How to add CORS to an Existing API

If you’ve ever tried to build a browser-based app that talks directly to an external API—like Notion’s or OpenAI’s—you’ve probably run into a frustrating wall called CORS. Today, we’re excited to share how Genie Nexus makes these problems disappear with a simple solution.

What is CORS, and Why Does It Block Me?

CORS, or Cross-Origin Resource Sharing, is a security feature built into browsers. It stops your web app (running at, say, myapp.com) from making requests to another domain (like api.notion.com)—unless that server explicitly says “yes, that’s allowed” by setting certain headers.

This is a good thing for security, but for developers, it’s often a pain. If the API you want to use doesn’t return the right CORS headers, your browser blocks the request. That’s why many devs set up backend proxies just to make these calls on behalf of the browser.

Genie Nexus to the Rescue

Genie Nexus makes this easy. With just a few clicks, you can:

• Generate a custom endpoint that mimics the API you’re trying to call.
• Handle the required OPTIONS preflight request by returning a static, CORS-allowed response.
• Rewrite the headers for real API calls (GET, POST, etc.) so the browser gets the CORS green light.

No servers. No backend code. Just a simple layer that speaks the browser’s language.

What This Unlocks

With CORS out of the way, you can now call APIs like:

• Notion: Update or read your pages directly from the browser.
• OpenAI: Use GPT or DALL·E right in your frontend app.
• Any third-party API that lacks CORS headers.

This means no more spinning up a Node server just to fetch data. Genie Nexus acts like a temporary API “shield,” letting your browser play nice with otherwise off-limits endpoints.

Try it out today and unlock the full power of your frontend—without the backend drama.

→ Start with a CORS-friendly endpoint on Genie Nexus