Systematic Debugging

Source: obra/superpowers (MIT)

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.

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
• "Just one quick fix" seems obvious
• You've already tried multiple fixes
• Previous fix didn't work
• You don't fully understand the issue

The Four Phases

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. 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 — What changed that could cause this? Git diff, recent commits, new dependencies, config changes.

4. Gather Evidence in Multi-Component Systems — For each component boundary: log what enters/exits, verify environment propagation, check state at each layer. Run once to gather evidence showing WHERE it breaks.

5. Trace Data Flow — 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 same codebase.
2. Compare Against References — If implementing a pattern, read reference implementation COMPLETELY.
3. Identify Differences — What's different between working and broken? List every difference.
4. Understand Dependencies — What other components does this need? What settings, config, environment?

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis — State clearly: "I think X is the root cause because Y"
2. Test Minimally — Make the SMALLEST possible change to test hypothesis. One variable at a time.
3. Verify Before Continuing — Did it work? Yes → Phase 4. Didn't work? Form NEW hypothesis.
4. When You Don't Know — Say "I don't understand X". Don't pretend. Ask for help.

Phase 4: Implementation

1. Create Failing Test Case — Simplest possible reproduction. MUST have before fixing.
2. Implement Single Fix — Address the root cause. ONE change at a time. No "while I'm here" improvements.
3. Verify Fix — Test passes now? No other tests broken? Issue actually resolved?
4. If 3+ Fixes Failed — STOP. Question the architecture. Discuss with your human partner before attempting more fixes.

Red Flags — STOP and Follow Process

• "Quick fix for now, investigate later"
• "Just try changing X and see if it works"
• "Add multiple changes, run tests"
• "It's probably X, let me fix that"
• "I don't fully understand but this might work"
• Proposing solutions before tracing data flow
• "One more fix attempt" (when already tried 2+)

ALL of these mean: 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