Got the same issue when testing via AWS Hosted UI. It defaults to sending the main app idHave double checked the redirect uri is correct. If I manually change the client id to the identifier of the Service ID for apple signing I still get the same error. AWS are currently trying to replicate the issue on their sideFAILS https://appleid.apple.com/auth/authorize?client_id=<serviceID_Identifier>&redirect_uri=<redirect_URI>FAILS https://appleid.apple.com/auth/authorize?client_id=<main_app_id>&redirect_uri=<redirect_URI>