How agent hooks work

When an agent hook fires:

1. Claude Code spawns a subagent with your prompt and the hook's JSON input
2. The subagent can use tools like Read, Grep, and Glob to investigate
3. After up to 50 turns, the subagent returns a structured \{ "ok": true/false \} decision
4. Claude Code allows the action if ok is true. If ok is false, Claude Code handles the block the same way as a prompt hook with continueOnBlock: true on that event, as listed under Response schema

Agent hooks are useful when verification requires inspecting actual files or test output, not just evaluating the hook input data alone.