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

Category	Symptoms	Investigation Steps
Null/Undefined	"Cannot read X of undefined"	Trace the variable origin
Type Errors	"X is not a function"	Check actual vs expected type
Async Issues	Race conditions, timing	Check promise handling, async/await
State Issues	Stale data, wrong state	Trace state mutations
Network	Timeouts, connection refused	Check endpoints, CORS, auth
Environment	Works locally, not in prod	Compare env vars, versions
Memory	Leaks, OOM	Profile memory usage
Concurrency	Deadlocks, race conditions	Check locks, shared state

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

Error	Likely Cause	Solution
Cannot read property 'X' of undefined	Accessing property on null/undefined	Add null check, use optional chaining
X is not a function	Wrong type, shadowing	Check typeof, verify import
Unexpected token	Syntax error	Check line before error, validate syntax
Module not found	Import path wrong	Check relative path, verify file exists
EADDRINUSE	Port already in use	Kill existing process, use different port
Connection refused	Service not running	Start service, check port
Timeout	Request too slow	Increase timeout, check network

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