Debugging Skill

Systematically analyze and fix bugs, errors, and unexpected behavior.

Debugging Process

1. Reproduce: exact steps, minimal repro, expected vs actual 기록.
2. Gather info: mcp__serena__find_referencing_symbols(usages), mcp__jetbrains__get_symbol_info(defs), mcp__context7__get-library-docs(docs); collect logs/stack traces.
3. Analyze: read errors, locate failure points, binary search/ logging로 범위 축소.
4. Fix: address root cause, preserve behavior, cover edge cases.
5. Verify: original 이슈 해결 확인, 관련 테스트 실행, 회귀 여부 점검.

Common Bugs

Syntax Errors

Error: Unexpected token
SyntaxError: invalid syntax

Actions:

• Check indicated line
• Verify brackets/parentheses balance
• Look for missing semicolons or commas

Runtime Errors

TypeError: Cannot read property 'x' of undefined
AttributeError: 'NoneType' has no attribute 'x'

Actions:

• Trace variable origin
• Add null checks
• Verify data flow

Logic Errors

• No error message, wrong result
• Off-by-one errors
• Incorrect conditions

Actions:

• Add logging to trace execution
• Step through logic manually
• Write test cases for expected behavior

Async Errors

UnhandledPromiseRejection
Race condition symptoms

Actions:

• Check async/await usage
• Verify Promise chains
• Look for shared state mutations

Import/Module Errors

ModuleNotFoundError
Cannot find module

Actions:

• Check package installation
• Verify import paths
• Check package.json/requirements.txt

Type Errors

Type 'A' is not assignable to type 'B'

Actions:

• Check type definitions
• Verify function signatures
• Look for type mismatches

Debugging Techniques

Print/Log Debugging

console.log('[DEBUG] variable:', JSON.stringify(variable, null, 2));
console.log('[DEBUG] function entered with args:', args);

import logging

logging.debug(f'variable: {variable}')

Binary Search Debugging

• Comment out half the suspicious code
• If bug persists, it's in the other half
• Repeat until isolated

Rubber Duck Debugging

• Explain the code line by line
• Often reveals assumptions and errors
• Document the explanation as comments

Git Bisect

git bisect start
git bisect bad HEAD
git bisect good <last-known-good-commit>
# Test each commit git bisect suggests

Stack Trace Analysis

• Read from bottom to top
• Focus on your code, not framework code
• Identify the trigger point

Bug Categories

Off-by-One Errors

// Bug: Array index out of bounds
for (let i = 0; i <= arr.length; i++)

// Fix: Use < instead of <=
for (let i = 0; i < arr.length; i++)

Null/Undefined Handling

// Bug: Accessing property of undefined
const name = user.profile.name;

// Fix: Optional chaining
const name = user?.profile?.name;

State Management

// Bug: Mutating shared state
users.push(newUser);

// Fix: Create new array
const updatedUsers = [...users, newUser];

Async Timing

// Bug: Not waiting for async
const data = fetchData();
process(data); // data is a Promise!

// Fix: Await the promise
const data = await fetchData();
process(data);

Output Format

## Bug Analysis

### Symptom

[What was observed]

### Root Cause

[Why it happened]

### Fix

(code changes with before/after)

### Impact

- [Files affected]
- [Potential side effects]
- [Preventive measures]

### Verification Method

(commands or tests to verify fix)

Edge Cases

• Intermittent bugs: Add logging, use stress testing
• Environment-specific: Check environment differences first
• No reproduction: Add monitoring and logging
• Third-party bugs: Document workaround, report upstream

Tips

• MCP tools are optional; omit if unavailable or add others as needed