phase-gated-debugging

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 // DEBUG marked 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

1. NEVER edit source code in phases 1-3 (except // DEBUG in phase 2)
2. NEVER proceed past phase 3 without user confirmation
3. ALWAYS reproduce before investigating
4. 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.