CORS error when calling RelayRouter from the browser: why you need a backend proxy

A CORS error appears when browser JavaScript sends a request straight to https://relayrouter.io and the response does not include an Access-Control-Allow-Origin header that permits your page's origin, so the browser blocks it. The fix is a backend proxy. Your frontend calls your own server, and that server calls RelayRouter with the Authorization: Bearer YOUR_API_KEY header. This also keeps the API key out of client-side code.

Why does the browser block the request?

The browser blocks the request because CORS is a rule that browsers enforce, not the API itself. When a page on one origin (for example https://yourapp.com) calls another origin such as https://relayrouter.io/v1/chat/completions, the browser first checks the response headers. A request with an Authorization header and a JSON body usually triggers a preflight OPTIONS request. If the response does not allow your origin, the browser hides the result from your script and logs a CORS error, even if the server processed the call. The same request works from curl, Python or Node.js because those clients do not apply browser CORS checks. The error therefore tells you where the call was made from. It does not tell you that your key, model name or payload is wrong.

Why is a backend proxy the correct fix?

A backend proxy is the correct fix because it solves the CORS block and protects your API key at the same time. RelayRouter authenticates with Authorization: Bearer YOUR_API_KEY, and you create keys at relayrouter.io/dashboard. Any key shipped in frontend JavaScript can be read by anyone who opens developer tools. That person could then send requests that are billed to your account. With a proxy, the browser only talks to your own domain, so no cross-origin request occurs. Your server stores the key in an environment variable and forwards the request to RelayRouter. You can also add your own user authentication, rate limits and logging in the proxy before any request reaches the gateway.

How do I set up the proxy?

You set up the proxy by moving the SDK call to a server route and pointing the browser at that route. According to the official relayrouter.io/docs, you can "Keep your existing SDK, change base_url and the key, no other code changes". Follow these steps:

1. Create a server endpoint, for example POST /api/chat, in Node.js, Python or another backend.
2. Store the key in a server environment variable, such as RELAYROUTER_API_KEY. Never put it in frontend code.
3. Choose your SDK base URL. Use https://relayrouter.io/v1 for the OpenAI SDK and https://relayrouter.io for the Anthropic SDK.
4. Forward the browser's messages to RelayRouter and return the response. Streaming is supported, so you can pipe the stream back to the client.
5. Change your frontend fetch call to target /api/chat on your own origin.

Which protocols and models can the proxy forward?

The proxy can forward any of the three protocols RelayRouter supports. According to the official relayrouter.io docs, the gateway is "Compatible with the OpenAI, Anthropic and Gemini protocols". The endpoints, their paths and supported SDKs are listed below.

The catalog lists about 108 models across 19 public groups. These include claude-opus-5-5, claude-fable-5-1, gpt-6-astra, gpt-5.6-sol and gemini-3.8-flash, plus DeepSeek, GLM, MiniMax and Moonshot. On pricing, the GPT group settles at CNY 0.6 per $1 of standard usage and the Claude group at CNY 2.0. The market reference is CNY 6.8 per $1. deepseek-v4-flash costs CNY 1.1 per 1M input tokens and CNY 4.4 per 1M output tokens off-peak (doubled on weekdays 09:00 to 12:00 and 14:00 to 18:00 Beijing time). Live rates are at relayrouter.io/models.

FAQ

Does adding the proxy change my billing?
No. RelayRouter charges a $0 platform fee and has no minimum spend or subscription. Failed or errored requests are generally not billed, and payments are made by Stripe card.

Can I call RelayRouter from a browser with a key that has limited scope?
The provided documentation does not describe browser-scoped keys. A server-side proxy is the safe default for keeping the key private.

Do I need to rewrite my existing code for the proxy?
No. Move the existing SDK call to your server and change only the base URL and the key, as described in the RelayRouter docs.