supported only with a template, an example and a test that builds and loads it in CI.

Stability

Preview (stability.md): API 0.1 and content 0.2 are served together; a mod built for 0.1.0 keeps loading on every 0.1.x game. Server mods are their own line, api = "0.3", experimental: they may change in any release.

Not there yet

What real mods need next, in the order modpacks and servers need it:

• Server mods in the game (api = "0.3", Server mods): the contract, runtime and SDK are here; next the server thread, LAN host and dedicated server start them and call their hooks, then entity events, game rules and server commands with typed arguments.
• World and blocks: read blocks, block states and biomes in loaded chunks; change them through the server's block log, so clients and the parity harnesses see vanilla's updates.
• Items and inventories: the player's inventory, held item and open container; item stacks with components.
• Entities: query and change entities near a position; spawn from a mod's own entity types (data first).
• Block entities and menus: ticking machines with slots and their screens.
• Screens and UI: widgets built from vanilla's, a screen the mod owns, a settings page under Options > Mods.
• Networking: a channel between a mod's two halves, client and server, typed by the mod's own WIT.
• Config: typed settings the game shows and stores for the mod, on the server too.
• Keys and the crosshair target: keybinds in Controls; what the player is looking at.
• Script mods and other languages: see Ways in.
• SDK: templates per kind of mod, typed wrappers in the SDK for every interface. Dev mods already reload when rebuilt, and cargo horizon-mod test runs a mod headless.

Changelog

• API 0.3 (server mods, experimental): world horizon-server-mod, capability players, the server-events hooks; SDK server::ServerMod; example welcome.
• SDK: cargo horizon-mod test and horizon_mods::testing, test scripts in TOML; the template has one.
• API 0.2 (content): data-driven blocks and items, tags, horizon:add_to, terrain adaptation.
• API 0.1.0: log, game, chat, hud, storage; init, tick, frame, command, chat.