Hooks: Event-Driven Automation

Hook Events

• Hooks are shell commands, HTTP endpoints, or LLM prompts that run automatically when the AI takes specific actions — think git hooks, but for AI tool usage. Declarative safety that can't be overridden by context loss.
• **SessionStart** — fires once when AI session begins. Check Node version, verify MCP connections, validate config files.
• **PreToolUse** — fires BEFORE a tool call. Block dangerous operations, validate parameters. Can REJECT the tool call — the AI must find another approach.
• **PostToolUse** — fires AFTER a tool call. Cache results, validate outputs, log actions. Can provide feedback to Claude via `additionalContext`.
• **Stop** — fires when the AI finishes. Can block termination to force continuation (check `stop_hook_active` to prevent infinite loops).
• Additional events (20+ total)
  • › **SubagentStart/Stop** — observe or block subagent lifecycle
  • › **UserPromptSubmit** — validate or enrich user prompts before processing
  • › **PostToolUseFailure** — catch tool errors and provide recovery guidance
  • › **FileChanged** — react to watched file changes (e.g., `.envrc`)
  • › **PermissionRequest** — auto-approve or deny permission dialogs
  • › See [official docs](https://code.claude.com/docs/en/hooks) for the full list

When to Use Each

1. 1 Real examples from our project
   • › PreToolUse blocks `git commit` on protected branches (development, main) — uses `if` field for precision
   • › PostToolUse saves Figma screenshots to disk automatically
   • › PostToolUse validates plugin file edits — uses `if: "Edit(**/.claude-plugin/**)"` to skip non-plugin edits
   • › PostToolUse logs Chrome DevTools screenshots asynchronously (`async: true`)
2. 2 Look at your project's hooks: `.claude/hooks/` or plugin `hooks.json` — if none exist, start with a branch protection hook
3. 3 Start with PreToolUse for safety — it's the most valuable hook type because it prevents mistakes structurally instead of relying on AI instructions
4. 4 Key difference: PreToolUse can block (exit **2** for blocking error), PostToolUse can observe and enrich. Exit 1 is a non-blocking error (verbose mode only)
5. 5 Use `statusMessage` to show spinner text while hooks run, and `async: true` for observational hooks that don't need to block