For agents

Rules for a coding agent making a Horizon mod. They are short on purpose; each links to its page.

1. Choose the shape. A content mod has no Cargo.toml. A code mod has Cargo.toml and src/ at the top and implements Hooks with #[mod_api::horizon_mod]. Never write raw injections first.
2. Use a hook or a Part method when one exists (search the rustdoc of horizon-mod-api). A per-block, per-item or per-entity moment is a Part method (Parts).
3. Otherwise follow the injection ladder: an API [[inject]] declaration naming a crate-root function and its mod_points record; use access = "internals" and declared internals only when the API cannot express it. Every injection comes with a hook request. API injections never rebuild the game: missing slots are refused, and source rebuilds and call-site injections require internals access or a hook request.
4. Names: NeoForge 26.3 names for hooks that have a counterpart; no on_ prefix; the event is &mut; handles come from the event; writes take host.
5. Layout: horizon-mod.toml, assets/, data/, Cargo.toml and src/ at the top. The id and version come only from horizon-mod.toml; ids are [a-z0-9_]. No native/ folder, no horizon-native.json.
6. Manifest fields: id, name, version, description, authors, api = "0.2", side, capabilities = ["registries"], [dependencies], top-level internals and [[inject]], [native] (game, horizon, mixins, optional module) or [wasm], [[jar]]. One of [native] or [wasm], never both. Unknown fields are refused (manifest).
7. Freshness errors: a changed function hash or call context refuses the injection; use inject update or reselect it with inject add. Never substitute a different call.
8. Data and assets are read through a reload hook, once per reload, never on the tick.
9. Missing hook? File a hook request on arismoko/horizon.