hook-dev

name	hook-dev
description	Use this skill when creating or refining Claude Code hooks. Hooks are shell commands that execute at specific lifecycle events (tool use, prompt submit, notifications, session events). Helps design event handlers for notifications, formatting, logging, feedback, and permission control. Automatically invoked when user requests "create a hook", "add automation", "event handler", "workflow automation", or mentions hook development.
allowed-tools	Read, Write, Edit, Grep, Bash(which:*), Bash(jq:*), Bash(osascript:*)

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

Feature	Hook	Skill	Command
Trigger	Lifecycle event	Context match	User invocation
Type	Shell command	AI capability	Prompt template
Use case	Automation, validation	Autonomous features	Reusable prompts
Control	Deterministic	LLM-driven	User-initiated

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.

Controlling Execution

Blocking with Exit Codes

For PreToolUse only - exit code 2 blocks execution:

#!/bin/bash
# Block edits to .env files
file_path=$(jq -r '.tool_input.file_path')
if [[ "$file_path" == ".env" ]]; then
  echo "ERROR: Cannot edit .env file" >&2
  exit 2  # Blocks the edit
fi
exit 0  # Allows the edit

Blocking with JSON Output

For PreToolUse, UserPromptSubmit, Stop, SubagentStop:

{
  "permissionDecision": "deny",  // PreToolUse: "allow", "deny", "ask"
  "decision": "block",            // UserPromptSubmit/Stop/SubagentStop
  "reason": "Explanation shown to user",
  "systemMessage": "Warning message",
  "updatedInput": {...},          // PreToolUse: modify tool parameters
  "continue": false,              // Stop Claude execution
  "stopReason": "User message",
  "suppressOutput": true          // Hide from transcript
}

Security Considerations

⚠️ CRITICAL: Hooks execute arbitrary shell commands automatically with your credentials.

Security checklist:

1. ✅ Review all hook commands before adding
2. ✅ Validate commands don't expose credentials
3. ✅ Avoid logging sensitive information
4. ✅ Test hooks in safe environment first
5. ✅ Limit hook scope with specific matchers
6. ✅ Always quote shell variables ("$VAR")
7. ✅ Use absolute paths and check for path traversal
8. ✅ Avoid processing .env, .git, credentials files

Hook configuration changes require explicit review via /hooks menu before affecting current sessions.

Practical Examples

1. Command Logging

Log all bash commands to audit file:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '\"\\(.tool_input.command) - \\(.tool_input.description // \\\"No description\\\")\"' | xargs -I {} sh -c 'echo \"$(date \"+%Y-%m-%d %H:%M:%S\") - {}\" >> ~/.claude-audit.log'"
          }
        ]
      }
    ]
  }
}

2. TypeScript/JavaScript Formatting

Auto-format after edits:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read file_path; if echo \"$file_path\" | grep -qE '\\.(ts|tsx|js|jsx)$'; then npx prettier --write \"$file_path\"; fi; }"
          }
        ]
      }
    ]
  }
}

3. Python Formatting

Format Python files with Black and isort:

{
  "hooks": {
    "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 \"$file_path\" && isort \"$file_path\"; fi; }"
          }
        ]
      }
    ]
  }
}

4. File Protection

Block edits to sensitive files:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read file_path; if echo \"$file_path\" | grep -qE '\\.(env|env\\..*|git/config|secrets\\.yaml)$'; then echo 'ERROR: Cannot edit protected file' >&2; exit 2; fi; }"
          }
        ]
      }
    ]
  }
}

5. macOS Notification

Alert when Claude awaits input:

{
  "hooks": {
    "Notification": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "osascript -e 'display notification \"Claude needs your input\" with title \"Claude Code\" sound name \"Glass\"'"
          }
        ]
      }
    ]
  }
}

6. Linux Notification

Desktop notification with notify-send:

{
  "hooks": {
    "Notification": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "notify-send 'Claude Code' 'Awaiting your input' -u normal -t 5000"
          }
        ]
      }
    ]
  }
}

7. Git Safety Check

Warn before force push:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.command' | { read cmd; if echo \"$cmd\" | grep -q 'git push --force'; then echo '⚠️ WARNING: Force push detected. Press Enter to continue or Ctrl+C to cancel' >&2; read; fi; }"
          }
        ]
      }
    ]
  }
}

8. Run Tests After Code Changes

{
  "hooks": {
    "Stop": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "if [ -f package.json ]; then npm run test:quick 2>&1 | head -20; fi"
          }
        ]
      }
    ]
  }
}

9. Session Context on Start

Load project info when session starts:

{
  "hooks": {
    "SessionStart": [
      {
        "matcher": "*",
        "hooks": [
          {
            "type": "command",
            "command": "echo \"📋 Project: $(basename $(pwd)) | Branch: $(git rev-parse --abbrev-ref HEAD 2>/dev/null || echo 'N/A') | Files changed: $(git status --short 2>/dev/null | wc -l)\""
          }
        ]
      }
    ]
  }
}

10. Multi-Language Formatting

Format multiple languages automatically:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read file_path; case \"$file_path\" in *.py) black \"$file_path\" && isort \"$file_path\";; *.ts|*.tsx|*.js|*.jsx) npx prettier --write \"$file_path\";; *.go) gofmt -w \"$file_path\";; *.rs) rustfmt \"$file_path\";; esac; }"
          }
        ]
      }
    ]
  }
}

Advanced Patterns

Conditional Execution

Execute only in specific conditions:

#!/bin/bash
# ~/.claude/hooks/conditional-formatter.sh

file_path=$(jq -r '.tool_input.file_path')

# Only format files in frontend/ directory
if [[ "$file_path" == frontend/* ]]; then
  case "$file_path" in
    *.ts|*.tsx)
      prettier --write "$file_path"
      ;;
    *.css)
      prettier --write "$file_path"
      ;;
  esac
fi

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit",
        "hooks": [
          {
            "type": "command",
            "command": "~/.claude/hooks/conditional-formatter.sh"
          }
        ]
      }
    ]
  }
}

Environment-Specific Hooks

Different behavior per environment:

#!/bin/bash
# Check NODE_ENV before running tests

if [[ "$NODE_ENV" == "production" ]]; then
  echo "⚠️ Production environment - running full test suite..."
  npm run test:full
else
  echo "Development environment - running quick tests"
  npm run test:quick
fi

Structured Logging with Context

Include git context in logs:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r --arg branch \"$(git rev-parse --abbrev-ref HEAD 2>/dev/null)\" --arg ts \"$(date -u +%Y-%m-%dT%H:%M:%SZ)\" '{timestamp: $ts, branch: $branch, tool: .tool_name, command: .tool_input.command}' >> ~/.claude-context.jsonl"
          }
        ]
      }
    ]
  }
}

Testing Your Hooks

1. Create Test Hook

Add to .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Read",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | xargs -I {} echo 'Reading: {}' >> /tmp/hook-test.log"
          }
        ]
      }
    ]
  }
}

2. Trigger the Hook

Ask Claude to read a file.

3. Verify Execution

cat /tmp/hook-test.log
# Should show: Reading: <file-path>

4. Test Blocking

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit",
        "hooks": [
          {
            "type": "command",
            "command": "jq -r '.tool_input.file_path' | { read fp; if [[ \"$fp\" == \"test.txt\" ]]; then echo 'Blocked test.txt' >&2; exit 2; fi; }"
          }
        ]
      }
    ]
  }
}

Attempt to edit test.txt - should be blocked.

Common Mistakes to Avoid

❌ Incorrect JSON structure:

{
  "hooks": [  // Wrong - should be object with event names as keys
    {"event": "PreToolUse", ...}
  ]
}

✅ Correct structure:

{
  "hooks": {
    "PreToolUse": [  // Event name as key
      {"matcher": "Edit", ...}
    ]
  }
}

❌ Using old environment variable syntax:

{
  "command": "black \"$file_path\""  // Variables don't work this way
}

✅ Using jq to extract from stdin:

{
  "command": "jq -r '.tool_input.file_path' | xargs -I {} black {}"
}

❌ Blocking in non-blocking events:

{
  "hooks": {
    "PostToolUse": [  // Can't block after execution
      {"matcher": "Edit", "hooks": [{"type": "command", "command": "exit 2"}]}
    ]
  }
}

❌ Logging sensitive data:

{
  "command": "jq '.' | curl -d @- https://external-log-service.com"
}

✅ Safe local logging:

{
  "command": "jq -r '{timestamp: now, tool: .tool_name}' >> ~/.local-audit.jsonl"
}

Resources

Reference the examples directory for:

• Complete hook configurations for common scenarios
• Security-focused hook patterns
• Platform-specific notification scripts
• Multi-step automation workflows

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Next Steps: After creating hooks, test in a safe environment, verify they don't leak credentials, check performance impact, and document for team members. Use /hooks menu to review and approve hook configuration changes.