---
name: systematic-debugging
description: Use when encountering any bug, test failure, or unexpected behavior, before proposing fixes
---

# Systematic Debugging

## Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

**Core principle:** ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

**Violating the letter of this process is violating the spirit of debugging.**

## The Iron Law

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

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

## When to Use

Use for ANY technical issue:
- Test failures
- Bugs in production
- Unexpected behavior
- Performance problems
- Build failures

## The Four Phases

### Phase 1: Root Cause Investigation

**BEFORE attempting ANY fix:**

1. **Read Error Messages Carefully** - They often contain the exact solution
2. **Reproduce Consistently** - Can you trigger it reliably?
3. **Check Recent Changes** - What changed that could cause this?
4. **Gather Evidence** - Add diagnostic instrumentation in multi-component systems
5. **Trace Data Flow** - See root-cause-tracing.md

### Phase 2: Pattern Analysis

1. **Find Working Examples** - Locate similar working code
2. **Compare Against References** - Read reference implementation COMPLETELY
3. **Identify Differences** - List every difference, however small
4. **Understand Dependencies** - What settings, config, environment?

### Phase 3: Hypothesis and Testing

1. **Form Single Hypothesis** - "I think X is the root cause because Y"
2. **Test Minimally** - One variable at a time
3. **Verify Before Continuing** - Did it work? If not, NEW hypothesis
4. **When You Don't Know** - Say "I don't understand X"

### Phase 4: Implementation

1. **Create Failing Test Case** - Use test-driven-development skill
2. **Implement Single Fix** - ONE change at a time
3. **Verify Fix** - Test passes? No other tests broken?
4. **If 3+ Fixes Failed** - Question the architecture

## 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 of these mean: STOP. Return to Phase 1.**

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Issue is simple" | Simple issues have root causes too. |
| "Emergency, no time" | Systematic debugging is FASTER than thrashing. |
| "Just try this first" | First fix sets the pattern. Do it right. |
| "I see the problem" | Seeing symptoms != understanding root cause. |

## Supporting Techniques

- **root-cause-tracing.md** - Trace bugs backward through call stack
- **defense-in-depth.md** - Add validation at multiple layers