systematic-debugging

Systematic Debugging

From obra/superpowers

Core Principle

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

Random fixes create new bugs. Understanding must precede solutions.

The Four Phases

Phase 1: Root Cause Investigation

Before ANY fix attempt:

1. Read error messages carefully

   • Full stack traces
   • Error codes and descriptions
   • Timestamps and context

2. Reproduce consistently

   • Document exact steps
   • Identify triggers
   • Note environmental factors

3. Check recent changes

   • Recent commits
   • Dependency updates
   • Configuration changes

4. Gather diagnostic evidence

   • Add logging at component boundaries
   • Check input/output at each step
   • Trace data flow backward to source

Phase 2: Pattern Analysis

1. Find similar working code

   • Search codebase for related functionality
   • Look for existing patterns

2. Read reference implementations completely

   • Don't skim
   • Understand the full context

3. Compare working vs broken

   • What's different?
   • What assumptions changed?

4. Understand dependencies

   • What does this code rely on?
   • What relies on this code?

Phase 3: Hypothesis and Testing

1. Form a SINGLE, SPECIFIC hypothesis

   • "The error occurs because X"
   • Not "maybe it's A or B or C"

2. Test with minimal changes

   • One variable at a time
   • Don't combine fixes

3. Accept results

   • If hypothesis is wrong, form new one
   • Don't force-fit evidence

4. Ask for help when stuck

   • After genuine investigation
   • With evidence gathered

Phase 4: Implementation

1. Create a failing test case FIRST

   • Reproduces the bug
   • Will pass when fixed

2. Implement SINGLE fix

   • Address root cause only
   • Don't fix "nearby" issues

3. Verify the fix

   • Test case passes
   • No regressions
   • Issue is resolved

The 3-Fix Rule

If ≥3 fixes fail: STOP and question the architecture

Three consecutive failed fixes signal:

• Architectural problem, not implementation bug
• Wrong mental model of the system
• Need to step back and reassess

Do NOT attempt more fixes. Reassess the approach.

Critical Red Flags

STOP and restart investigation if you:

• Propose fixes before understanding the problem
• Add multiple changes simultaneously
• Skip investigation for "quick fixes"
• Make 3+ failed fix attempts
• Say "let's just try this"

Debugging Checklist

## Investigation
- [ ] Read full error message/stack trace
- [ ] Reproduced issue consistently
- [ ] Checked recent changes
- [ ] Added diagnostic logging
- [ ] Traced data flow

## Analysis
- [ ] Found similar working code
- [ ] Compared working vs broken
- [ ] Understood all dependencies

## Hypothesis
- [ ] Formed single specific hypothesis
- [ ] Tested with minimal change
- [ ] Accepted results honestly

## Fix
- [ ] Created failing test case
- [ ] Implemented single fix
- [ ] Verified fix works
- [ ] No regressions

Example Workflow

Issue: User login fails silently

Phase 1 - Investigation:
- Error: "null reference at AuthService.validate()"
- Reproduced: happens with specific user emails
- Recent change: added email normalization

Phase 2 - Analysis:
- Working logins use lowercase emails
- Failing logins have mixed case
- Normalization strips @ symbol incorrectly

Phase 3 - Hypothesis:
- "Email normalization regex is wrong"
- Test: log email before/after normalization
- Result: "[email protected]" → "testexample.com"
- Confirmed!

Phase 4 - Fix:
- Write test: normalize("[email protected]") == "[email protected]"
- Fix regex to preserve @
- Verify: test passes, logins work