Hook Dev Skill

This skill helps create production-ready Claude Code hooks following Anthropic's official specifications.

What are Hooks?

Hooks are user-defined shell commands that execute at various points in Claude Code's lifecycle. They provide:

• Deterministic control: Ensure actions happen automatically, not relying on LLM decisions
• Workflow automation: Format code, run linters, log commands
• Custom notifications: Alert when Claude needs input
• Permission systems: Block sensitive file modifications
• Feedback loops: Validate code changes against conventions

Hook vs Skill vs Command

When to use Hooks: Automatic formatting, logging, notifications, permission control, code validation

Hook Events

Ten lifecycle events trigger hooks:

1. PreToolUse

Executes after Claude creates tool parameters but before processing.

Use cases:

• Block edits to sensitive files (.env, production configs)
• Validate bash commands before execution
• Request confirmation for destructive operations
• Log all tool usage for auditing
• Modify tool inputs programmatically

Blocking capability: Yes - exit code 2 blocks with error fed to Claude; JSON output with "permissionDecision": "deny" also blocks

2. PostToolUse

Runs immediately after successful tool completion.

Use cases:

• Format code after edits (Prettier, Black, gofmt)
• Run linters after file modifications
• Sync changes to external systems
• Update indexes or caches

Blocking capability: No - tool already executed

3. UserPromptSubmit

Fires when users submit prompts, before Claude processes them.

Use cases:

• Inject context automatically (git status, env variables)
• Log user interactions
• Validate prompt safety
• Add project-specific context

Blocking capability: Yes - JSON output with "decision": "block" prevents submission

4. PermissionRequest

Activates when permission dialogs appear to users.

Use cases:

• Auto-approve safe operations
• Auto-deny dangerous operations
• Log permission requests
• Provide context for decisions

Blocking capability: Yes - can allow, deny, or pass through to user

5. Notification

Triggers when Claude Code sends notifications.

Use cases:

• Custom notification sounds
• Desktop notifications (macOS, Linux, Windows)
• Send to Slack/Discord
• Visual alerts

Blocking capability: No - notification already generated

6. Stop

Activates when the main agent finishes responding.

Use cases:

• Run tests after code generation
• Update documentation
• Commit changes automatically
• Trigger deployments

Blocking capability: Yes - JSON output with "decision": "block" prevents stop

7. SubagentStop

Runs when subagent tasks complete.

Use cases:

• Log subagent results
• Validate subagent outputs
• Trigger next workflow steps
• Aggregate subagent data

Blocking capability: Yes - JSON output with "decision": "block" prevents stop

8. PreCompact

Executes before context window compaction operations.

Use cases:

• Save conversation state
• Export conversation history
• Archive important context

Blocking capability: Yes - can prevent compaction

9. SessionStart

Triggers at session initialization or resumption.

Use cases:

• Load project context
• Initialize environment variables
• Display project status
• Check for updates

Blocking capability: No - session already started

10. SessionEnd

Runs when sessions terminate.

Use cases:

• Save session state
• Cleanup temporary files
• Export logs
• Trigger cleanup scripts

Blocking capability: No - session already ending

Hook Configuration

Hooks are configured in .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'Editing file' >> /tmp/claude-audit.log"
          }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read file_path; if echo \"$file_path\" | grep -q '\\.py$'; then black \"$file_path\"; fi; }"
          }
        ]
      }
    ]
  }
}

Configuration Structure

{
  "hooks": {
    "EventName": [                     // Event type (PreToolUse, PostToolUse, etc.)
      {
        "matcher": "ToolPattern",      // Tool name, regex (pipe-separated), or "*" for all
        "hooks": [                     // Array of hooks for this matcher
          {
            "type": "command",         // "command" or "prompt"
            "command": "shell command", // Shell command to execute
            "timeout": 60              // Optional timeout in seconds (default: 60)
          }
        ]
      }
    ]
  }
}

Configuration Locations

• User-level: ~/.claude/settings.json (all projects)
• Project-level: .claude/settings.json (this project, team-shared)
• Local: .claude/settings.local.json (uncommitted, project-specific overrides)
• Plugin-level: plugin-name/hooks/hooks.json (bundled with plugin)

Matchers

Tool Matching

Exact match:

{
  "matcher": "Edit"
}

Regex pattern (use pipe for OR):

{
  "matcher": "Edit|Write|MultiEdit"
}

Wildcard (match all tools):

{
  "matcher": "*"
}

Common tool names: Bash, Read, Write, Edit, MultiEdit, Glob, Grep, Task, WebFetch, WebSearch, TodoRead, TodoWrite, NotebookRead, NotebookEdit

MCP tools: Use mcp__<server>__<tool> pattern (e.g., mcp__github__create_issue)

Accessing Tool Data

Hook input arrives via stdin as JSON. Use jq to extract values:

# Access file_path from Edit tool
jq -r '.tool_input.file_path'

# Access bash command
jq -r '.tool_input.command'

# Check if file matches pattern
jq -r '.tool_input.file_path' | { read file_path; if echo "$file_path" | grep -q '\.py$'; then echo "Python file"; fi; }

# Access tool response (PostToolUse only)
jq -r '.tool_response'

Common JSON fields:

• .session_id - Current session identifier
• .transcript_path - Path to conversation transcript
• .cwd - Current working directory
• .tool_name - Name of the tool being used
• .tool_input - Tool parameters (varies by tool)
• .tool_response - Tool output (PostToolUse only)
• .hook_event_name - Name of the event

Environment variables:

• $CLAUDE_PROJECT_DIR - Project root absolute path
• $CLAUDE_ENV_FILE - For SessionStart hooks to persist environment variables
• $CLAUDE_CODE_REMOTE - "true" if remote execution, empty if local
• ${CLAUDE_PLUGIN_ROOT} - Plugin directory path (plugin hooks only). Use this to reference scripts bundled with your plugin:

  {
    "type": "command",
    "command": "${CLAUDE_PLUGIN_ROOT}/scripts/process.sh"
  }
  

Hook Types

Command Hooks

Execute bash scripts with stdin JSON input:

{
  "type": "command",
  "command": "jq -r '.tool_input.file_path' | xargs -I {} prettier --write {}"
}

Exit codes:

• 0 - Success; stdout shown to user (except UserPromptSubmit shows to Claude)
• 2 - Blocking error; stderr fed to Claude as context
• Other - Non-blocking error; stderr shown to user

Prompt Hooks

Send input to LLM (Haiku) for decisions (PreToolUse, Stop, SubagentStop, UserPromptSubmit):

{
  "type": "prompt",
  "prompt": "Analyze this tool use and decide if it should be allowed: {{input}}"
}

Returns JSON with decision fields.

Cont