CORS
The browser blocked this page's API request. Check whether CORS is not allowing this site.
app.example.com requests api.example.com, the server must allow that page origin in its response before the browser gives the data to script. It protects cross-origin reads in browsers; the server still decides whether an API can be called and whether a user has permission.http://localhost:3000Content-Type: application/jsonWaiting for response headers…An origin comes from the address: A different protocol, domain, or port can mean a different origin. A local page at localhost:3000 calling localhost:4000 is a common cross-origin case too.
The server declares the allowed scope: Before the browser exposes a response, it checks CORS response headers. A server should allow the real sites, methods, and credentials needed—not every origin just to remove an error.
You can say this to an AI Agent
Investigate the CORS error when `http://localhost:3000` calls `http://localhost:4000`. Allow only the origin, methods, and credentials needed by this development page; do not open every origin. Then make a real browser request to verify it.
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