Hooks: Events & System — Claude Code Recap Card M11

What is a hook

A hook is a shell script (or any command) executed automatically when Claude Code triggers an event. Comparable to git hooks, but for the agent environment. The script receives a JSON via stdin describing the event context.

Main events

Event	Moment	Can block
`SessionStart`	Startup or resume	No
`UserPromptSubmit`	Before prompt processing	Yes
`PreToolUse`	Before a tool call	Yes
`PostToolUse`	After a tool succeeds	No
`Notification`	Claude sends a notification	No
`Stop`	Claude finishes responding	Yes
`SubagentStop`	A sub-agent ends	Yes

Agent Teams events (v2.1.32+)

Event	Use
`TeammateIdle`	A team member is about to go idle
`TaskCompleted`	A task is about to be marked complete
`ConfigChange`	A config file changed during the session

These events allow implementing quality gates at the agent team level: preventing a task from being marked completed if tests fail, for example.

Configuration in settings.json

{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Edit|Write",
      "hooks": [{
        "type": "command",
        "command": ".claude/hooks/auto-format.sh",
        "timeout": 10000,
        "async": true
      }]
    }],
    "PreToolUse": [{
      "matcher": "Bash",
      "hooks": [{
        "type": "command",
        "command": ".claude/hooks/security.sh"
      }]
    }]
  }
}

Exit codes

Code	Meaning
`0`	Success, continue normally
`2`	Block the action (stderr message visible to Claude)
Other non-0	Error surfaced to Claude

For PreToolUse hooks, exit 2 prevents the tool from executing. Claude reads the stderr message and can adapt its behavior.

Synchronous vs Asynchronous

Synchronous hooks (default): Claude waits for the script to finish. Suited for validation and type checking. Asynchronous hooks (async: true): Claude continues immediately. Suited for formatting, notifications, and logging, where feedback is not needed for the next step.

stdin data flow

# Example JSON received on stdin (PostToolUse)
{
  "tool_name": "Edit",
  "tool_input": { "file_path": "src/auth.ts" },
  "tool_response": { "success": true },
  "hook_event_name": "PostToolUse"
}

The Stop event also includes a last_assistant_message field since v2.1.47, giving direct access to Claude’s last response without parsing transcripts.