systematic-debugging by heyitsnoah

---
name: systematic-debugging
description: ALWAYS use before attempting any fix. Never jump to solutions - investigate root cause first. Use when encountering any technical issue, bug, test failure, or unexpected behavior.
version: 2.0.0
---

# 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)
- Manager wants it fixed NOW (systematic is faster than thrashing)

## 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 (CI → build → signing, API → service →
   database):**

   **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
   ```

   **Example (multi-layer system):**

   ```bash
   # Layer 1: Workflow
   echo "=== Secrets available in workflow: ==="
   echo "IDENTITY: ${IDENTITY:+SET}${IDENTITY:-UNSET}"

   # Layer 2: Build script
   echo "=== Env vars in build script: ==="
   env | grep IDENTITY || echo "IDENTITY not in environment"

   # Layer 3: Signing script
   echo "=== Keychain state: ==="
   security list-keychains
   security find-identity -v

   # Layer 4: Actual signing
   codesign --sign "$IDENTITY" --verbose=4 "$APP"
   ```

   **This reveals:** Which layer fails (secrets → workflow ✓, workflow → build ✗)

5. **Trace Data Flow**

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

   **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
   - What works that's similar to what's broken?

2. **Compare Against References**
   - If implementing pattern, 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
   - 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
   - Automated test if possible
   - One-off test script if no framework
   - MUST have before fixing

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**
   - 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 (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 "massive 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 the user before attempting more fixes**

   This is NOT a failed hypothesis - this is a wrong architecture.

## 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"
- "Pattern says X but I'll adapt it differently"
- "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.                       |
| "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. |

## 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    |

## Technique: Root Cause Tracing

When bugs manifest deep in the call stack, trace backward to find the original
trigger.

### The Tracing Process

1. **Observe the Symptom**

   ```
   Error: git init failed in /Users/jesse/project/packages/core
   ```

2. **Find Immediate Cause** - What code directly causes this?

   ```typescript
   await execFileAsync('git', ['init'], { cwd: projectDir })
   ```

3. **Ask: What Called This?**

   ```typescript
   WorktreeManager.createSessionWorktree(projectDir, sessionId)
     → called by Session.initializeWorkspace()
     → called by Session.create()
     → called by test at Project.create()
   ```

4. **Keep Tracing Up** - What value was passed?
   - `projectDir = ''` (empty string!)
   - Empty string as `cwd` resolves to `process.cwd()`

5. **Find Original Trigger** - Where did empty string come from?
   ```typescript
   const context = setupCoreTest() // Returns { tempDir: '' }
   Project.create('name', context.tempDir) // Accessed before beforeEach!
   ```

### Adding Stack Traces

When you can't trace manually, add instrumentation:

```typescript
async function gitInit(directory: string) {
  const stack = new Error().stack
  console.error('DEBUG git init:', {
    directory,
    cwd: process.cwd(),
    nodeEnv: process.env.NODE_ENV,
    stack,
  })
  await execFileAsync('git', ['init'], { cwd: directory })
}
```

**Tips:**

- Use `console.error()` in tests (logger may be suppressed)
- Log before the dangerous operation, not after it fails
- Include context: directory, cwd, environment variables
- `new Error().stack` shows complete call chain

### Finding Which Test Causes Pollution

If something appears during tests but you don't know which test, use bisection:

```bash
# Run tests one-by-one, stop at first polluter
for f in src/**/*.test.ts; do
  npm test "$f" && [ -d .git ] && echo "POLLUTER: $f" && break
done
```

**NEVER fix just where the error appears.** Trace back to find the original
trigger.

## Technique: Defense-in-Depth Validation

After finding root cause, validate at EVERY layer data passes through. Make the
bug structurally impossible.

### Why Multiple Layers

- Single validation: "We fixed the bug"
- Multiple layers: "We made the bug impossible"

Different layers catch different cases:

- Entry validation catches most bugs
- Business logic catches edge cases
- Environment guards prevent context-specific dangers
- Debug logging helps when other layers fail

### The Four Layers

**Layer 1: Entry Point Validation** - Reject invalid input at API boundary

```typescript
function createProject(name: string, workingDirectory: string) {
  if (!workingDirectory || workingDirectory.trim() === '') {
    throw new Error('workingDirectory cannot be empty')
  }
  if (!existsSync(workingDirectory)) {
    throw new Error(`workingDirectory does not exist: ${workingDirectory}`)
  }
}
```

**Layer 2: Business Logic Validation** - Ensure data makes sense for operation

```typescript
function initializeWorkspace(projectDir: string, sessionId: string) {
  if (!projectDir) {
    throw new Error('projectDir required for workspace initialization')
  }
}
```

**Layer 3: Environment Guards** - Prevent dangerous operations in specific
contexts

```typescript
async function gitInit(directory: string) {
  if (process.env.NODE_ENV === 'test') {
    const normalized = normalize(resolve(directory))
    const tmpDir = normalize(resolve(tmpdir()))
    if (!normalized.startsWith(tmpDir)) {
      throw new Error(`Refusing git init outside temp dir during tests`)
    }
  }
}
```

**Layer 4: Debug Instrumentation** - Capture context for forensics

```typescript
async function gitInit(directory: string) {
  logger.debug('About to git init', {
    directory,
    cwd: process.cwd(),
    stack: new Error().stack,
  })
}
```

### Applying Defense-in-Depth

When you find a bug:

1. **Trace the data flow** - Where does bad value originate? Where used?
2. **Map all checkpoints** - List every point data passes through
3. **Add validation at each layer** - Entry, business, environment, debug
4. **Test each layer** - Try to bypass layer 1, verify layer 2 catches it

**Don't stop at one validation point.** Add checks at every layer.

## Real-World Impact

From debugging sessions:

- Systematic approach: 15-30 minutes to fix
- Random fixes approach: 2-3 hours of thrashing
- First-time fix rate: 95% vs 40%
- New bugs introduced: Near zero vs common