Code Investigate Skill

Trigger

• Keywords: investigate code, how feature works, trace implementation, dual confirmation, deep dive, how code works, what this code does, code research

When NOT to Use

• Just need quick lookup (use Grep/Glob directly)
• Code review (use codex-review)
• System verification (use feature-verify)
• Git history tracking (use git-investigate)

Core Principle

Codex must explore independently. Feeding Claude's conclusions to Codex is prohibited.

┌─────────────────┐     ┌─────────────────┐
│ Claude Explores  │     │ Codex Explores   │
│ Independently    │     │ Independently    │
│   (Phase 1-2)   │     │    (Phase 3)     │
└────────┬────────┘     └────────┬────────┘
         │                       │
         ▼                       ▼
   ┌───────────┐           ┌───────────┐
   │ Claude    │           │ Codex     │
   │ Conclusion│           │ Conclusion│
   └─────┬─────┘           └─────┬─────┘
         │                       │
         └───────────┬───────────┘
                     ▼
              ┌─────────────┐
              │ Consolidated│
              │   Report    │
              │  (Phase 4)  │
              └─────────────┘

Workflow

Phase	Name	Action	Output
1	Claude Explore	Grep/Glob/Read to search code	Related files list
2	Claude Conclude	Analyze logic, form understanding	Initial conclusion (internal)
3	Codex Explore	Invoke Codex MCP to explore independently	Codex analysis report
4	Integrate	Compare both perspectives, mark differences	Consolidated report

Codex Invocation Rules

Required Parameters

Parameter	Value	Description
sandbox	read-only	Force read-only
approval-policy	never	Auto-execute
cwd	Project root	Exploration start point

Correct Approach

mcp__codex__codex({
  prompt: `# Code Investigation Task

## Question
${userQuestion}

## Project Info
- Path: ${cwd}
- Tech Stack: {FRAMEWORK} + TypeScript + {DATABASE}

Please **independently explore** the codebase and answer:
1. What files are related?
2. How does the core logic work?
3. What is the data flow?
4. What are the key dependencies?

Please grep/read and explore on your own, then provide your analysis.`,
  cwd: '/path/to/project',
  sandbox: 'read-only',
  'approval-policy': 'never',
});

Prohibited Approaches

Pattern	Problem	Example
Feeding conclusion	Claude's findings leak to Codex	Claude found these files: ${findings}
Leading question	Presupposes answer	I think the problem is in cache, verify
Scope restriction	Prevents independent exploration	Only look at src/service/

Output

## Investigation Report
- **Claude findings**: <independent analysis>
- **Codex findings**: <independent analysis>
- **Integrated conclusion**: <merged findings>
- **Confidence**: High / Medium / Low

Verification Checklist

Check	Standard
Claude independent conclusion	Phase 2 forms conclusion, not output to user
Codex prompt is clean	Contains only question + project path, no Claude findings
Report perspectives separated	Claude / Codex conclusions presented separately
Integration is complete	Marks agreement, differences, possible gaps

References

File	Purpose	When to Read
references/prompts.md	Codex prompt templates	Before Phase 3
references/output-template.md	Report format	During Phase 4

Examples

Feature Investigation

Input: Investigate how order processing works
Phase 1: Grep "processOrder" -> Read src/service/order/*.ts
Phase 2: Form understanding: Controller -> Service -> Repository write
Phase 3: Codex explores independently (only given question + path)
Phase 4: Consolidated report -> mark both perspectives

Mechanism Understanding

Input: How does the API caching mechanism work?
Phase 1: Grep "cache" + "portfolio" -> Read related files
Phase 2: Understand Redis TTL + fallback mechanism
Phase 3: Codex investigates independently
Phase 4: Compare differences -> output consolidated report

Problem Diagnosis

Input: Why is token price sometimes null?
Phase 1: Search price-related logic + error handling
Phase 2: Identify possible fallback paths
Phase 3: Codex diagnoses independently
Phase 4: Synthesize both findings -> list possible causes