Debugger Skill

Director Mode Lite - Debugging Specialist

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5-Step Root-Cause Method

1. CAPTURE      → Collect the full error, reproduce, define expected vs actual
2. ISOLATE      → Narrow to the smallest failing case
3. HYPOTHESIZE  → List and rank likely causes
4. INVESTIGATE  → Test hypotheses, inspect state
5. FIX & VERIFY → Apply the minimal fix, confirm, prevent recurrence

Step 1: Capture

• Collect the complete error message and stack trace
• Can reproduce the issue with clear steps
• Identify the input that triggered the error
• Know expected vs actual behavior

Step 2: Isolate

• Identify the failure location from the stack trace
• Which file(s) and function(s) are involved?
• When did it start? (git bisect, git log)
• Narrow down to the smallest reproducible case

Step 3: Hypothesize

• List possible causes based on evidence
• Rank hypotheses by likelihood
• Design a test to confirm or eliminate each

Step 4: Investigate

• Analyze error messages and logs carefully
• Add strategic debug logging; inspect variable state at key points
• Check for: null/undefined values, type mismatches, race conditions, resource exhaustion, external service failures

Common Bug Patterns

Pattern	Signs	Common Fix
Null/Undefined	Cannot read property of undefined	Add null checks / optional chaining
Off-by-one	Loop runs one too many/few times	Check loop bounds
Race condition	Intermittent failures	Add synchronization
Type coercion	"1" + 1 = "11"	Explicit type conversion
Async issues	Promise { <pending> }	Await/handle promises

By language and domain:

• JavaScript/TypeScript: undefined is not a function (binding), Cannot read property of null (optional chaining), promise rejections (async/await handling), type errors (strict mode)
• Python: AttributeError (init/typos), TypeError (type validation), ImportError (paths, circular imports), KeyError (dict defaults)
• Database: connection timeouts (pool exhaustion), constraint violations (validation, foreign keys), deadlocks (transaction ordering)
• API/Network: 4xx (request validation, auth), 5xx (server-side, resource limits), timeouts (network, long-running queries)

Investigation Tools

# Search for error message
grep -r "error message" src/

# Find recent changes
git log --oneline -20
git diff HEAD~5

# Check specific function
grep -r "functionName" src/

Step 5: Fix & Verify

Apply the solution:

• Make the minimal change; don't refactor while fixing; one fix per commit

Confirm the fix:

• Original issue is resolved
• No new issues introduced
• Tests pass; add a test to prevent recurrence
• Manual verification done

Output Format

## Debug Report

### Issue
[Description of the bug]

### Reproduction Steps
1. Step one
2. Step two
3. Observe error

### Root Cause
[Explanation of why this happens]

### Location
- **File**: `src/utils/parser.ts`
- **Line**: 45-52
- **Function**: `parseInput()`

### Fix Applied
[Description of the fix]

### Verification
- [x] Issue resolved
- [x] Tests pass
- [x] No regression