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.

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

The Four Phases

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
2. Reproduce Consistently
   • Can you trigger it reliably?
   • What are the exact steps?
   • 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

4. Gather Evidence in Multi-Component Systems

   When system has multiple components (API → service → database):

   # For EACH component boundary:
   # - Log what data enters component
   # - Log what data exits component
   # - Verify environment/config propagation
   
   # Run once to gather evidence showing WHERE it breaks
   # THEN analyze evidence to identify failing 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

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
3. Identify Differences
   • What's different between working and broken?
   • List every difference, however small

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis
   • State clearly: "I think X is the root cause because Y"
   • 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

Phase 4: Implementation

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. Rule of 3: Question Architecture

   If 3+ fixes have failed:

   • STOP attempting more fixes
   • Each fix revealing new problems = architectural issue
   • Discuss with user before continuing

   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"
• "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

Quick Reference

Integration with Omni Workflow

• Use this skill BEFORE attempting fixes in /forge
• If debugging during QA, document findings for forge to fix
• For integration issues, use existing fixtures to create regression tests