Seamless Integration of ChatGPT Plus/Pro into Claude Code Without API Keys

In the evolving landscape of AI-assisted development, efficiency and cost-effectiveness are paramount. An exciting development has emerged for users who operate both ChatGPT Plus/Pro and Claude Code, allowing for smoother workflows and potential savings. This article explores how you can now leverage your ChatGPT Plus or Pro subscription directly within Claude Code—no API key required.

The Challenge Prior to This Solution

Many developers and AI enthusiasts found themselves paying twice—once for ChatGPT Plus (or Pro) at around $20/month, and again for an OpenAI API key—to run GPT models behind Claude. This setup often led to redundant expenses and management overhead, especially when both services are in active use.

Introducing a Game-Changer: Modyak’s ChatGPT Subscription Provider

A new, free macOS application called Modyak addresses this challenge head-on. Modyak now includes a feature that enables users to route Claude Code inference requests through their existing ChatGPT subscription, eliminating the need for separate API keys.

How does it work?

  • Subscription Integration: After installing Modyak, users select the ChatGPT subscription tile within the app.

  • Authentication Simplified:

  • If you use Codex CLI, Modyak reuses your existing authentication stored in ~/.codex/auth.json, allowing for quick setup without a web browser.

  • New users are guided through an OAuth PKCE flow against auth.openai.com. A single sign-in completes the process.

  • Terminal Access: Once configured, issuing the claude command in your terminal interacts with your ChatGPT plan, routing requests seamlessly.

Technical Workflow and Data Security

Under the hood, the system establishes a local proxy connection:

  • Request Flow:

  • Claude Code messages → Local proxy → Chat Completion API → Responses API → Codex backend → responses returned to your terminal

  • Streaming: The process streams data continuously to ensure real-time interaction.

  • Security:

  • Refresh tokens are securely stored within the macOS Keychain.
  • The proxy only accesses short-lived access tokens via a loopback endpoint, ensuring that your typed data remains private and isolated.

Key Features and Benefits

  • Privacy: Requests made through Claude Code are not stored or visible on chatgpt.com, thanks to store: false on requests.
  • Model Synchronization: The model picker dynamically reads ~/.codex/models_cache.json, reflecting any models currently available in OpenAI’s Codex interface. No need for app updates when new models are introduced.
  • Cost-Effective: By routing inference through your existing ChatGPT subscription, users avoid extra API charges.

Legal and Usage Clarifications

Is this approach permitted? Absolutely. OpenAI explicitly permits ChatGPT subscribers to access their plans through third-party clients, including integrations like this. Conversely, using an Anthropic subscription within Codex CLI or similar clients is not permitted under their terms, making this setup a one-way integration favoring ChatGPT users.

Limitations and Considerations

This solution is not a workaround to bypass rate limits or serve as an unauthorized backdoor. It employs the same OAuth flow, backend infrastructure, and headers as the official Codex CLI, ensuring compliance with provider policies.

Final Thoughts

This development signifies a substantial upgrade for developers who want an integrated, cost-efficient workflow. By utilizing Modyak’s innovative routing, ChatGPT Plus/Pro users can work seamlessly within Claude Code without additional API keys or expenses.

For more information and to try this setup yourself, visit Modyak.app. Embrace a more streamlined development environment with this elegant solution to optimize your AI tooling.

Disclaimer: Always ensure your usage complies with OpenAI’s and Anthropic’s terms of service.

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