debug-hooks

SKILL.md

Debug Hooks

Systematic workflow for debugging Claude Code hooks.

When to Use

"Hook isn't firing"

"Hook produces wrong output"

"SessionEnd not working"

"PostToolUse hook not triggering"

"Why didn't my hook run?"

Workflow

1. Check Outputs First (Observe Before Editing)

# Check project cache
ls -la $CLAUDE_PROJECT_DIR/.claude/cache/

Check specific outputs

ls -la $CLAUDE_PROJECT_DIR/.claude/cache/learnings/

Check for debug logs

tail $CLAUDE_PROJECT_DIR/.claude/cache/*.log 2>/dev/null

Also check global (common mistake: wrong path)

ls -la ~/.claude/cache/ 2>/dev/null

### 2. Verify Hook Registration

```bash
# Project settings
cat $CLAUDE_PROJECT_DIR/.claude/settings.json | grep -A 20 '"SessionEnd"\|"PostToolUse"\|"UserPromptSubmit"'

# Global settings (hooks merge from both)
cat ~/.claude/settings.json | grep -A 20 '"SessionEnd"\|"PostToolUse"\|"UserPromptSubmit"'

3. Check Hook Files Exist

# Shell wrappers
ls -la $CLAUDE_PROJECT_DIR/.claude/hooks/*.sh

# Compiled bundles (if using TypeScript)
ls -la $CLAUDE_PROJECT_DIR/.claude/hooks/dist/*.mjs

4. Test Hook Manually

# SessionEnd hook
echo '{"session_id": "test-123", "reason": "clear", "transcript_path": "/tmp/test"}' | \
  $CLAUDE_PROJECT_DIR/.claude/hooks/session-end-cleanup.sh

# PostToolUse hook (Write tool example)
echo '{"tool_name": "Write", "tool_input": {"file_path": "test.md"}, "session_id": "test-123"}' | \
  $CLAUDE_PROJECT_DIR/.claude/hooks/handoff-index.sh

5. Check for Silent Failures

If using detached spawn with stdio: 'ignore':

// This pattern hides errors!
spawn(cmd, args, { detached: true, stdio: 'ignore' })

Fix: Add temporary logging:

const logFile = fs.openSync('.claude/cache/debug.log', 'a');
spawn(cmd, args, {
  detached: true,
  stdio: ['ignore', logFile, logFile]  // capture stdout/stderr
});

6. Rebuild After Edits

If you edited TypeScript source, you MUST rebuild:

cd $CLAUDE_PROJECT_DIR/.claude/hooks
npx esbuild src/session-end-cleanup.ts \
  --bundle --platform=node --format=esm \
  --outfile=dist/session-end-cleanup.mjs

Source edits alone don't take effect - the shell wrapper runs the bundled .mjs.

Common Issues

Symptom
Likely Cause
Fix

Hook never runs
Not registered in settings.json
Add to correct event in settings

Hook runs but no output
Detached spawn hiding errors
Add logging, check manually

Wrong session ID
Using "most recent" query
Pass ID explicitly

Works locally, not in CI
Missing dependencies
Check npx/node availability

Runs twice
Registered in both global + project
Remove duplicate

Debug Checklist

 Outputs exist? (ls -la .claude/cache/)

 Registered? (grep -A10 '"hooks"' .claude/settings.json)

 Files exist? (ls .claude/hooks/*.sh)

 Bundle current? (ls -la .claude/hooks/dist/)

 Manual test works? (echo '{}' | ./hook.sh)

 No silent failures? (check for stdio: 'ignore')

Source Sessions

Derived from 10 sessions (83% of all learnings):

a541f08a, 1c21e6c8, 6a9f2d7a, a8bd5cea, 2ca1a178, 657ce0b2, 3998f3a2, 2a829f12, 0b46cfd7, 862f6e2c