Skill 018

Hooks provide programmatic control over Claude's behavior without modifying core code, enabling project-specific automation, safety checks, and workflow customization.

.claude/hooks.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"\\(.tool_input.command) - \\(.tool_input.description // \\\"No description\\\")\"' >> ~/.claude/bash-log.txt"
          }
        ]
      }
    ]
  }
}

This hook:

• Fires before (PreToolUse) every Bash tool use
• Executes a command (not an LLM prompt)
• Logs command + description to a file

Blocking hooks can return "decision": "block" to prevent the action. See references/hook-types.md for detailed use cases.

Use when:

• Simple validation (check file exists)
• Logging (append to file)
• External tools (formatters, linters)
• Desktop notifications

Input: JSON via stdin Output: JSON via stdout (optional)

{
  "type": "command",
  "command": "/path/to/script.sh",
  "timeout": 30000
}

Use when:

• Complex decision logic
• Natural language validation
• Context-aware checks
• Reasoning required

Input: Prompt with $ARGUMENTS placeholder Output: JSON with decision and reason

{
  "type": "prompt",
  "prompt": "Evaluate if this command is safe: $ARGUMENTS\n\nReturn JSON: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
}

{
  "matcher": "Bash",           // Exact match
  "matcher": "Write|Edit",     // Multiple tools (regex OR)
  "matcher": "mcp__.*",        // All MCP tools
  "matcher": "mcp__memory__.*" // Specific MCP server
}

No matcher: Hook fires for all tools

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [...]  // No matcher - fires on every user prompt
      }
    ]
  }
}

Example output (blocking hooks):

{
  "decision": "approve" | "block",
  "reason": "Why this decision was made"
}

See references/input-output-schemas.md for complete schemas for each hook type.

Example:

{
  "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/validate.sh"
}

Block destructive git commands:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Check if this command is destructive: $ARGUMENTS\n\nBlock if it contains: 'git push --force', 'rm -rf', 'git reset --hard'\n\nReturn: {\"decision\": \"approve\" or \"block\", \"reason\": \"explanation\"}"
          }
        ]
      }
    ]
  }
}

Auto-format code after edits:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write $CLAUDE_PROJECT_DIR",
            "timeout": 10000
          }
        ]
      }
    ]
  }
}

Add context at session start:

{
  "hooks": {
    "SessionStart": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "echo '{\"hookSpecificOutput\": {\"hookEventName\": \"SessionStart\", \"additionalContext\": \"Current sprint: Sprint 23. Focus: User authentication\"}}'"
          }
        ]
      }
    ]
  }
}

This shows which hooks matched, command execution, and output. See references/troubleshooting.md for common issues and solutions.

Command vs Prompt hooks: references/command-vs-prompt.md

• Decision tree: which type to use
• Command hook patterns and examples
• Prompt hook patterns and examples
• Performance considerations

Matchers and patterns: references/matchers.md

• Regex patterns for tool matching
• MCP tool matching patterns
• Multiple tool matching
• Debugging matcher issues

Input/Output schemas: references/input-output-schemas.md

• Complete schema for each hook type
• Field descriptions and types
• Hook-specific output fields
• Example JSON for each event

Working examples: references/examples.md

• Desktop notifications
• Command validation
• Auto-formatting workflows
• Logging and audit trails
• Stop logic patterns
• Session context injection

Troubleshooting: references/troubleshooting.md

• Hooks not triggering
• Command execution failures
• Prompt hook issues
• Permission problems
• Timeout handling
• Debug workflow

• Infinite loop prevention: Check stop_hook_active flag in Stop hooks to prevent recursive triggering
• Timeout configuration: Set reasonable timeouts (default: 60s) to prevent hanging
• Permission validation: Ensure hook scripts have executable permissions (chmod +x)
• Path safety: Use absolute paths with $CLAUDE_PROJECT_DIR to avoid path injection
• JSON validation: Validate hook config with jq before use to catch syntax errors
• Selective blocking: Be conservative with blocking hooks to avoid workflow disruption

Testing protocol:

# Always test with debug flag first
claude --debug

# Validate JSON config
jq . .claude/hooks.json

• Valid JSON in .claude/hooks.json (validated with jq)
• Appropriate hook event selected for the use case
• Correct matcher pattern that matches target tools
• Command or prompt that executes without errors
• Proper output schema (decision/reason for blocking hooks)
• Tested with --debug flag showing expected behavior
• No infinite loops in Stop hooks (checks stop_hook_active flag)
• Reasonable timeout set (especially for external commands)
• Executable permissions on script files if using file paths