systematic-debugging

name: systematic-debugging description: Enforces a systematic debugging process to find root causes before attempting fixes. Use when encountering any bug, test failure, or unexpected behavior.

Systematic Debugging

When to use this skill

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 this skill

Use for any technical issue: test failures, bugs, 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
• A previous fix didn't work
• You don't fully understand the issue

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.

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 — git diff, recent commits, new dependencies, config/env changes.

4. Gather evidence in multi-component systems

   When system has multiple components (CI → build → signing, API → service → DB), add diagnostic instrumentation before proposing fixes:

   # Layer 1: Workflow
   echo "IDENTITY: ${IDENTITY:+SET}${IDENTITY:-UNSET}"
   
   # Layer 2: Build script
   env | grep IDENTITY || echo "IDENTITY not in environment"
   
   # Layer 3: Signing
   security list-keychains
   security find-identity -v
   

   Run once to gather evidence showing WHERE it breaks, then investigate that layer.

5. Trace data flow — see root-cause-tracing.md for the full backward tracing technique. Quick version: where does the bad value originate? What called this with the bad value? Trace up until you find the source. Fix at source, not symptom.

Phase 2: Pattern Analysis

1. Find working examples of similar code in the codebase
2. Read reference implementations completely — don't skim
3. List every difference between working and broken, however small
4. Understand dependencies, config, environment assumptions

Phase 3: Hypothesis and Testing

1. Form one hypothesis: "I think X is the root cause because Y"
2. Make the smallest possible change to test it — one variable at a time
3. Verify — did it work? Yes → Phase 4. No → form a new hypothesis. Don't stack fixes.
4. If you don't know → say so. Don't pretend. Ask or research.

Phase 4: Implementation

1. Create failing test case — simplest possible reproduction. Use test-driven-development skill.
2. Implement single fix — address root cause only. No "while I'm here" improvements.
3. Verify fix — test passes? No other tests broken?
4. If fix doesn't work:
   • Count how many fixes you've tried
   • < 3: Return to Phase 1 with new information
   • ≥ 3: Stop and question the architecture (see below)

When 3+ Fixes Fail: Question Architecture

Pattern indicating an architectural problem:

• Each fix reveals new shared state/coupling in a different place
• Fixes require massive refactoring
• Each fix creates new symptoms elsewhere

Stop and ask: Is this pattern fundamentally sound? Are we stuck through sheer inertia? Should we refactor vs. continue fixing symptoms?

Discuss with your human partner before attempting more fixes.

Red Flags — Stop and Follow Process

If you catch yourself thinking any of these, return to Phase 1:

• "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"
• "Here are the main problems: [lists fixes without investigation]"
• "One more fix attempt" (when already tried 2+)
• Each fix reveals a new problem in a different place

Common Rationalizations

Supporting Files

Related skills: test-driven-development, verification-before-completion