Phase-Gated Debugging
Overview
AI coding agents see an error and immediately edit code. They guess at fixes, get it wrong, and spiral. This skill enforces a strict 5-phase protocol where you CANNOT edit source code until the root cause is identified and confirmed.
Based on claude-debug (full plugin with PreToolUse hook enforcement).
When to Use
Use this skill when:
- a bug keeps getting "fixed" without resolving the underlying issue
- you need to slow an agent down and force disciplined debugging before code edits
- the failure is intermittent, a regression, performance-related, or otherwise hard to isolate
- you want an explicit user confirmation checkpoint before any fix is applied
The Protocol
Phase 1: REPRODUCE
Run the failing command/test. Capture the exact error. Run 2-3 times for consistency.
- Do NOT read source code
- Do NOT hypothesize
- Do NOT edit any files
Phase 2: ISOLATE
Read code. Add diagnostic logging marked // DEBUG. Re-run with diagnostics. Binary search to narrow down.
- Only
// DEBUGmarked logging is allowed - Do NOT fix the bug even if you see it
Phase 3: ROOT CAUSE
Analyze WHY at the isolated location. Use "5 Whys" technique. Remove debug logging.
State: "This is my root cause analysis: [explanation]. Do you agree, or should I investigate further?"
WAIT for user confirmation. Do NOT proceed without it.
Phase 4: FIX
Remove all // DEBUG lines. Apply minimal change addressing confirmed root cause.
- Only edit files related to root cause
- Do NOT refactor unrelated code
Phase 5: VERIFY
Run original failing test — must pass. Run related tests. For intermittent bugs, run 5+ times. If verification fails: root cause was wrong, go back to Phase 2.
Bug-Type Strategies
| Type | Technique |
|---|---|
| Crash/Panic | Stack trace backward — trace the bad value to its source |
| Wrong Output | Binary search — log midpoint, halve search space each iteration |
| Intermittent | Compare passing vs failing run logs — find ordering divergence |
| Regression | git bisect — find the offending commit |
| Performance | Timing at stage boundaries — find the bottleneck |
Key Rules
- NEVER edit source code in phases 1-3 (except
// DEBUGin phase 2) - NEVER proceed past phase 3 without user confirmation
- ALWAYS reproduce before investigating
- ALWAYS verify after fixing
Example
User request:
a bug keeps getting "fixed" without resolving the underlying issue; diagnose it and return the concrete next action.
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.