Document: SKILL.md Role: Error Diagnosis and Troubleshooting Assistant Priority: High - Problem solving automation Applies To: Error diagnosis and debugging in any project Optimized For: Claude 4.5 (Sonnet/Opus) Last Updated: 2025-12-28 This skill is auto-discovered by Claude when users encounter errors or need debugging help. It provides structured diagnosis and resolution following the team's troubleshooting process.

Troubleshoot Skill

This skill diagnoses and fixes errors following structured troubleshooting process.

Source of Truth

• Decision Tree: `process.md` (Troubleshooting Decision Tree)
• Output Format: `output-formats.md` (Troubleshooting Format)

When to Activate

This skill activates in these scenarios:

1. Error reported: User pastes error message or stack trace
2. Problem description: "왜 안돼?", "에러 났어", "오류 발생"
3. Debug request: "디버깅 해줘", "문제 해결해줘"
4. Confusion: "이거 왜 이래?", "안 돼", "작동 안 해"

Troubleshooting Principles

• Understand before fixing: Read the error message carefully
• Reproduce first: Ensure the issue can be consistently reproduced
• Fix root cause: Avoid band-aid solutions that hide symptoms
• 3-attempt rule: Try different approaches after 3 failed attempts
• Document findings: Help prevent similar issues in the future

Instructions

Step 1: Classify the Problem

Identify the issue type to apply the correct diagnosis approach:

Issue Type	Symptoms	First Action
Compilation/Build	Error during build, syntax error	Read error message, check recent changes
Runtime	Crash during execution, exception	Get stack trace, identify error location
Logic Bug	Wrong behavior, unexpected output	Define expected vs actual behavior
Performance	Slow, timeout, high resource usage	Measure and profile first
Test Failure	Test not passing	Check if test or code is wrong

Step 2: Apply Diagnosis Process

For Compilation/Build Errors

1. Read the error message - Does it clearly indicate the problem?
2. Check recent changes - Did this work before your changes?
3. Verify environment:
   • Dependencies installed? (npm install, pip install)
   • Correct version? (Node, Python, etc.)
   • Environment variables set?
   • Build cache corrupted? (try clean build)

For Runtime Errors

1. Locate the error - Get full stack trace
2. Understand the error:
   • What is the exact error type?
   • What operation was being performed?
   • What were the input values?
3. Reproduce consistently - Write a test that reproduces the error
4. Fix systematically - Minimal fix, verify with test

For Logic Bugs

1. Define expected behavior - What should happen?
2. Isolate the issue:
   • Add logging at key points
   • Trace data flow
   • Narrow down to specific function/line
3. Understand why:
   • Logic error → Review algorithm, check conditions
   • Data error → Check input validation
   • State error → Review state management
   • Integration error → Check external dependencies

For Test Failures

1. Understand the failure:
   • Expected (new code breaks old behavior)?
   • Unexpected (existing code now failing)?
2. Categorize:
   • Flaky test → Fix test (mocking, timeouts)
   • Wrong assumption → Update test
   • Actual bug → Fix the code
   • Environment issue → Fix configuration

NEVER:

• ❌ Delete failing tests
• ❌ Comment out assertions
• ❌ Add try-catch to hide errors

Step 3: Format Output

Use this structured format:

## 에러 진단

### ❌ 에러 메시지
\`\`\`
[Full error message]
\`\`\`

### 🔍 원인 분석

**직접적 원인**:
[에러가 발생한 직접적 이유]

**근본 원인**:
[왜 그런 상황이 발생했는지]

**발생 위치**:
- File: [file_path:line_number]
- Function: [function_name]
- Context: [what was being done]

---

## 해결 방법

### Option 1: [즉시 해결] (권장)

**수정 내용**:
\`\`\`[language]
[Fix code]
\`\`\`

**장점**: [benefits]
**단점**: [tradeoffs]

### Option 2: [대안]

**수정 내용**:
\`\`\`[language]
[Alternative fix]
\`\`\`

**장점**: [benefits]
**단점**: [tradeoffs]

---

## 검증 단계

1. [Step 1 to verify fix]
2. [Step 2 to verify fix]

## 재발 방지

- [Preventive measure 1]
- [Preventive measure 2]

Simplified Output (for quick fixes)

When the fix is obvious and simple:

## 진단 결과

**원인**: [간단한 원인 설명]

**해결**:
\`\`\`[language]
[Fix code]
\`\`\`

**확인**: [검증 방법]

3-Attempt Rule

If unresolved after 3 attempts, stop and:

1. Document what failed:

   • What was attempted
   • Specific error messages
   • Estimated cause of failure

2. Research alternatives:

   • Find 2-3 similar implementations
   • Document alternative approaches

3. Try different angle:

   • Different library/framework features?
   • Different architectural pattern?
   • Remove abstraction instead of adding?

4. Ask for help with:

   • Full error message
   • Steps to reproduce
   • What's been tried so far

Common Error Patterns

NullPointerException / undefined is not a function

**원인**: 객체가 초기화되지 않은 상태에서 접근
**해결**: null check 추가 또는 optional chaining 사용

Module not found / Import error

**원인**: 패키지 미설치 또는 경로 오류
**해결**:
1. 패키지 설치 확인: `npm install` / `pip install`
2. import 경로 확인
3. 상대/절대 경로 구분

Permission denied

**원인**: 파일/디렉토리 접근 권한 부족
**해결**:
1. 파일 권한 확인: `ls -la`
2. 소유자 확인
3. 필요시 권한 변경 (주의 필요)

Response Language

• Diagnosis and explanation: Korean (한국어)
• Code examples: English (comments, variable names)
• Error messages: Keep original (for searchability)

See Also

• process.md - Full troubleshooting decision tree
• output-formats.md - Output format templates
• quality-assurance.md - Test failure handling