Codex Skill for Claude Code

Give Claude Code a "second opinion" by letting it consult OpenAI's Codex CLI for independent verification.

What It Does

When Claude Code creates a plan, Codex automatically reviews it before you approve. Two AIs checking each other's work catches more edge cases.

Automatic review on:

• Every plan Claude creates (via hook)
• Architecture decisions
• Implementation approaches

Manual use for:

• Researching unfamiliar APIs or libraries
• Verifying complex code patterns
• Getting alternative perspectives

Prerequisites

Install Codex CLI:

npm install -g @openai/codex

Configure your OpenAI API key.

Supported Models

Model	Context	Speed	Best for
gpt-5.4	272k	Standard	Most capable — deep analysis, architecture, novel problems
gpt-5.3-codex-spark	128k	Ultra-fast (1000+ tok/s)	Quick queries, fact checks (default)
gpt-5.3-codex	272k	Standard	General-purpose coding tasks
gpt-5.2-codex	272k	Standard	Older alternative
gpt-5.1-codex-max	272k	Standard	Older alternative
gpt-5.1-codex-mini	272k	Fast	Budget option

Installation

1. Install the skill

Clone and copy the skill to your Claude Code skills directory:

git clone https://github.com/cathrynlavery/codex-skill.git
mkdir -p ~/.claude/skills/codex
cp codex-skill/skills/codex/SKILL.md ~/.claude/skills/codex/

2. Set up automatic plan review (recommended)

Copy the hook script:

cp hooks/plan-review.sh ~/.claude/hooks/
chmod +x ~/.claude/hooks/plan-review.sh

Add to your ~/.claude/settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "ExitPlanMode",
        "hooks": [
          {
            "type": "command",
            "command": "~/.claude/hooks/plan-review.sh",
            "timeout": 120
          }
        ]
      }
    ]
  }
}

Now every time Claude finishes a plan, Codex reviews it before you approve.

How It Works

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   Claude    │────▶│ ExitPlanMode│────▶│   Codex     │
│ creates plan│     │   (hook)    │     │  reviews    │
└─────────────┘     └─────────────┘     └─────────────┘
                                               │
                                               ▼
                                        ┌─────────────┐
                                        │ You approve │
                                        │ with context│
                                        └─────────────┘

Codex reviews for:

• Potential issues or risks
• Missing steps
• Better alternatives
• Edge cases not addressed

Manual Usage

Invoke directly:

/codex

Or ask Claude:

"Can you verify this approach with Codex?" "Get a second opinion on this architecture"

The skill uses gpt-5.3-codex-spark by default for speed. For complex questions, it switches to gpt-5.4 with high reasoning effort automatically.

Example Output

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
🔍 CODEX SECOND OPINION ON PLAN
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
✓ LGTM - Plan covers the main implementation steps.

Minor suggestions:
• Consider adding error handling for the API timeout case
• Step 3 could be split into separate DB migration and code changes
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

License

MIT