Prepares:

• Firebase project
• [Firebase Authentiation tab] Google SignIn enabled

Steps:

1. In Firebase console, create android app, following steps.
2. Install Google Sigin package
3. [Firebase Console] Add fingerprint:

# excute bellow cmd to get your keys's fingerprint (SHA1 & SHA256)
# in project root
cd android && ./gradlew signingReport

Example result:

Then Copy SHA1 & SHA256 above to your firebase console android app.

3.1. [Important] Add fingerprint for your Production app:

• Go to play console > App signing key certificate then copy SHA1 & SHA256 to your firebase console android app: https://play.google.com/console/u/0/developers/DEVELOPER_ID/app/APP_ID/keymanagement

4. Code: e.g.: login.dart

  Future<void> _signInWithGoogle() async {
    FocusScope.of(context).unfocus();
    if (mounted) {
      setState(() {
        _isLoading = true;
      });
    }
    try {
      if (Platform.isAndroid) {
        await _googleSignIn.initialize(
						/* optional serverClientId: normally in your downloaded android/app/google-services.json file, use WEB client id which "client_type": 3 */
						serverClientId: "YOUR_oauth_client.client_id",
        );
      }
      final GoogleSignInAccount googleUser = await _googleSignIn.authenticate();
      final GoogleSignInAuthentication googleAuth = googleUser.authentication;
      final AuthCredential credential = GoogleAuthProvider.credential(idToken: googleAuth.idToken);
      UserCredential userCredential = await _auth.signInWithCredential(credential);
      if (kDebugMode) {
        print("Google Sign In Successful: ${userCredential.user?.uid}");
      }
      if (mounted) {
        Navigator.of(
          context,
        ).pushReplacement(MaterialPageRoute(builder: (context) => const MainScreen()));
      }
    } catch (e) {
      if (mounted) {
        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.toString())));
      }
      if (kDebugMode) {
        print("Failed to sign in with Google: $e");
      }
    } finally {
      if (mounted) setState(() => _isLoading = false);
    }
  }

Or modern code:

  Future<void> _signInWithGoogle() async {
    FocusScope.of(context).unfocus();
    if (mounted) {
      setState(() {
        _isLoading = true;
      });
    }
    try {
      final GoogleSignIn signIn = GoogleSignIn.instance;
      unawaited(
        signIn
            .initialize(
							/* optional serverClientId: normally in your downloaded android/app/google-services.json file, 
							use WEB client id which "client_type": 3 */
              serverClientId: "YOUR_oauth_client.client_id",
            )
            .then((_) {
              signIn.authenticationEvents.listen(
                (event) async {
                  final GoogleSignInAccount googleUser;
                  switch (event) {
                    case GoogleSignInAuthenticationEventSignIn():
                      googleUser = event.user;
                    case GoogleSignInAuthenticationEventSignOut():
                      // TODO: Handle this case.
                      throw UnimplementedError();
                  }
                  final GoogleSignInAuthentication googleAuth = googleUser.authentication;
                  final AuthCredential credential = GoogleAuthProvider.credential(
                    idToken: googleAuth.idToken,
                  );
                  UserCredential userCredential = await _auth.signInWithCredential(credential);
                  if (kDebugMode) {
                    print("Google Sign In Successful: ${userCredential.user?.uid}");
                  }
                  if (mounted) {
                    Navigator.of(
                      context,
                    ).pushReplacement(MaterialPageRoute(builder: (context) => const MainScreen()));
                  }
                },
                onError: (e) => {
                  if (mounted && !e.toString().contains("cancelled by the user"))
                    {
                      ScaffoldMessenger.of(
                        context,
                      ).showSnackBar(SnackBar(content: Text(e.toString()))),
                    },
                },
                onDone: () {
                  setState(() {
                    _isLoading = false;
                  });
                },
              );

              /// This example always uses the stream-based approach to determining
              /// which UI state to show, rather than using the future returned here,
              /// if any, to conditionally skip directly to the signed-in state.
              signIn.attemptLightweightAuthentication();
            }),
      );
    } catch (e) {
      if (mounted) {
        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.toString())));
      }
      if (kDebugMode) {
        print("Failed to sign in with Google: $e");
      }
    } finally {
      if (mounted) setState(() => _isLoading = false);
    }
  }

5. Your google sigin should worked as bellow:

Screen_Recording_20251001_172033.mp4

Also see some issues if have any questions:

• google_sign_in 7.1.0 required serverClientId for Android
• https://pub.dev/packages/google_sign_in_android#integration