systematic-debugging

name	systematic-debugging
description	Use when encountering any bug, test failure, build error, or unexpected behavior - 4-phase root cause process before proposing fixes

Systematic Debugging

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

Phase 1: Root Cause Investigation

1. Read error messages carefully - stack traces, line numbers, error codes
2. Reproduce consistently - exact steps, every time?
3. Check recent changes - git diff, recent commits, new dependencies
4. Gather evidence in multi-component systems - log data at each component boundary (API -> service -> DB)
5. Trace data flow backward - where does bad value originate? Keep tracing up until source

Phase 2: Pattern Analysis

1. Find working examples in codebase - what works that's similar?
2. Compare against references - read reference implementation COMPLETELY
3. Identify differences - list EVERY difference, however small
4. Understand dependencies - what settings, config, environment?

Phase 3: Hypothesis and Testing

1. Form single hypothesis: "I think X is the root cause because Y"
2. Test minimally - ONE variable at a time
3. Verify - did it work? No -> form NEW hypothesis
4. Don't pretend to know - say "I don't understand X" and research more

Phase 4: Implementation

1. Create failing test case first - simplest reproduction, automated if possible
2. Implement single fix - ONE change, no "while I'm here" improvements
3. Verify - test passes? No other tests broken? Issue resolved?
4. If 3+ fixes failed - STOP. Question architecture. Discuss with human.

Red Flags - STOP

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "Add multiple changes, run tests"
• Proposing solutions before tracing data flow
• Each fix reveals new problem in different place

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