Here is one more suggestion that may improve Kakao’s openid authentication flow.
As described in the docs, if you want to log in a user with openid, you may redirect them with the scope set to “openid account_email profile_image profile_nickname”, for example.
But if the user has never been linked to the Kakao Application, then the user sees the Consent prompt.
Even if the user consents to the scope, the “openid” scope is not included in the subsequent callback.
Standard openid clients, like ones used by Firebase auth, will throw an error because there is no “openid” scope so no id_token is returned to the callback.
If “openid” is part of the original scope, please consider passing it on to the callback if the user consents to the other Consent Items. Of course, you will want to test that account_email, profile_image, and profile_nickname all have been consented to by the user because they will be automatically included in the openid token.