Systematic Debugging

Core Principle

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

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

The Iron Law

If you haven't completed root cause investigation, 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

Quick Reference: The Four Phases

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

Red Flags - STOP and Follow Process

If you notice yourself considering:

• "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 (not just symptoms)

Topic Guides

📖 @references/systematic.md - Four-Phase Framework

• Complete systematic debugging process
• Phase 1: Root cause investigation
• Phase 2: Pattern analysis
• Phase 3: Hypothesis and testing
• Phase 4: Implementation
• Multi-component system diagnostics
• 3+ fix failure → architectural questioning
• Human partner signals you're doing it wrong
• Common rationalizations to avoid

🔍 @references/root-cause.md - Backward Tracing

• Trace bugs backward through call stack
• Finding original trigger vs fixing symptoms
• Adding stack trace instrumentation
• Finding which test causes pollution
• Real example: empty projectDir

🛡️ @references/defense-in-depth.md - Multi-Layer Validation

• Validate at EVERY layer data passes through
• Layer 1: Entry point validation
• Layer 2: Business logic validation
• Layer 3: Environment guards
• Layer 4: Debug instrumentation
• Making bugs structurally impossible

✅ @references/verification.md - Evidence Before Claims

• The Iron Law: No completion claims without fresh verification
• The Gate Function (5 steps before any claim)
• Common failures and red flags
• Regression test red-green cycle
• Why this matters (trust, quality, honesty)

Common Patterns

Multi-Component Diagnostic

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

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

# Layer 3: Target script
echo "=== State at execution: ==="
./script.sh --verbose

# Reveals: Which layer fails

Stack Trace Addition

// Before problematic operation
async function operation(param: string) {
  const stack = new Error().stack;
  console.error('DEBUG operation:', {
    param,
    cwd: process.cwd(),
    env: process.env.NODE_ENV,
    stack,
  });
  
  await doRiskyThing(param);
}

Defense-in-Depth Pattern

// Layer 1: Entry validation
function createThing(path: string) {
  if (!path || path.trim() === '') {
    throw new Error('path cannot be empty');
  }
  
  // Layer 2: Business logic
  if (!existsSync(path)) {
    throw new Error(`path does not exist: ${path}`);
  }
  
  // Layer 3: Environment guard
  if (process.env.NODE_ENV === 'test') {
    if (!path.startsWith(tmpdir())) {
      throw new Error('Test operations must use temp dir');
    }
  }
  
  // Layer 4: Debug instrumentation
  logger.debug('Creating thing', { path, caller: new Error().stack });
  
  // Now safe to proceed
}

Integration

This skill works with:

• systematic-thinking - Deep analysis for complex bugs
• code-review - Catch bugs before they ship
• validate-plan - Ensure fix approach is sound

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

---

The Bottom Line:

1. Find root cause BEFORE proposing fixes
2. Trace backward to original trigger
3. Validate at EVERY layer
4. Verify BEFORE claiming success

This is non-negotiable.