Systematic Debugging

---
name: systematic-debugging
description: "Use when encountering any bug, test failure, or unexpected behavior, before proposing fixes. Enforces 4-phase root cause investigation: no fixes without understanding the root cause first."
argument-hint: "Describe the bug, error, or unexpected behavior to debug."
---

# 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
- Integration issues

**Use this ESPECIALLY when:**
- Under time pressure (emergencies make guessing tempting)
- "Just one quick fix" seems obvious
- You've already tried multiple fixes
- Previous fix didn't work
- You don't fully understand the issue

## The Four Phases

You MUST complete each phase before proceeding to the next.

### Phase 1: Root Cause Investigation

**BEFORE attempting ANY fix:**

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

2. **Reproduce Consistently**
   - Can you trigger it reliably?
   - What are the exact steps?
   - Does it happen every time?
   - If not reproducible → gather more data, don't guess

3. **Check Recent Changes**
   - What changed that could cause this?
   - Git diff, recent commits
   - New dependencies, config changes
   - Environmental differences

4. **Gather Evidence in Multi-Component Systems**
   - For EACH component boundary: log what enters and exits
   - Run once to gather evidence showing WHERE it breaks
   - THEN analyze evidence to identify failing component
   - THEN investigate that specific component

5. **Trace Data Flow**
   - Where does bad value originate?
   - What called this with bad value?
   - Keep tracing up until you find the source
   - Fix at source, not at symptom

### Phase 2: Pattern Analysis

**Find the pattern before fixing:**

1. **Find Working Examples** — locate similar working code in same codebase
2. **Compare Against References** — if implementing pattern, read reference implementation COMPLETELY
3. **Identify Differences** — list every difference between working and broken, however small
4. **Understand Dependencies** — what other components, settings, config, environment?

### Phase 3: Hypothesis and Testing

**Scientific method:**

1. **Form Single Hypothesis** — "I think X is the root cause because Y"
2. **Test Minimally** — make the SMALLEST possible change to test hypothesis. One variable at a time.
3. **Verify Before Continuing** — did it work? Yes → Phase 4. Didn't work? Form NEW hypothesis.
4. **When You Don't Know** — say "I don't understand X". Don't pretend to know.

### Phase 4: Implementation

**Fix the root cause, not the symptom:**

1. **Create Failing Test Case** — simplest possible reproduction. MUST have before fixing.
2. **Implement Single Fix** — address the root cause identified. ONE change at a time. No "while I'm here" improvements.
3. **Verify Fix** — test passes now? No other tests broken? Issue actually resolved?
4. **If Fix Doesn't Work** — STOP. Count: How many fixes have you tried?
   - If < 3: Return to Phase 1, re-analyze with new information
   - **If ≥ 3: STOP and question the architecture (see step 5)**

5. **If 3+ Fixes Failed: Question Architecture**
   - Is this pattern fundamentally sound?
   - Are we "sticking with it through sheer inertia"?
   - Should we refactor architecture vs. continue fixing symptoms?
   - **Discuss with your human partner before attempting more fixes**

## Red Flags - STOP and Follow Process

If you catch yourself thinking:
- "Quick fix for now, investigate later"
- "Just try changing X and see if it works"
- "Add multiple changes, run tests"
- "Skip the test, I'll manually verify"
- "It's probably X, let me fix that"
- "I don't fully understand but this might work"
- Proposing solutions before tracing data flow
- **"One more fix attempt" (when already tried 2+)**
- **Each fix reveals new problem in different place**

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

## Quick Reference

| Phase | Key Activities | Success Criteria |
|-------|---------------|------------------|
| **1. Root Cause** | Read errors, reproduce, check changes, gather evidence | 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 |

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Issue is simple, don't need process" | Simple issues have root causes too. Process is fast for simple bugs. |
| "Emergency, no time for process" | Systematic debugging is FASTER than guess-and-check thrashing. |
| "Just try this first, then investigate" | First fix sets the pattern. Do it right from the start. |
| "I'll write test after confirming fix works" | Untested fixes don't stick. Test first proves it. |
| "Multiple fixes at once saves time" | Can't isolate what worked. Causes new bugs. |
| "I see the problem, let me fix it" | Seeing symptoms ≠ understanding root cause. |
| "One more fix attempt" (after 2+ failures) | 3+ failures = architectural problem. Question pattern, don't fix again. |

## Related Skills

- **superpowers:test-driven-development** — For creating failing test case (Phase 4, Step 1)
- **superpowers:verification-before-completion** — Verify fix worked before claiming success