for tool events: PreToolUse, PostToolUse, PermissionRequest.

### Hook Input (stdin JSON)
```json
{
  "session_id": "abc123",
  "tool_name": "Write",
  "tool_input": { "file_path": "/path/to/file.txt", "content": "..." },
  "tool_response": { "success": true }  // PostToolUse only
}
```

### Hook JSON Output

Hooks can return JSON to control behavior:

```json
{
  "systemMessage": "Warning shown to user in UI",
  "continue": false,
  "stopReason": "Message shown when blocking",
  "additionalContext": "Context injected back to model",
  "decision": "approve" | "block"
}
```

**Fields:**
- `systemMessage` - Display a message to the user (all hooks)
- `continue` - Set to `false` to block/stop (default: true)
- `stopReason` - Message shown when `continue` is false
- `additionalContext` - Text injected into model context (event-specific)
- `decision` - "approve" or "block" for PreToolUse hooks

### Common Patterns

**Auto-format after writes:**
```json
{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "command",
        "command": "jq -r '.tool_response.filePath // .tool_input.file_path' | xargs prettier --write 2>/dev/null || true"
      }]
    }]
  }
}
```

**Log all bash commands:**
```json
{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Bash",
      "hooks": [{
        "type": "command",
        "command": "jq -r '.tool_input.command' >> ~/.claude/bash-log.txt"
      }]
    }]
  }
}
```

**Stop hook that displays message to user:**

Command must output JSON with `systemMessage` field:
```bash
# Example command that outputs: {"systemMessage": "Session complete!"}
echo '{"systemMessage": "Session complete!"}'
```

**Run tests after code changes:**
```json
{
  "hooks": {
    "PostToolUse": [{
      "matcher": "Write|Edit",
      "hooks": [{
        "type": "command",
        "command": "jq -r '.tool_input.file_path // .tool_response.filePath' | grep -E '\\.(ts|js)$' && npm test || true"
      }]
    }]
  }
}
```
WŸ