jbro/systematic-debugging

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, unexpected behavior, performance problems, build failures, integration issues.

Never skip when: issue seems simple, you're under time pressure, or you've already tried multiple fixes. Systematic debugging is FASTER than thrashing. Emergencies make guessing tempting — resist.

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? If not → gather more data, don't guess

3. Check Recent Changes

   • What changed that could cause this? Git diff, recent commits, config changes

4. Gather Evidence in Multi-Component Systems

   WHEN system has multiple components:

   Add diagnostic instrumentation at each component boundary before proposing fixes:

   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
   
   Run once to gather evidence → identify failing component → investigate that component
   

   Example (CI → build → signing pipeline):

   echo "=== Secrets in workflow: ===" && echo "IDENTITY: ${IDENTITY:+SET}${IDENTITY:-UNSET}"
   echo "=== Env vars in build: ===" && env | grep IDENTITY || echo "IDENTITY not in environment"
   echo "=== Keychain state: ===" && security find-identity -v
   

   This reveals which layer fails (workflow ✓, workflow → build ✗).

5. Trace Data Flow

   WHEN error is deep in call stack:

   Read root-cause-tracing.md in this skill's directory for the complete backward tracing technique.

   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

1. Find Working Examples — locate similar working code in the same codebase
2. Compare Against References — if implementing a pattern, read the reference COMPLETELY; don't skim
3. Identify Differences — list every difference between working and broken, however small
4. Understand Dependencies — settings, config, environment assumptions

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis — "I think X is the root cause because Y"; write it down
2. Test Minimally — smallest possible change to test the hypothesis; one variable at a time
3. Verify Before Continuing — worked? → Phase 4. Didn't work? → new hypothesis
4. When You Don't Know — say so; don't pretend; ask for help or research more

Phase 4: Implementation

Fix the root cause, not the symptom:

1. Create Failing Test Case — simplest possible reproduction; use the test-driven-development skill

2. Implement Single Fix — one change; no "while I'm here" improvements

3. Verify Fix — test passes; no other tests broken

4. If Fix Doesn't Work — STOP; count fixes tried:

   • < 3: return to Phase 1 with new information
   • ≥ 3: STOP — question the architecture (step 5)

5. If 3+ Fixes Failed: Question Architecture

   Pattern indicating architectural problem:

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

   STOP: discuss with your human partner before attempting more fixes. This is a wrong architecture, not a failed hypothesis.

Red Flags — STOP and Follow Process

If you catch yourself thinking any of these:

• "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 a different place
• "Emergency, no time for process" (systematic IS faster)
• "I see the problem" (symptoms ≠ root cause)

ALL of these mean: STOP. Return to Phase 1.

If 3+ fixes failed: Question the architecture (Phase 4.5).

your human partner's Signals You're Doing It Wrong

Watch for these redirections:

• "Is that not happening?" — you assumed without verifying
• "Will it show us...?" — you should have added evidence gathering
• "Stop guessing" — you're proposing fixes without understanding
• "Ultrathink this" — question fundamentals, not just symptoms
• "We're stuck?" (frustrated) — your approach isn't working

When you see these: STOP. Return to Phase 1.

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 |

When Process Reveals "No Root Cause"

If investigation reveals the issue is truly environmental, timing-dependent, or external:

1. Document what you investigated
2. Implement appropriate handling (retry, timeout, error message)
3. Add monitoring/logging for future investigation

Note: 95% of "no root cause" cases are incomplete investigation.

Supporting Techniques

Available in this directory:

• root-cause-tracing.md — trace bugs backward through call stack to find original trigger
• defense-in-depth.md — add validation at multiple layers after finding root cause
• condition-based-waiting.md — replace arbitrary timeouts with condition polling

Related skills:

• test-driven-development — for creating the failing test case (Phase 4, Step 1)
• verification-before-completion — verify fix worked before claiming success