Writing Hooks

Writing Hooks

Overview

Writing hooks IS creating automated quality gates.

Hooks run on Claude Code events (PreToolUse, PostToolUse, etc.) and can block actions with exit code 2.

Core principle: Hooks enforce what humans forget. Fast checks only—slow hooks kill productivity.

Task Initialization (MANDATORY)

Follow task initialization protocol.

Tasks: 0. Fetch latest official hook spec

1. Analyze requirements
2. RED - Test without hook
3. GREEN - Write hook script
4. Configure settings.json
5. Validate behavior
6. Test blocking
7. REFACTOR - Quality review

Announce: "Created 8 tasks (0–7). Starting execution..."

TDD Mapping for Hooks

Task 0: Fetch Latest Official Spec

Goal: Pull the current Anthropic hook spec before designing — never trust cached memory.

Action:

Skill tool: fetching-claude-docs
  component: hook
  question: "hook events (PreToolUse, PostToolUse, etc.), matcher syntax,
             exit code contract, settings.json schema, additionalContext field,
             security considerations"

Verification: Received YAML with source: https://code.claude.com/docs/en/hooks.md and non-empty spec_excerpt. Use as authoritative reference; if any rule in this SKILL conflicts with the fetched spec, the fetched spec wins.

Task 1: Analyze Requirements

Goal: Understand what quality gate to create.

Questions to answer:

• What violation should be blocked?
• Which files should be checked?
• What tool/command performs the check?
• What event triggers the hook?
• What is the project's primary language? (check package.json, go.mod, Cargo.toml, pyproject.toml, *.csproj, etc.)

Event Selection:

Verification: Can describe the violation and the command to detect it.

Task 2: RED - Test Without Hook

Goal: Write code WITHOUT the hook. Observe violations that slip through.

Process:

1. Ask agent to write code in the target file type
2. Intentionally introduce the violation (bad format, type error, etc.)
3. Observe that Claude Code doesn't catch it
4. Document the specific violation

Verification: Documented at least 1 violation that should have been caught.

Task 3: GREEN - Write Hook Script

Goal: Create Python script that catches the violations you documented.

Hook Structure

.claude/hooks/
├── eslint_check.py
├── prettier_check.py
└── typecheck.py

Exit Code Contract

Hook Template

See references/static-checks.md for complete hook templates. For performance-optimized security hooks, see references/performance-optimization.md. Key pattern: read JSON from stdin, filter by extension, run check, exit 2 to block.

Stop Event Self-Verify (Ralph Wiggum / L-Thread)

When the agent tries to end its turn, block until deterministic checks pass — turning a one-shot agent into a loop that won't quit until the work is actually done. Use for long-running tasks, refactors, migrations, anything where "I think I'm done" is unreliable.

See references/stop-event-self-verify.md for the hook script and settings.json registration. Critical rule: always check stop_hook_active to bail after one retry — otherwise the agent loops forever on unfixable failures.

Critical Requirements

• Fast - Under 5 seconds, hooks run synchronously
• Performance constraints - Total execution < 30s, parallel processing support
• Graceful degradation - Handle timeouts without blocking development
• Filter files - Only check relevant extensions
• Limit output - First 5-10 errors, not all
• Use Python - Cross-platform, wrapped shell commands. In settings.json command, prefer uv → fallback python3 → fallback python (see cross-platform-scripts.md)
• Progressive checks - Fast/standard/thorough modes based on context
• Cross-platform - Must work on macOS, Linux, AND Windows

Windows Compatibility (MANDATORY)

Important: Read cross-platform-scripts.md for full cross-platform rules covering paths, shell commands, line endings, and common pitfalls.

Verification:

• Script is executable (chmod +x) — skip on Windows
• Returns exit code 0 for valid files
• Returns exit code 2 for violations
• Runs under 5 seconds
• Uses pathlib.Path for all path operations
• No shell=True with string commands (use list args)
• No hardcoded /tmp/ or path separators
• settings.json command uses three-runner fallback template (uv → python3 --version → python); python3 probed by --version to avoid Windows Microsoft Store stub

Task 4: Configure settings.json

Goal: Register hook in .claude/settings.json.

Important: Use .claude/settings.json, NOT settings.local.json. Settings are team-shared.

Configuration Format

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [{
          "type": "command",
          "command": "\"${CLAUDE_PROJECT_DIR}\"/.claude/hooks/eslint_check.py",
          "timeout": 30
        }]
      }
    ]
  }
}

Matcher Patterns

Verification:

• Hook registered in correct event
• Matcher targets correct tools
• Command path uses ${CLAUDE_PROJECT_DIR} (braced — see cross-platform-scripts.md)
• Timeout is reasonable (5-30 seconds)

Task 5: Validate Behavior

Goal: Verify hook script works correctly.

Test command:

echo '{"tool_input":{"file_path":"test.ts"}}' | .claude/hooks/your_hook.py
echo $?  # Should be 0 (pass) or 2 (block)

Checklist:

• Valid file returns exit code 0
• Invalid file returns exit code 2
• Errors go to stderr, info to stdout
• Completes under 5 seconds

Verification: Manual test passes for both valid and invalid inputs.

Task 6: Test Blocking

Goal: Verify hook actually blocks bad code in Claude Code.

Process:

1. Ask Claude to write code with the violation
2. Observe hook blocks the action
3. Verify error message is fed back to Claude
4. Claude should fix and retry

Verification:

• Hook blocks violating code
• Claude receives feedback and fixes the issue

Task 7: REFACTOR - Quality Review

Goal: Have hook reviewed by hook-reviewer subagent.

Agent tool:
- subagent_type: "rcc:hook-reviewer"
- prompt: "Review hook at [path/to/hook] with settings at [path/to/.claude/settings.json]"

Interpret YAML output:

• pass: true → Hook complete
• pass: false → Fix all issues listed, re-run reviewer, repeat until pass: true

This is the REFACTOR phase: Close loopholes identified by reviewer.

Verification: hook-reviewer returns YAML with pass: true.

Common Hook Patterns

See references/static-checks.md for linting, type checking, and auto-fix examples.

Red Flags - STOP

These thoughts mean you're rationalizing. STOP and reconsider:

• "Hooks are overkill for this project"
• "I'll just remember to run linting"
• "5 seconds is too strict"
• "Check all files, not just changed ones"
• "Skip testing, the logic is simple"
• "Use settings.local.json for team hooks"

All of these mean: You're about to create a weak hook. Follow the process.

Common Rationalizations

References

• references/static-checks.md - Complete hook examples
• references/events.md - Hook events reference
• references/performance-optimization.md - Performance-optimized security hooks with parallel execution
• references/ai-security-checks.md - AI-powered security validation patterns
• Scaffold script: ${CLAUDE_SKILL_DIR}/scripts/add_hook.py — scaffolds a new hook file and registers it in settings.json