building-hooks

<skill_overview> Hooks encode business rules at application level; start with observation, add automation, enforce only when patterns clear. </skill_overview>

<rigidity_level> MEDIUM FREEDOM - Follow progressive enhancement (observe → automate → enforce) strictly. Hook patterns are adaptable, but always start non-blocking and test thoroughly. </rigidity_level>

<quick_reference>

Phase	Approach	Example
1. Observe	Non-blocking, report only	Log edits, display reminders
2. Automate	Background tasks, non-blocking	Auto-format, run builds
3. Enforce	Blocking only when necessary	Block dangerous ops, require fixes

Most used events: UserPromptSubmit (before processing), Stop (after completion)

Critical: Start Phase 1, observe for a week, then Phase 2. Only add Phase 3 if absolutely necessary. </quick_reference>

<when_to_use> Use hooks for:

• Automatic quality checks (build, lint, format)
• Workflow automation (skill activation, context injection)
• Error prevention (catching issues early)
• Consistent behavior (formatting, conventions)

Never use hooks for:

• Complex business logic (use tools/scripts)
• Slow operations that block workflow (use background jobs)
• Anything requiring LLM reasoning (hooks are deterministic) </when_to_use>

<hook_lifecycle_events>

Event	When Fires	Use Cases
UserPromptSubmit	Before Claude processes prompt	Validation, context injection, skill activation
Stop	After Claude finishes	Build checks, formatting, quality reminders
PostToolUse	After each tool execution	Logging, tracking, validation
PreToolUse	Before tool execution	Permission checks, validation
ToolError	When tool fails	Error handling, fallbacks
SessionStart	New session begins	Environment setup, context loading
SessionEnd	Session closes	Cleanup, logging
Error	Unhandled error	Error recovery, notifications
</hook_lifecycle_events>

<progressive_enhancement>

Phase 1: Observation (Non-Blocking)

Goal: Understand patterns before acting

Examples:

• Log file edits (PostToolUse)
• Display reminders (Stop, non-blocking)
• Track metrics

Duration: Observe for 1 week minimum

---

Phase 2: Automation (Background)

Goal: Automate tedious tasks

Examples:

• Auto-format edited files (Stop)
• Run builds after changes (Stop)
• Inject helpful context (UserPromptSubmit)

Requirement: Fast (<2 seconds), non-blocking

---

Phase 3: Enforcement (Blocking)

Goal: Prevent errors, enforce standards

Examples:

• Block dangerous operations (PreToolUse)
• Require fixes before continuing (Stop, blocking)
• Validate inputs (UserPromptSubmit, blocking)

Requirement: Only add when patterns clear from Phase 1-2 </progressive_enhancement>

<common_hook_patterns>

Pattern 1: Build Checker (Stop Hook)

Problem: TypeScript errors left behind

Solution:

#!/bin/bash
# Stop hook - runs after Claude finishes

# Check modified repos
modified_repos=$(grep -h "edited" ~/.claude/edit-log.txt | cut -d: -f1 | sort -u)

for repo in $modified_repos; do
  echo "Building $repo..."
  cd "$repo" && npm run build 2>&1 | tee /tmp/build-output.txt

  error_count=$(grep -c "error TS" /tmp/build-output.txt || echo "0")

  if [ "$error_count" -gt 0 ]; then
    if [ "$error_count" -ge 5 ]; then
      echo "⚠️  Found $error_count errors - consider error-resolver agent"
    else
      echo "🔴 Found $error_count TypeScript errors:"
      grep "error TS" /tmp/build-output.txt
    fi
  else
    echo "✅ Build passed"
  fi
done

Configuration:

{
  "event": "Stop",
  "command": "~/.claude/hooks/build-checker.sh",
  "description": "Run builds on modified repos",
  "blocking": false
}

Result: Zero errors left behind

---

Pattern 2: Auto-Formatter (Stop Hook)

Problem: Inconsistent formatting

Solution:

#!/bin/bash
# Stop hook - format all edited files

edited_files=$(tail -20 ~/.claude/edit-log.txt | grep "^/" | sort -u)

for file in $edited_files; do
  repo_dir=$(dirname "$file")
  while [ "$repo_dir" != "/" ]; do
    if [ -f "$repo_dir/.prettierrc" ]; then
      echo "Formatting $file..."
      cd "$repo_dir" && npx prettier --write "$file"
      break
    fi
    repo_dir=$(dirname "$repo_dir")
  done
done

echo "✅ Formatting complete"

Result: All code consistently formatted

---

Pattern 3: Error Handling Reminder (Stop Hook)

Problem: Claude forgets error handling

Solution:

#!/bin/bash
# Stop hook - gentle reminder

edited_files=$(tail -20 ~/.claude/edit-log.txt | grep "^/")

risky_patterns=0
for file in $edited_files; do
  if grep -q "try\|catch\|async\|await\|prisma\|router\." "$file"; then
    ((risky_patterns++))
  fi
done

if [ "$risky_patterns" -gt 0 ]; then
  cat <<EOF
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
📋 ERROR HANDLING SELF-CHECK
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

⚠️  Risky Patterns Detected
   $risky_patterns file(s) with async/try-catch/database operations

   ❓ Did you add proper error handling?
   ❓ Are errors logged appropriately?

   💡 Consider: Sentry.captureException(), proper logging
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
EOF
fi

Result: Claude self-checks without blocking

---

Pattern 4: Skills Auto-Activation

See: xpowers:skills-auto-activation for complete implementation

Summary: Analyzes prompt keywords, injects skill activation reminder before Claude processes. </common_hook_patterns>

<hook_composition>

Naming for Order Control

Multiple hooks for same event run in alphabetical order by filename.

Use numeric prefixes:

hooks/
├── 00-log-prompt.sh       # First (logging)
├── 10-inject-context.sh   # Second (context)
├── 20-activate-skills.sh  # Third (skills)
└── 99-notify.sh           # Last (notifications)

Hook Dependencies

If Hook B depends on Hook A's output:

1. Option 1: Numeric prefixes (A before B)
2. Option 2: Combine into single hook
3. Option 3: File-based communication

Example:

# 10-track-edits.sh writes to edit-log.txt
# 20-check-builds.sh reads from edit-log.txt

</hook_composition>

<testing_hooks>

Test in Isolation

# Manually trigger
bash ~/.claude/hooks/build-checker.sh

# Check exit code
echo $?  # 0 = success

Test with Mock Data

# Create mock log
echo "/path/to/test/file.ts" > /tmp/test-edit-log.txt

# Run with test data
EDIT_LOG=/tmp/test-edit-log.txt bash ~/.claude/hooks/build-checker.sh

Test Non-Blocking Behavior

• Hook exits quickly (<2 seconds)
• Doesn't block Claude
• Provides clear output

Test Blocking Behavior

• Blocking decision correct
• Reason message helpful
• Escape hatch exists

Debugging

Enable logging:

set -x  # Debug output
exec 2>~/.claude/hooks/debug.log

Check execution:

tail -f ~/.claude/logs/hooks.log

Common issues:

• Timeout (>10 second default)
• Wrong working directory
• Missing environment variables
• File permissions </testing_hooks>

Developer adds blocking hook immediately without observation # Developer frustrated by TypeScript errors # Creates blocking Stop hook immediately:

#!/bin/bash npm run build

if [ $? -ne 0 ]; then echo "BUILD FAILED - BLOCKING" exit 1 # Blocks Claude fi

<why_it_fails>

• No observation period to understand patterns
• Blocks even for minor errors
• No escape hatch if hook misbehaves
• Might block during experimentation
• Frustrates workflow when building is slow
• Haven't identified when blocking is actually needed </why_it_fails>

**Phase 1: Observe (1 week)**

#!/bin/bash
# Non-blocking observation
npm run build 2>&1 | tee /tmp/build.log

if grep -q "error TS" /tmp/build.log; then
  echo "🔴 Build errors found (not blocking)"
fi

After 1 week, review:

• How often do errors appear?
• Are they usually fixed quickly?
• Do they cause real problems or just noise?

Phase 2: If errors are frequent, automate

#!/bin/bash
# Still non-blocking, but more helpful
npm run build 2>&1 | tee /tmp/build.log

error_count=$(grep -c "error TS" /tmp/build.log || echo "0")

if [ "$error_count" -ge 5 ]; then
  echo "⚠️  $error_count errors - consider using error-resolver agent"
elif [ "$error_count" -gt 0 ]; then
  echo "🔴 $error_count errors (not blocking):"
  grep "error TS" /tmp/build.log | head -5
fi

Phase 3: Only if observation shows blocking is necessary

Never reached - non-blocking works fine!

What you gain:

• Understood patterns before acting
• Non-blocking keeps workflow smooth
• Helpful messages without friction
• Can experiment without frustration

Hook is slow, blocks workflow #!/bin/bash # Stop hook that's too slow

Run full test suite (takes 45 seconds!)

npm test

Run linter (takes 10 seconds)

npm run lint

Run build (takes 30 seconds)

npm run build

Total: 85 seconds of blocking!

<why_it_fails>

• Hook takes 85 seconds to complete
• Blocks Claude for entire duration
• User can't continue working
• Frustrating, likely to be disabled
• Defeats purpose of automation </why_it_fails>

**Make hook fast (<2 seconds):**

#!/bin/bash
# Stop hook - fast checks only

# Quick syntax check (< 1 second)
npm run check-syntax

if [ $? -ne 0 ]; then
  echo "🔴 Syntax errors found"
  echo "💡 Run 'npm test' manually for full test suite"
fi

echo "✅ Quick checks passed (run 'npm test' for full suite)"

Or run slow checks in background:

#!/bin/bash
# Stop hook - trigger background job

# Start tests in background
(
  npm test > /tmp/test-results.txt 2>&1
  if [ $? -ne 0 ]; then
    echo "🔴 Tests failed (see /tmp/test-results.txt)"
  fi
) &

echo "⏳ Tests running in background (check /tmp/test-results.txt)"

What you gain:

• Hook completes instantly
• Workflow not blocked
• Still get quality checks
• User can continue working

Hook has no error handling, fails silently #!/bin/bash # Hook with no error handling

file=$(tail -1 ~/.claude/edit-log.txt) prettier --write "$file"

<why_it_fails>

• If edit-log.txt missing → hook fails silently
• If file path invalid → prettier errors not caught
• If prettier not installed → silent failure
• No logging, can't debug
• User has no idea hook ran or failed </why_it_fails>

**Add error handling:**

#!/bin/bash
set -euo pipefail  # Exit on error, undefined vars

# Log execution
echo "[$(date)] Hook started" >> ~/.claude/hooks/formatter.log

# Validate input
if [ ! -f ~/.claude/edit-log.txt ]; then
  echo "[$(date)] ERROR: edit-log.txt not found" >> ~/.claude/hooks/formatter.log
  exit 1
fi

file=$(tail -1 ~/.claude/edit-log.txt | grep "^/.*\.ts$")

if [ -z "$file" ]; then
  echo "[$(date)] No TypeScript file to format" >> ~/.claude/hooks/formatter.log
  exit 0
fi

if [ ! -f "$file" ]; then
  echo "[$(date)] ERROR: File not found: $file" >> ~/.claude/hooks/formatter.log
  exit 1
fi

# Check prettier exists
if ! command -v prettier &> /dev/null; then
  echo "[$(date)] ERROR: prettier not installed" >> ~/.claude/hooks/formatter.log
  exit 1
fi

# Format
echo "[$(date)] Formatting: $file" >> ~/.claude/hooks/formatter.log
if prettier --write "$file" 2>&1 | tee -a ~/.claude/hooks/formatter.log; then
  echo "✅ Formatted $file"
else
  echo "🔴 Formatting failed (see ~/.claude/hooks/formatter.log)"
fi

What you gain:

• Errors logged and visible
• Graceful handling of missing files
• Can debug when issues occur
• Clear feedback to user
• Hook doesn't fail silently

**Hooks run with your credentials and have full system access.**

Best Practices

1. Review code carefully - Hooks execute any command
2. Use absolute paths - Don't rely on PATH
3. Validate inputs - Don't trust file paths blindly
4. Limit scope - Only access what's needed
5. Log actions - Track what hooks do
6. Test thoroughly - Especially blocking hooks

Dangerous Patterns

❌ Don't:

# DANGEROUS - executes arbitrary code
cmd=$(tail -1 ~/.claude/edit-log.txt)
eval "$cmd"

✅ Do:

# SAFE - validates and sanitizes
file=$(tail -1 ~/.claude/edit-log.txt | grep "^/.*\.ts$")
if [ -f "$file" ]; then
  prettier --write "$file"
fi

<critical_rules>

Rules That Have No Exceptions

1. Start with Phase 1 (observe) → Understand patterns before acting
2. Keep hooks fast (<2 seconds) → Don't block workflow
3. Test thoroughly → Hooks have full system access
4. Add error handling and logging → Silent failures are debugging nightmares
5. Use progressive enhancement → Observe → Automate → Enforce (only if needed)

Common Excuses

All of these mean: STOP. Follow progressive enhancement.

• "Hook is simple, don't need testing" (Untested hooks fail in production)
• "Blocking is fine, need to enforce" (Start non-blocking, observe first)
• "I'll add error handling later" (Hook errors silent, add now)
• "Hook is slow but thorough" (Slow hooks block workflow, optimize)
• "Need access to everything" (Minimal permissions only) </critical_rules>

<verification_checklist> Before deploying hook:

• Tested in isolation (manual execution)
• Tested with mock data
• Completes quickly (<2 seconds for non-blocking)
• Has error handling (set -euo pipefail)
• Has logging (can debug failures)
• Validates inputs (doesn't trust blindly)
• Uses absolute paths
• Started with Phase 1 (observation)
• If blocking: has escape hatch

Can't check all boxes? Return to development and fix. </verification_checklist>

**This skill covers:** Hook creation and patterns

Related skills:

• xpowers:skills-auto-activation (complete skill activation hook)
• xpowers:verification-before-completion (quality hooks automate this)
• xpowers:testing-anti-patterns (avoid in hooks)

Hook patterns support:

• Automatic skill activation
• Build verification
• Code formatting
• Error prevention
• Workflow automation

**Detailed guides:** - [Complete hook examples](resources/hook-examples.md) - [Hook pattern library](resources/hook-patterns.md) - [Testing strategies](resources/testing-hooks.md)

Official documentation:

• Anthropic Hooks Guide

When stuck:

• Hook failing silently → Add logging, check ~/.claude/hooks/debug.log
• Hook too slow → Profile execution, move slow parts to background
• Hook blocking incorrectly → Return to Phase 1, observe patterns
• Testing unclear → Start with manual execution, then mock data