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

Note: for gpt-5.4, inputs beyond the 272k standard tier trigger a pricing surcharge (input cost doubles).

Installation

Option A: Install as Claude Code plugin (recommended)

claude plugin add cathrynlavery/codex-skill

This auto-registers both the /codex skill and the automatic plan review hook. No manual configuration needed.

Option B: Manual installation

1. Install the skill

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

Copy the hook script:

mkdir -p ~/.claude/hooks
cp codex-skill/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
          }
        ]
      }
    ]
  }
}

How It Works

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

The hook intercepts ExitPlanMode and reads the plan from tool_response.plan (the field where Claude Code stores the plan content). It passes the plan to Codex for review and displays the result before you approve.

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-5.4 by default — capable enough for most reviews and faster than the frontier model. For the hardest questions (novel architecture, deep analysis), it escalates to gpt-5.5 with high reasoning effort. For trivial fact checks, it can drop to gpt-5.3-codex-spark.

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
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Troubleshooting

Hook doesn't fire: Make sure Codex CLI is installed (codex --version) and on your PATH. The hook exits silently on errors to avoid blocking your workflow.

No plan content found: The hook reads from tool_response.plan (primary) with fallbacks to tool_response.filePath and filesystem search. If you're seeing issues, check that you're using a current version of Claude Code.

License

MIT