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 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

Don't skip when:

• Issue seems simple (simple bugs have root causes too)
• You're in a hurry (rushing guarantees rework)
• Manager wants it fixed NOW (systematic is faster than thrashing)

Focused path

Use this shorter path when all are true:

• A single failing test or assertion is identified (e.g. from CI logs)
• You can reproduce it locally and stably
• The change surface is clear (e.g. files touched in the failing change)

Focused steps:

1. Read the failure and relevant diff/context (understand WHAT failed and WHY at the failing locus)
2. Create or tighten a regression test (wukong-code:test-driven-development)
3. Implement the fix at the root cause for that failure—no bundled unrelated changes
4. Run focused checks, then the broader suite needed for the claim

The Iron Law still holds: no fix before you understand the failing cause. Focused path skips the full multi-phase narration, not investigation.

Use the full Four Phases instead when:

• The failure crosses service/process boundaries
• Root cause is unclear after one focused attempt
• Reproduction is inconsistent

The Four Phases

When Focused path does not apply, you MUST complete each phase before proceeding to the next.

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read error messages carefully (stack traces, line numbers, error codes)
2. Reproduce consistently (exact steps; if not reproducible, gather more data)
3. Check recent changes (git diff, commits, dependencies, config, environment)
4. Gather evidence at component boundaries in multi-component systems
5. Trace data flow to source (see root-cause-tracing.md)

Phase 2: Pattern Analysis

1. Find working examples similar to what's broken
2. Compare against references — read completely, don't skim
3. Identify every difference between working and broken
4. Understand dependencies, config, environment, assumptions

Phase 3: Hypothesis and Testing

1. Form single hypothesis: "I think X because Y"
2. Test minimally — one variable at a time
3. Verify before continuing; if failed, form NEW hypothesis
4. When you don't know — say so, ask for help, research more

Phase 4: Implementation

1. Create failing test case (use wukong-code:test-driven-development)
2. Implement single fix at root cause — no bundled changes
3. Verify fix (test passes, no regressions, issue resolved)
4. If fix doesn't work: STOP. Return to Phase 1, or if ≥3 fixes failed, question architecture with your human partner

For full phase detail, multi-component instrumentation examples, and architecture-questioning guidance, Read skills/systematic-debugging/references/rationalizations-and-depth.md

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"
• "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 (see skills/systematic-debugging/references/rationalizations-and-depth.md, Phase 4.5 / architecture questioning section)

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
• "Ultra-think 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

When Process Reveals "No Root Cause"

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

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

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

Supporting Techniques

These techniques are part of systematic debugging and 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:

• wukong-code:test-driven-development - For creating failing test case (Phase 4, Step 1)
• wukong-code:verification-before-completion - Verify fix worked before claiming success

For rationalizations, phase depth, and real-world impact data, Read skills/systematic-debugging/references/rationalizations-and-depth.md