Debug

/debug

Systematic debugging workflow: reproduce, isolate, fix, and verify.

Usage

/debug <issue> [--logs <path>] [--trace]

Arguments

• issue: Description of the bug, error message, or issue number
• --logs <path>: Path to relevant log files
• --trace: Enable verbose tracing during investigation

Instructions

When this skill is invoked:

Agent Behavior

Autonomy:

• Complete the full debug cycle end-to-end
• Make reasonable hypotheses based on error patterns
• Implement and verify fixes without prompting

Thoroughness:

• Exhaust all likely causes before asking for help
• Document findings as you investigate
• Create regression tests for fixed bugs

Methodology:

• Follow scientific method: hypothesize → test → conclude
• Never change multiple things at once
• Always verify the fix resolves the original issue

Debug Process

Phase 1: Understand

1. Gather information:

   • Read error message/stack trace carefully
   • Identify the failing component
   • Note when the issue started (if known)

2. Read relevant code in parallel:

   • File where error occurs
   • Related files (imports, dependencies)
   • Recent changes to affected area

3. Check logs if available:

   # Recent logs
   tail -100 {log_path}
   
   # Search for errors
   grep -i "error\|exception\|fail" {log_path}
   

Phase 2: Reproduce

1. Create minimal reproduction:

   • Identify exact steps to trigger the bug
   • Reduce to smallest failing case
   • Document reproduction steps

2. Write a failing test:

   # Create test that captures the bug
   {test_command} tests/unit/test_bug_reproduction.py -v
   

3. Verify test fails for the right reason

Phase 3: Isolate

1. Form hypotheses (list 3-5 likely causes):

   • Hypothesis 1: [Most likely cause]
   • Hypothesis 2: [Second most likely]
   • Hypothesis 3: [Edge case possibility]

2. Test each hypothesis:

   • Add logging/print statements
   • Use debugger breakpoints
   • Check variable states

3. Binary search if issue location unclear:

   • Comment out half the code
   • Determine which half contains the bug
   • Repeat until isolated

4. Check common causes:

   • Null/undefined values
   • Off-by-one errors
   • Race conditions
   • Type mismatches
   • Missing error handling
   • Configuration issues

Phase 4: Fix

1. Implement minimal fix:

   • Change only what's necessary
   • Don't refactor during bug fix
   • Keep the fix focused

2. Verify fix:

   # Run the failing test
   {test_command} tests/unit/test_bug_reproduction.py -v
   
   # Run related tests
   {test_command} tests/unit/test_{module}.py -v
   
   # Run full test suite
   {test_command} tests/ -v
   

3. Add regression test if not already covered

Phase 5: Document

1. Update issue/ticket with:

   • Root cause analysis
   • Fix description
   • Prevention measures

2. Create commit with clear message:

   fix(scope): description of what was fixed
   
   Root cause: [explanation]
   Solution: [what was changed]
   
   Closes #issue_number
   

Capture Knowledge

If this fix took more than 15 minutes to diagnose, or the root cause was non-obvious, run /compound to capture the solution. This prevents future re-discovery of the same problem and builds the project's knowledge base in docs/solutions/.

Debug Checklist

• Error message understood
• Issue reproduced locally
• Failing test written
• Root cause identified
• Minimal fix implemented
• All tests pass
• Regression test added
• Fix documented
• Knowledge captured with /compound (if non-trivial fix)

Common Debug Patterns

Null Reference:

# Symptom: "Cannot read property X of undefined"
# Check: Is the object initialized? Is it returned from async?

Race Condition:

# Symptom: Intermittent failures, works sometimes
# Check: Async operations, shared state, missing awaits

Type Error:

# Symptom: "Expected X but got Y"
# Check: API response shape, serialization, type coercion

Configuration:

# Symptom: Works locally, fails in CI/prod
# Check: Environment variables, file paths, permissions

Example Output

$ /debug "TypeError: Cannot read property 'id' of undefined in UserService"

====== Phase 1: Understand ======

Error: TypeError: Cannot read property 'id' of undefined
Location: src/services/user.py:45
Stack trace points to: getUserById() method

Reading relevant files...
- src/services/user.py
- src/api/users.py
- tests/unit/test_user.py

====== Phase 2: Reproduce ======

Reproduction steps:
1. Call GET /api/users/999 (non-existent ID)
2. Service returns undefined instead of null
3. Caller tries to access .id property

Writing failing test...
Created: tests/unit/test_user_not_found.py

Test result: FAILED (as expected)

====== Phase 3: Isolate ======

Hypotheses:
1. getUserById returns undefined for missing users
2. Database query doesn't handle empty results
3. Null check missing in API layer

Testing hypothesis 1...
Confirmed: getUserById returns undefined, not null

Root cause: Missing null check after database query

====== Phase 4: Fix ======

Fix: Add explicit null return when user not found

src/services/user.py:45
- return result
+ return result if result else None

Running tests...
- test_user_not_found: PASSED
- test_user_service: PASSED (15/15)
- Full suite: PASSED (142/142)

====== Phase 5: Document ======

Root cause: Database query returned undefined for missing records,
but callers expected null. This caused a TypeError when accessing
properties on the undefined value.

Fix: Added explicit null return with proper type hints.

Ready to commit:
fix(user): handle missing user in getUserById

Closes #789