Is it possible to handle sign-in from the server side if the user is enrolled in multi-factor authentication?

I am dealing with firebase authentication from the server-side using REST APIs But there are some users who have enrolled in Multi-Factor authentication using a Phone provider, is it still possible to handle the authentication method from the server-side?

The Admin SDKs for Firebase don't have any methods to sign the user in, as that should typically done client-side.

Firebase Authentication does has a REST API that you can call from anywhere, and that supports sign in with anonymous, email/password and OAuth providers. But as far as I can tell that doesn't support signing in with Phone Number.

Looking at Google Cloud Identity Platform, that offers features like MFA and multi-tenant provisioning a different pricing model, that does seem to have a documented REST API for signing in with phone number so you might want to check if whether MFA also works from the REST API there. If it does, it might be also worth to check if you can call that from a regular Firebase Authentication project, of whether it requires you to upgrade to GCIP.