Debugger

An advanced debugging specialist that helps diagnose and resolve code issues systematically.

When This Skill Activates

Activates when you:

• Report an error or bug
• Mention "debug this" or "help debug"
• Describe unexpected behavior
• Ask why something isn't working

Debugging Process

Phase 1: Understand the Problem

1. Reproduce the issue

   • What are the exact steps to reproduce?
   • What is the expected behavior?
   • What is the actual behavior?
   • What error messages appear?

2. Gather context

   # Check recent changes
   git log --oneline -10
   
   # Check error logs
   tail -f logs/error.log
   
   # Check environment
   env | grep -i debug
   

Phase 2: Isolate the Issue

1. Locate the error source

   • Stack trace analysis
   • Error code lookup
   • Log correlation

2. Narrow down scope

   • Binary search (comment out half)
   • Minimize reproduction case
   • Identify affected components

Phase 3: Analyze the Root Cause

Common Error Categories

Phase 4: Form Hypotheses

For each potential cause:

1. Form a hypothesis
2. Create a test to validate
3. Run the test
4. Confirm or reject

Phase 5: Fix and Verify

1. Implement the fix
2. Add logging if needed
3. Test the fix
4. Add regression test

Debugging Commands

General Debugging

# Find recently modified files
find . -type f -mtime -1 -name "*.js" -o -name "*.ts" -o -name "*.py"

# Grep for error patterns
grep -r "ERROR\|FATAL\|Exception" logs/

# Search for suspicious patterns
grep -r "TODO\|FIXME\|XXX" src/

# Check for console.log left in code
grep -r "console\.log\|debugger" src/

Language-Specific

JavaScript/TypeScript:

# Run with debug output
NODE_DEBUG=* node app.js

# Check syntax
node -c file.js

# Run tests in debug mode
npm test -- --inspect-brk

Python:

# Run with pdb
python -m pdb script.py

# Check syntax
python -m py_compile script.py

# Verbose mode
python -v script.py

Go:

# Race detection
go run -race main.go

# Debug build
go build -gcflags="-N -l"

# Profile
go test -cpuprofile=cpu.prof

Common Debugging Patterns

Pattern 1: Divide and Conquer

# When you don't know where the bug is:
def process():
    step1()
    step2()
    step3()
    step4()

# Comment out half:
def process():
    step1()
    # step2()
    # step3()
    # step4()

# If bug disappears, uncomment half of commented:
def process():
    step1()
    step2()
    # step3()
    # step4()

# Continue until you isolate the bug

Pattern 2: Add Logging

// Before (mysterious failure):
async function getUser(id: string) {
  const user = await db.find(id);
  return transform(user);
}

// After (with logging):
async function getUser(id: string) {
  console.log('[DEBUG] getUser called with id:', id);
  const user = await db.find(id);
  console.log('[DEBUG] db.find returned:', user);
  const result = transform(user);
  console.log('[DEBUG] transform returned:', result);
  return result;
}

Pattern 3: Minimal Reproduction

// Complex code with bug:
function processBatch(items, options) {
  // 100 lines of complex logic
}

// Create minimal reproduction:
function processBatch(items, options) {
  console.log('Items:', items.length);
  console.log('Options:', options);
  // Test with minimal data
  return processBatch([items[0]], options);
}

Error Message Analysis

Common Error Messages

Debugging Checklist

• I can reproduce the issue consistently
• I have identified the exact error location
• I understand the root cause
• I have a proposed fix
• The fix doesn't break existing functionality
• I've added a test to prevent regression

Scripts

Generate a debug report:

python scripts/debug_report.py <error-message>

References

• references/checklist.md - Debugging checklist
• references/patterns.md - Common debugging patterns
• references/errors.md - Error message reference