Generally available in Claude Code 2.1.287+ (on by default) as of Addy Osmani's October 1, 2026 getting-started guide on claude.dev. A mod is a normal plugin with one module listed in hooks/hooks.json; the module exports register(on) and uses a $ host API (no DOM, no Node). Hooks form a middleware chain and can observe, rewrite, or answer events. First-party features including AGENTS.md support and the /diff pane are themselves mods. Distinct from settings hooks, which run a shell command per event. Install only from publishers you trust: a mod has the same access as Claude Code.