Multiple Redirect URI

Yes. A clientID supports multple callbacks

Commonly the issue is that redirect_uri in your outbound redirect is missing the ending / or you didn’t URIEncode.

So uou said “my redirect is http://localhost/auth
And

http://localhost/auth != http://localhost/auth/

The have to exact match and the common issue people do is the miss the / on the end leading to a mismatch. Either from not writing the / in code or not URIEncoding and the URI becoming confused when a URI is in a URI