I subclassed CredentialsAuthProvider to return a custom response from CredentialsAuthProvider.OnAuthenticated (a HttpResult containing my response DTO)
A user posts to this providers endpoint while already having logged in with an earlier request.
In this scenario:
CredentialsAuthProvider.IsAuthorized gets a session with its IsAuthenticated flag set (as the client still has the session cookies from the authenticated first session) and I return true.
AuthenticateService.Authenticate will return the null response.
AuthenticateService.Post will return a default authentication response.
In this case I’d like to be able to return my custom response instead of the default one.
Or am I missing a vital hook somewhere that I can leverage
It’s default behavior and it can’t be overridden now. If user already authenticated then AuthenticateService.Authenticate returns null and to change this behavior need to change ServiceStack public API. You should ask @mythz if he thinks that the change is useful (I personally think it makes sense) it could be added.