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)

The Four Phases

You MUST complete each phase before proceeding to the next. Each phase below is the orchestrator-level summary; the full step-by-step walk-through, including multi-component evidence-gathering, the architecture-questioning escape hatch, and the "no root cause" edge case, lives in four-phases.md in this directory.

Phase 1: Root Cause Investigation

Read errors carefully, reproduce consistently, check recent changes, and gather evidence at every component boundary in multi-layer systems before forming any theory. If the error is deep in a call stack, trace data flow backward to the source.

See four-phases.md for the full Phase 1 procedure and root-cause-tracing.md for the backward tracing technique.

Phase 2: Pattern Analysis

Find similar working code, read reference implementations completely (no skimming), enumerate every difference between working and broken, and understand all dependencies before fixing.

See four-phases.md for the full Phase 2 procedure.

Phase 3: Hypothesis and Testing

Form one specific hypothesis ("I think X is the root cause because Y"), make the smallest possible change to test it, change one variable at a time, and form a new hypothesis on failure rather than stacking fixes.

See four-phases.md for the full Phase 3 procedure.

Phase 4: Implementation

Create a failing test case before fixing anything, implement one targeted fix at the root cause (no bundled refactoring), and verify. If 3+ fixes have failed: stop and question the architecture. That pattern means the design is wrong, not the hypothesis.

See four-phases.md for the full Phase 4 procedure including the architecture-questioning protocol.

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

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 Phase 4 in four-phases.md).

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.

Common Rationalizations

Excuse	Reality
"Issue is simple, don't need process"	Simple issues have root causes too. Process is fast for simple bugs.
"Emergency, no time for process"	Systematic debugging is FASTER than guess-and-check thrashing.
"Just try this first, then investigate"	First fix sets the pattern. Do it right from the start.
"I'll write test after confirming fix works"	Untested fixes don't stick. Test first proves it.
"Multiple fixes at once saves time"	Can't isolate what worked. Causes new bugs.
"Reference too long, I'll adapt the pattern"	Partial understanding guarantees bugs. Read it completely.
"I see the problem, let me fix it"	Seeing symptoms ≠ understanding root cause.
"One more fix attempt" (after 2+ failures)	3+ failures = architectural problem. Question pattern, don't fix again.

Supporting Techniques

These references in this directory deepen specific phases:

• four-phases.md - Full step-by-step walk-through of Phases 1 to 4, the architecture-questioning protocol, the "no root cause" edge case, and real-world impact data.
• root-cause-tracing.md - Trace bugs backward through the call stack to find the original trigger (Phase 1, step 5).
• defense-in-depth.md - Add validation at multiple layers once root cause is fixed (Phase 4 follow-up).
• condition-based-waiting.md - Replace arbitrary timeouts with condition polling when debugging flaky timing (Phase 1 evidence gathering).

Related Skills

• workbench:test-driven-development - For creating the failing test case in Phase 4, Step 1.
• workbench:verification-before-completion - Verify the fix worked before claiming success.