debugger

---
name: debugger
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
- 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

**Don't skip when:**
- Issue seems simple (simple bugs have root causes too)
- You're in a hurry (rushing guarantees rework)

## 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**

   **WHEN system has multiple components:**

   **BEFORE proposing fixes, add diagnostic instrumentation:**
   ```
   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer

   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** — Read reference implementation COMPLETELY
3. **Identify Differences** — List every difference, however small
4. **Understand Dependencies** — What other components does this need?

### 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
3. **Verify Before Continuing** — Did it work? Yes → Phase 4. No → 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. ONE change at a time.
3. **Verify Fix** — Test passes? No other tests broken? Issue actually resolved?
4. **If Fix Doesn't Work** — STOP. Count fixes tried. If ≥ 3: question the architecture.

5. **If 3+ Fixes Failed: Question Architecture**
   - Is this pattern fundamentally sound?
   - Should we refactor architecture vs. continue fixing symptoms?
   - Discuss with user 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"
- "It's probably X, let me fix that"
- "I don't fully understand but this might work"
- "One more fix attempt" (when already tried 2+)

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

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Issue is simple, don't need process" | Simple issues have root causes too. |
| "Emergency, no time for process" | Systematic debugging is FASTER than guess-and-check. |
| "Just try this first, then investigate" | First fix sets the pattern. Do it right from the start. |
| "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. |

## 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 |

**Related skills:**
- `oac:verification-before-completion` — Verify fix worked before claiming success
- `oac:test-generation` — For creating failing test cases (Phase 4, Step 1)