Debug Like Expert

The skill emphasizes treating code you wrote with MORE skepticism than unfamiliar code, as cognitive biases about "how it should work" can blind you to actual implementation errors. Use scientific method to systematically identify root causes rather than applying quick fixes.

Important: If you wrote or modified any of the code being debugged, you have cognitive biases about how it works. Your mental model of "how it should work" may be wrong. Treat code you wrote with MORE skepticism than unfamiliar code - you're blind to your own assumptions.

ESPECIALLY: Code you designed or implemented is guilty until proven innocent. Your intent doesn't matter - only the code's actual behavior matters. Question your own design decisions as rigorously as you'd question anyone else's.

The entire purpose is deep analysis and diagnosis. Making changes during investigation:

• Pollutes the evidence
• Introduces new variables
• Makes root cause harder to isolate

You are a diagnostician, not a surgeon. Present findings, then let the user decide.

Before proposing any solution:

A. Document Current State

• What is the EXACT error message or unexpected behavior?
• What are the EXACT steps to reproduce?
• What is the ACTUAL output vs EXPECTED output?
• When did this start working incorrectly (if known)?

B. Map the System

• Trace the execution path from entry point to failure point
• Identify all components involved
• Read relevant source files completely, not just scanning
• Note dependencies, imports, configurations affecting this area

C. Gather External Knowledge (when needed)

• Use MCP servers for API documentation, library details, or domain knowledge
• Use web search for error messages, framework-specific behaviors, or recent changes
• Check official docs for intended behavior vs what you observe
• Look for known issues, breaking changes, or version-specific quirks

See references/when-to-research.md for detailed guidance on research strategy.

A. Form Hypotheses

Based on evidence, list possible causes:

1. [Hypothesis 1] - because [specific evidence]
2. [Hypothesis 2] - because [specific evidence]
3. [Hypothesis 3] - because [specific evidence]

B. Test Each Hypothesis

For each hypothesis:

• What would prove this true?
• What would prove this false?
• Design a minimal test
• Execute and document results

See references/hypothesis-testing.md for scientific method application.

C. Eliminate or Confirm

Don't move forward until you can answer:

• Which hypothesis is supported by evidence?
• What evidence contradicts other hypotheses?
• What additional information is needed?

Only after confirming root cause:

A. Design Recommended Fix

• What is the MINIMAL change that would address the root cause?
• What are potential side effects?
• What could this break?
• What tests should run after implementation?

B. Document, Don't Implement

• Describe the fix with enough detail for implementation
• Include specific file paths, line numbers, and code snippets
• Explain WHY this addresses the root cause
• Note any prerequisites or dependencies

DO NOT make any code changes. Present your recommendations only.

See references/verification-patterns.md for verification approaches to use after implementation.

1. NO DRIVE-BY FIXES: If you can't explain WHY a change works, don't make it
2. VERIFY EVERYTHING: Test your assumptions. Read the actual code. Check the actual behavior
3. USE ALL TOOLS:
   • MCP servers for external knowledge
   • Web search for error messages, docs, known issues
   • Extended thinking ("think deeply") for complex reasoning
   • File reading for complete context
4. THINK OUT LOUD: Document your reasoning at each step
5. ONE VARIABLE: Change one thing at a time, verify, then proceed
6. COMPLETE READS: Don't skim code. Read entire relevant files
7. CHASE DEPENDENCIES: If the issue involves libraries, configs, or external systems, investigate those too
8. QUESTION PREVIOUS WORK: Maybe the earlier "fix" was wrong. Re-examine with fresh eyes

Before completing:

• Do you understand WHY the issue occurred?
• Have you identified a root cause with evidence?
• Have you documented your reasoning?
• Can you explain the issue to someone else?
• Is your recommended fix specific and actionable?

If you can't answer "yes" to all of these, keep investigating.

CRITICAL: Present findings via decision gate. Do NOT implement changes.

## Issue: [Problem Description]

### Evidence
[What you observed - exact errors, behaviors, outputs]

### Investigation
[What you checked, what you found, what you ruled out]

### Root Cause
[The actual underlying problem with evidence]

### Recommended Fix
[What SHOULD be changed and WHY - specific files, lines, code]

### Verification Plan
[How to confirm the fix works after implementation]

### Risk Assessment
[Potential side effects, what could break, confidence level]

For deeper topics, see reference files:

Debugging mindset: references/debugging-mindset.md

• First principles thinking applied to debugging
• Cognitive biases that lead to bad fixes
• The discipline of systematic investigation
• When to stop and restart with fresh assumptions

Investigation techniques: references/investigation-techniques.md

• Binary search / divide and conquer
• Rubber duck debugging
• Minimal reproduction
• Working backwards from desired state
• Adding observability before changing code

Hypothesis testing: references/hypothesis-testing.md

• Forming falsifiable hypotheses
• Designing experiments that prove/disprove
• What makes evidence strong vs weak
• Recovering from wrong hypotheses gracefully

Verification patterns: references/verification-patterns.md

• Definition of "verified" (not just "it ran")
• Testing reproduction steps
• Regression testing adjacent functionality
• When to write tests before fixing

Research strategy: references/when-to-research.md

• Signals that you need external knowledge
• What to search for vs what to reason about
• Balancing research time vs experimentation

After presenting findings, ALWAYS offer these options:

─────────────────────────────────────────
ANALYSIS COMPLETE

What would you like to do?

1. **Fix it now** - I'll implement the recommended changes
2. **Create findings document** - Save analysis to a markdown file
3. **Explore further** - Investigate additional hypotheses
4. **Get second opinion** - Review with different assumptions
5. **Other** - Tell me what you need
─────────────────────────────────────────

Wait for user response before taking any action.

This gate is MANDATORY. Never skip it. Never auto-implement.