ModSmith

Claude Code ships its own mod authoring: the plugin-authoring skill and the generated declarations (claude-code.d.ts, written beside every mod as .claude-plugin/types/claude-code/index.d.ts and by /plugin-types). Those are the API. This skill does not re-teach it. It adds four things on top:

1. Patterns that work, as complete templates you copy and change.
2. Cost and prompt-cache review: what a mod costs per turn, what breaks the cache, and how to vet someone else's mod before installing it.
3. Composition: how mods share a site, state and the cache.
4. Decision notes: every mod ships a DECISIONS.md of what was chosen, what was rejected and what is still unverified.

Harnesses go out of date fast. When anything here disagrees with the generated declarations for the running build, the declarations win. Load the built-in plugin-authoring skill for any API question.

Step 1: Surface the unknowns