Systematic Debugging

Core principle: Find root cause before attempting fixes. Symptom fixes are failure.

The Four Phases

Complete each phase before proceeding to the next.

Debugging Progress:
- [ ] Phase 1: Root Cause Investigation
- [ ] Phase 2: Pattern Analysis
- [ ] Phase 3: Hypothesis Testing
- [ ] Phase 4: Implementation

Phase 1: Root Cause Investigation

Before attempting ANY fix:

1. Read error messages carefully – Stack traces often contain the solution
2. Reproduce consistently – If not reproducible, gather more data
3. Check recent changes – Git diff, new dependencies, config changes
4. Trace data flow backward – Find where invalid data originates

For multi-component systems: Add diagnostic logging at each component boundary before proposing fixes. See references/debugging-techniques.md for instrumentation patterns.

For log-heavy investigations: Use Skill(ce:reading-logs) for efficient analysis.

Phase 2: Pattern Analysis

1. Find working examples of similar code in the codebase
2. Compare working vs broken – list every difference
3. Read reference implementations completely, not just skimming

Phase 3: Hypothesis Testing

1. Form single hypothesis: “X is the root cause because Y”
2. Make the SMALLEST possible change to test it
3. Verify before continuing – if wrong, form NEW hypothesis

Phase 4: Implementation

1. Create failing test case first
2. Implement single fix at root cause
3. Apply defense-in-depth – Validate at multiple layers
4. Verify fix and run tests

If 3+ fixes have failed: Stop fixing symptoms. Question the architecture.

Red Flags

Stop and return to Phase 1 if you catch yourself:

• Proposing fixes without completing investigation
• “Just try changing X and see if it works”
• Adding multiple changes at once
• “It’s probably X, let me fix that” (without evidence)
• Each fix reveals new problems in different places

Tactical Techniques

For specific debugging methods, see references/debugging-techniques.md:

• Binary search / git bisect
• Minimal reproduction
• Strategic logging
• Runtime assertions
• Differential analysis
• Multi-component instrumentation
• Backward tracing

Reporting Format

## Root Cause
[1-3 sentences explaining underlying issue]
Located in: `file.ts:123`

## What Was Wrong
[Specific problem - mutation, race condition, missing validation, etc.]

## The Fix
[Changes made and why they address root cause]

## Verification
- [x] Bug reproduced and confirmed fixed
- [x] Existing tests pass
- [x] Added regression test