Debugging Skill

Guide for debugging with targeted log injection and runtime analysis.

When to Activate

Suggest /debug-tools:debug when users describe:

• "X is not working"
• "Getting error Y when doing Z"
• "Something broke after [change]"
• Silent failures or unexpected behavior

Log Format

console.log('[DEBUG] [file:line] description', { values });

• [DEBUG] - Prefix for grep and cleanup
• [file:line] - Location for navigation
• description - What this log checks
• { values } - Relevant data (no sensitive info)

Log Patterns

React/Next.js

// Lifecycle
console.log('[DEBUG] [Component.tsx:10] mount', { props });

// Effect
useEffect(() => {
  console.log('[DEBUG] [Component.tsx:15] effect run', { deps });
  return () => console.log('[DEBUG] [Component.tsx:17] cleanup');
}, [deps]);

// State
console.log('[DEBUG] [Component.tsx:25] before setState', { current: state });

Node.js/Express

// Request
console.log('[DEBUG] [route.ts:10] request', { method: req.method, path: req.path });

// Error
console.log('[DEBUG] [service.ts:30] caught error', { name: err.name, message: err.message });

API Calls

console.log('[DEBUG] [api.ts:10] fetch start', { url, method });
console.log('[DEBUG] [api.ts:15] fetch done', { status: res.status, ok: res.ok });

Common Bug Patterns

Pattern	Symptom	Check
Null access	"Cannot read property X of undefined"	Optional chaining, defaults
Race condition	Works sometimes, fails randomly	Async ordering, state timing
Stale closure	Using old values in callbacks	useCallback deps, event bindings
API mismatch	Data not displaying	Response shape, null handling
Silent error	Nothing happens	Empty catch blocks, missing error state

Confidence Scoring

Score	Meaning	Action
>= 70	High - clear evidence	Report as probable cause
50-69	Medium - possible	Suggest logs to confirm
< 50	Low - speculation	Do not report

Cleanup

After debugging, all [DEBUG] logs are removed automatically.

Manual check:

grep -rn '\[DEBUG\]' . --include='*.ts' --include='*.tsx' --include='*.js' --include='*.jsx'

MCP Integration

MCP	Provides
Console Ninja	Runtime values, test status, coverage
Chrome DevTools	Network inspection, browser console, DOM state