PlatformException (PlatformException(sign_in_failed, com.google.android.gms.common.api.ApiException: 12500: , null))

The OP, @Mircea, specifically mentioned error 12500. This is how I overcame that error:

I have spent the last few days running into the same problem. I refactored my code dozens of times, added break points, reset the SHA-1, SHA-256 and did just about every other 'code' based solution I could find. I was constantly running into the error:

E/flutter (11935): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: PlatformException(sign_in_failed, com.google.android.gms.common.api.ApiException: 12500: , null)

I was losing my mind so I decided to start with fresh eyes and move slowly through solutions one more time. I noticed an obscure post nestled deep in a thread that mentioned something about having a support email. It was not until I added a said support email that things "magically" started working. I've attached a screenshot below of the Firebase settings for my project.

You need to complete "Oauth consent screen"

I went to this page (https://console.developers.google.com/apis/credentials)Do not forget to select your project on the list (top left). Then go on "Oauth consent screen"

I filled the application logo and the application homepage link and aplication privacy policy link (with the project.firebaseapp.com) and saved.

Now it's working, I can login.

This must be explained in the documentation. People are going to be crazy and to don't try flutter with this kind of lack of documentation.

In my case, I have not enabled Google Sign In method.