# Systematic Debugging

> Four-phase debugging process - root cause first, then fix. Use when encountering bugs or unexpected behavior.

- Skill: `itseffi/systematic-debugging` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add itseffi/systematic-debugging`
- Raw SKILL.md: https://api.skillmd.com/api/skills/itseffi/systematic-debugging/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: itseffi (https://skillmd.com/u/itseffi)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/itseffi/systematic-debugging

---


# Systematic Debugging

Use when encountering any bug, test failure, or unexpected behavior, before proposing fixes.

## The Iron Law

```
NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST
```

If you haven't completed Phase 1, you cannot propose fixes.

## The Four Phases

### Phase 1: Root Cause Investigation

**BEFORE attempting ANY fix:**

1. **Read Error Messages Carefully**
   - Don't skip past errors or warnings
   - Read stack traces completely
   - Note line numbers, file paths, error codes

2. **Reproduce Consistently**
   - Can you trigger it reliably?
   - What are the exact steps?
   - If not reproducible, gather more data

3. **Check Recent Changes**
   - Git diff, recent commits
   - New dependencies, config changes
   - Environmental differences

4. **Trace Data Flow**
   - Where does bad value originate?
   - What called this with bad value?
   - Keep tracing up until you find the source

### Phase 2: Pattern Analysis

1. **Find Working Examples**
   - Locate similar working code in same codebase

2. **Compare Against References**
   - Read reference implementation completely
   - Don't skim - read every line

3. **Identify Differences**
   - What's different between working and broken?
   - List every difference, however small

### Phase 3: Hypothesis and Testing

1. **Form Single Hypothesis**
   - State clearly: "I think X is the root cause because Y"
   - Write it down

2. **Test Minimally**
   - Make the SMALLEST possible change
   - One variable at a time

3. **Verify Before Continuing**
   - Did it work? Yes -> Phase 4
   - Didn't work? Form NEW hypothesis
   - DON'T add more fixes on top

### Phase 4: Implementation

1. **Create Failing Test Case**
   - Use the `tdd.md` skill

2. **Implement Single Fix**
   - ONE change at a time
   - No "while I'm here" improvements

3. **Verify Fix**
   - Test passes now?
   - No other tests broken?

4. **If 3+ Fixes Failed: Question Architecture**
   - Is this pattern fundamentally sound?
   - Should we refactor vs continue fixing symptoms?
   - Discuss before attempting more fixes

## Red Flags - STOP and Follow Process

- "Quick fix for now, investigate later"
- "Just try changing X and see if it works"
- "I don't fully understand but this might work"
- Proposing solutions before tracing data flow
- "One more fix attempt" (when already tried 2+)

**ALL mean: STOP. Return to Phase 1.**

## Quick Reference

| Phase | Key Activities | Success Criteria |
|-------|---------------|------------------|
| 1. Root Cause | Read errors, reproduce, check changes | Understand WHAT and WHY |
| 2. Pattern | Find working examples, compare | Identify differences |
| 3. Hypothesis | Form theory, test minimally | Confirmed or new hypothesis |
| 4. Implementation | Create test, fix, verify | Bug resolved, tests pass |

## When to Use

Use this skill when the task directly matches the workflow described above.

## When Not to Use

Do not use this skill when the request is unrelated, low-stakes, or better handled by a simpler direct response.