anthropic api cors error

When building web applications, you might encounter a Cross-Origin Resource Sharing (CORS) block when calling the Anthropic API. This occurs because Anthropic explicitly blocks direct client-side requests from web browsers to protect your API keys from exposure. To fix this, you must adjust how your application communicates with Claude.

Stop calling the API from the frontend

Browsers enforce CORS policies to prevent unauthorised sites from making requests. If you attempt to invoke the Anthropic API directly from client-side JavaScript, the browser blocks the response. Additionally, exposing your API key in frontend code is a critical security risk, as anyone can inspect your network traffic and steal your credentials. You must never hardcode your API key in client-side code.

Build a secure backend proxy server

The correct way to interface with Claude is through a lightweight backend proxy server. Your frontend application makes a request to your own server, and your server securely forwards that request to Anthropic. Because backend-to-backend communication does not run inside a browser, it completely bypasses CORS restrictions. You can build this easily using Node.js, Python, or Go.

Set up serverless functions

If you do not want to manage a dedicated backend server, you can deploy a serverless function using platforms like Vercel, AWS Lambda, or Netlify. These functions act as secure intermediaries. Put your API key in the serverless environment variables, parse the request from your frontend, make the fetch request to Anthropic, and return the response back to your web app with the appropriate header permissions.

Validate your proxy headers

If you are already using a proxy server but still see a CORS block, the issue lies in your server configuration. Ensure your custom backend explicitly sends the 'Access-Control-Allow-Origin' header to permit your specific web domain. Do not use wildcard headers in production, as this compromises security. If you suspect broader platform issues are stopping your requests, you can read our guide on how to handle an /fix/anthropic-api-529-overloaded status.