Debug Mode
debug
Systematic debugging with hypothesis-driven investigation. Use when something is broken, tests are failing, unexpected behavior occurs, or errors need investigation. Triggers on: 'this is broken', 'debug', 'why is this failing', 'unexpected error', 'not working', 'bug', 'fix this issue', 'investi...
SKILL.md
Full skill instructions
Debug Mode
Systematic bug investigation and resolution.
Core Approach
"Don't guess. Form hypotheses. Test them."
The 4-Phase Process
Phase 1: Assessment 🔍
Goal: Understand and reproduce
- What is the expected behavior?
- What is the actual behavior?
- Can you reliably reproduce?
- What changed recently?
Key Questions:
- When did this start happening?
- Does it happen consistently or intermittently?
- What are the exact inputs that trigger it?
- What error messages or symptoms appear?
Phase 2: Investigation 🔬
Goal: Isolate and trace
- Trace execution from entry point
- Identify where expected diverges from actual
- Form hypotheses about root cause
- Test hypotheses systematically
Techniques:
- Add strategic logging/prints
- Use debugger breakpoints
- Simplify inputs to minimal reproduction
- Check boundary conditions
Phase 3: Resolution 🔧
Goal: Fix minimally and verify
- Implement the smallest fix that addresses root cause
- Don't fix symptoms, fix the disease
- Add regression test
- Verify fix doesn't break other things
If fix doesn't work:
- Count: How many fixes attempted?
- If < 3: Return to Phase 1, re-analyze with new information
- If ≥ 3: STOP. Question your understanding of the system.
Phase 4: Quality ✅
Goal: Prevent recurrence
- Add test covering the bug
- Document if the cause was non-obvious
- Consider if similar bugs exist elsewhere
- Clean up debug code
Debugging Checklist
- [ ] **Reproduced**: Can trigger bug consistently
- [ ] **Isolated**: Know which component is failing
- [ ] **Root Cause**: Understand WHY it fails
- [ ] **Fixed**: Minimal change addresses cause
- [ ] **Tested**: Regression test added
- [ ] **Clean**: Debug code removed
Hypothesis Template
**Hypothesis**: [What you think is wrong]
**Test**: [How you'll verify]
**Result**: [What happened]
**Conclusion**: [Confirmed/Rejected/Needs more info]
Common Root Causes
| Symptom | Often Caused By |
|---|---|
| Works locally, fails in CI | Environment differences, missing deps |
| Intermittent failure | Race condition, timing, external dependency |
| Wrong output | Logic error, wrong variable, off-by-one |
| Crash/exception | Null/None access, type mismatch, missing data |
| Performance issue | N+1 queries, missing index, memory leak |
Rationalization Prevention
| Excuse | Reality |
|---|---|
| "I'll just add a quick fix" | Quick fixes hide root cause. Follow Phase 1 first. |
| "It's probably X" | "Probably" isn't evidence. Test the hypothesis. |
| "This is too simple to debug formally" | Simple bugs waste the most time when undiagnosed. |
| "I've tried 3 things, might as well try a 4th" | STOP. Return to Phase 1. Re-analyze with new info. |
| "It works now, not sure why" | If you don't know why it works, it will break again. |
Red Flags - STOP and Re-Assess
- Adding a 3rd fix attempt without returning to Phase 1
- Saying "should work now" without verification
- Fixing a symptom because root cause is unclear
- Skipping reproduction because "I know what's wrong"
- Multiple hypotheses being tested simultaneously
Debug Report Format
## Debug Report
### Bug Summary
- **Expected**: [what should happen]
- **Actual**: [what happens instead]
- **Severity**: [critical/high/medium/low]
### Reproduction
1. [Step to reproduce]
2. [Step to reproduce]
3. [Observe bug]
**Minimal reproduction**: [simplest case that triggers bug]
### Investigation
| Hypothesis | Test | Result |
| ---------- | -------------- | -------------------------- |
| [theory] | [what I tried] | ✅ Confirmed / ❌ Rejected |
### Root Cause
[What's actually wrong and why]
### Fix Applied
- **File**: `path/to/file.py`
- **Change**: [what was modified]
- **Why**: [how this fixes the root cause]
### Verification
- [ ] Bug no longer reproduces
- [ ] Existing tests pass
- [ ] Regression test added: `test_name`
- [ ] No debug code left behind
### Prevention
[How to prevent similar bugs in the future]