Error 12501 authenticating with google sign-in

Well, this is very embarrassing, but I figured it out:

GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                    .requestIdToken(AuthenticatedActivity.this.getResources().getString(R.string.server_client_id))
                    .requestEmail().build();

I was sending it the resource ID instead of dereferenced string resource.

Obviously first check your release sha1 key is correct or not. But if still it is not working and you ar using google play services 8.4.0 (i.e.compile 'com.google.android.gms:play-services:8.4.0'), the issue could be solved by modifying GoogleSignInOption object. Instead of:

GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                    .requestEmail()  
       .requestIdToken("YOUR_WEB_API_ID.apps.googleusercontent.com")
                    .build();

You have to use :

GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestScopes(new Scope(Scopes.PLUS_LOGIN))
                .requestScopes(new Scope(Scopes.PLUS_ME))
                .requestEmail()
                .build();

This solves error returning statusCode=INTERNAL_ERROR OR statusCode=Error 12501 OR statusCode=Error 12500. Then this gso object could be used for creating GoogleApiClient as shown below:

 mGoogleApiClient = new GoogleApiClient.Builder(this)
                .enableAutoManage(this, this)
                .addApi(Auth.GOOGLE_SIGN_IN_API,gso)
               // .addApi(Plus.API, null)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
               // .addScope(Plus.SCOPE_PLUS_LOGIN)
                .build(); 

See this for detail: Google signin not working on release version of android

Make sure you added the SHA-1 fingerprint for your (release) signing key to the Firebase console

Find your SHA1 key: keytool -exportcert -list -v -alias <your-key-name> -keystore <path-to-production-keystore>

Add it to the firebase console: go to https://console.firebase.google.com, select your app, select settings.

In my project I had a different applicationId in my gradle file than packagename in my manifest.xml and that was the source of my problem.

The android key I had to created needed to have the applicationId fqdn and NOT the package name (contrary to what google tells you) for it to work for me.

Thought I'd leave that here in case it saves time to someone.