Debug Protocol Skill

Purpose

To replace "Shotgun Debugging" (randomly changing code) with a deterministic, scientific process. Goal: Fix the root cause, not just the symptom.

When to Use

• Trigger: When a test fails, an exception is thrown, or output is unexpected.
• Agent: Primarily used by execution mode.
• Support:
  • Call quality to write the reproduction test.
  • Call researcher if the error message is obscure/undocumented.

The Scientific Method (4-Step Protocol)

Phase 1: Observation (The MRE)

Rule: If you cannot reproduce it, you cannot fix it.

1. Isolate: Create a standalone script or test case that fails 100% of the time.
2. Minimize: Remove all code not strictly necessary to produce the error.
3. Log: Do not guess variables. Log them.

Phase 2: Localization (The "Wolf Fence")

Rule: Divide and Conquer.

1. Binary Search: Is the data wrong at the Database? No? At the API? No? At the UI?
2. Trace: Follow the data flow. Find the exact line where the reality diverges from expectation.

Phase 3: Hypothesis (The "5 Whys")

Rule: Do not fix the symptom.

1. Identify: "Variable X is null."
2. Ask Why: "Why is X null?" -> "Because the API returned 404."
3. Ask Why: "Why 404?" -> "Because the ID was undefined."
4. Ask Why: "Why undefined?" -> "Because of a typo in the frontend."
5. Root Cause: Typo in the frontend payload.

Phase 4: Resolution & Verification

Rule: Prove it.

1. Apply Fix: Correct the root cause.
2. Verify: Run the MRE from Phase 1. It must pass.
3. Regression: Run the full test suite. Ensure nothing else broke.

Debugging Scratchpad Template

Copy this into context during a debug session.

## 🐞 Debug Session: [Error Name]

### 1. The Reproduction (MRE)
- [ ] Created `reproduce_issue.ts`
- [ ] Failure is deterministic (Happens every time)
- **Error Message**: `[Paste exact error]`

### 2. Localization (Wolf Fence)
- [ ] Inputs are correct? (Yes/No)
- [ ] Intermediate state correct? (Yes/No)
- **Found at**: File `X`, Line `Y`.

### 3. Root Cause Analysis (5 Whys)
1. Why? [Answer]
2. Why? [Answer]
3. **Root Cause**: [The fundamental flaw]

### 4. Resolution
- **Action**: [What did you change?]
- **Verification**: `npm test -- reproduce_issue.ts` -> ✅ PASS

Anti-Patterns (Must Avoid)

• ❌ "It should work": Code does not care what it "should" do. It does what it is told. Look at what it is doing.
• ❌ Console Log Spray: Don't leave console.log('here') all over the code. Use structured logging or remove them after finding the bug.
• ❌ The "Blind Fix": Applying a StackOverflow solution without understanding why it applies to your specific context. (Call researcher first).
• ❌ Ignoring Warning Logs: Often the error is preceded by a warning 10 lines up. Read the whole log.

Integration with Agents

• quality agent: If you are stuck on Phase 1 (Reproduction), delegate to quality. "I see the error, but I can't write a test for it. Quality Agent, please create a Cypress test for this UI bug."
• researcher agent: If you are stuck on Phase 3 (Hypothesis). "I have the error 'Heap Out of Memory'. Researcher, what are the common causes for this in Node.js 18?"