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:

• Read error messages carefully
• Full stack traces
• Error codes and descriptions
• Timestamps and context

• Reproduce consistently
• Document exact steps
• Identify triggers
• Note environmental factors

• Check recent changes
• Recent commits
• Dependency updates
• Configuration changes

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

Phase 2: Pattern Analysis

• Find similar working code
• Search codebase for related functionality
• Look for existing patterns

• Read reference implementations completely
• Don't skim
• Understand the full context

• Compare working vs broken
• What's different?
• What assumptions changed?

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

Phase 3: Hypothesis and Testing

• Form a SINGLE, SPECIFIC hypothesis
• "The error occurs because X"
• Not "maybe it's A or B or C"

• Test with minimal changes
• One variable at a time
• Don't combine fixes

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

• Ask for help when stuck
• After genuine investigation
• With evidence gathered

Phase 4: Implementation

• Create a failing test case FIRST
• Reproduces the bug
• Will pass when fixed

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

• 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