Systematic Debugging

---
name: systematic-debugging
description: >
  Systematic 4-phase debugging process with Iron Law: Root Cause Investigation →
  Pattern Analysis → Hypothesis & Testing → Implementation. Use when encountering
  any bug, test failure, unexpected behavior, or build failure — BEFORE proposing
  fixes. Auto-trigger: load when /coder VERIFY fails 3x consecutively.
  Keywords: bug, test failing, stuck, not working, can't fix, root cause, 3x fail,
  test loop, flaky test, unexpected behavior, debugging, investigate.
  Do NOT use for pre-implementation planning (that is /planner territory).
---

# 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 (including 3x fail loop in /coder VERIFY)
- 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)
- 3x test fails — this is the MOST important time to use this skill

## 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
   - Read stack traces completely
   - Note line numbers, file paths, error codes
   - Run: `go test -v -run TestXxx ./...` to see full output

2. **Reproduce Consistently**
   - Can you trigger it reliably?
   - What are the exact steps?
   - `go test -count=1 -run TestXxx ./...` (disables cache, ensures fresh run)
   - If not reproducible → gather more data, don't guess

3. **Check Recent Changes**
   - What changed that could cause this?
   - `git diff HEAD~1`, recent commits
   - New dependencies, config changes
   - Environmental differences

4. **Gather Evidence in Multi-Component Systems**

   **WHEN system has multiple components (handler → service → repository):**

   **BEFORE proposing fixes, add diagnostic instrumentation:**
   ```
   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify context/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
   ```

   **Example (Go multi-layer diagnostic):**
   ```go
   // Layer 1: Handler
   func (h *UserHandler) GetUser(w http.ResponseWriter, r *http.Request) {
       id := chi.URLParam(r, "id")
       slog.Debug("handler.GetUser", "id", id)

       // Layer 2: Service
       user, err := h.svc.GetUser(r.Context(), id)
       slog.Debug("service.GetUser result", "id", id, "err", err)
   }

   // Layer 3: Repository (add temporarily in Find method)
   // slog.Debug("repo.Find called", "id", id, "stack", string(debug.Stack()))
   ```

   **This reveals:** Which layer receives invalid data

5. **Trace Data Flow**

   **WHEN error is deep in call stack:**

   See `root-cause-tracing.md` for the complete backward tracing technique.

   **Quick version:**
   - 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
   - `grep -r "func.*GetUser" ./internal/` to find similar functions

2. **Compare Against References**
   - Read reference implementation COMPLETELY
   - Don't skim — read every line
   - Understand the pattern fully before applying

3. **Identify Differences**
   - What's different between working and broken?
   - List every difference, however small
   - Don't assume "that can't matter"

4. **Understand Dependencies**
   - What other components does this need?
   - What settings, config, environment?
   - What assumptions does it make?

### Phase 3: Hypothesis and Testing

**Scientific method:**

1. **Form Single Hypothesis**
   - State clearly: "I think X is the root cause because Y"
   - Write it down
   - Be specific, not vague

2. **Test Minimally**
   - Make the SMALLEST possible change to test hypothesis
   - One variable at a time
   - `go test -v -run TestXxx -count=1 ./...`
   - Don't fix multiple things at once

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

4. **When You Don't Know**
   - Say "I don't understand X"
   - Don't pretend to know
   - Ask for help
   - Research more

### Phase 4: Implementation

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

1. **Create Failing Test Case**
   - Simplest possible reproduction
   - `go test -v -run TestXxx -count=1 ./...` must FAIL before fix
   - Use tdd-rules skill for proper failing test creation (loads tdd-shapes/<LANGUAGE>.md per cascade)
   - For concurrency bugs: `go test -race -run TestXxx ./...`

2. **Implement Single Fix**
   - Address the root cause identified
   - ONE change at a time
   - No "while I'm here" improvements
   - No bundled refactoring

3. **Verify Fix**
   - `go test -v -run TestXxx -count=1 ./...` passes?
   - `go test ./...` — 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 (step 5 below)**
   - DON'T attempt Fix #4 without architectural discussion

5. **If 3+ Fixes Failed: Question Architecture**

   **Pattern indicating architectural problem:**
   - Each fix reveals new shared state/coupling/problem in different place
   - Fixes require major refactoring to implement
   - Each fix creates new symptoms elsewhere

   **STOP and question fundamentals:**
   - 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"
- "It's probably X, let me fix that"
- "I don't fully understand but this might work"
- "Here are the main problems: [lists fixes without investigation]"
- 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.**

**If 3+ fixes failed:** Question the architecture (see Phase 4.5)

## 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 (use tdd-rules skill). |
| "Multiple fixes at once saves time" | Can't isolate what worked. Causes new bugs. |
| "Reference too long, I'll adapt the pattern" | Partial understanding guarantees bugs. Read it completely. |
| "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. |
| "`go test` is slow, I'll skip verification" | Unverified fixes are not fixes. Run `go test -count=1 -run TestXxx ./...`. |

## Integration with /coder VERIFY Phase

**Auto-trigger:** Load this skill when tests fail 3x consecutively in /coder VERIFY.

**Entry point:** Start at Phase 1 with the failing `go test -v ./...` output as evidence.

**Exit:** Once root cause found and single fix applied, return to VERIFY:
`go vet ./... && make fmt && make lint && make test`

If investigation reveals architectural issue → escalate to user (do NOT attempt Fix #4).

## 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 |

## Supporting Techniques

- **`root-cause-tracing.md`** — Trace bugs backward through call stack to find original trigger
- **`defense-in-depth.md`** — Add validation at multiple layers after finding root cause
- **`condition-based-waiting.md`** — Replace arbitrary `time.Sleep` with condition polling

**Related skills:**
- **tdd-rules** — For creating failing test case (Phase 4, Step 1; per-language idioms in tdd-shapes/<LANGUAGE>.md)