A browser security rule that controls which websites can request data from your server.
CORS (Cross-Origin Resource Sharing) is a server-side security mechanism that controls which websites can make requests to your API. By default, browsers block requests from different domains (origins) to protect users.
CORS headers tell the browser "this origin is allowed to access my resources." Without proper CORS configuration, your Frontend cannot fetch data from your Backend API. Middleware typically handles CORS configuration, adding the right headers to responses.
CORS issues are one of the most common debugging headaches for new developers. When you see a CORS error, the Network Tab in DevTools shows the blocked request with a specific Status Code and error message. Supabase, Edge Functions, and REST APIs all need CORS configured. The fix is always on the server side — adding allowed origins, methods, and headers. In Edge Functions, you must include CORS headers in every response, including error responses. Claude Code can generate the correct CORS headers when you describe the error you see in the console.
CORS is enforced through HTTP Headers on the HTTP Response. When Client-Side code calls a different origin, the browser checks these headers before allowing your app to read the response.
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