Why does Firebase Google sign-in fail in my MAUI app due to token issues?

I'm attempting to authenticate with Firebase using the Google provider in a MAUI app, but the sign-in fails with an INVALID_IDP_RESPONSE error. The token is generated correctly and inspected at https://jwt.io, and even works in Postman, but the issue arises during the Firebase sign-in process.

Below is the code that creates the credential and attempts to sign in:

CSHARPCopied
if (requestCode == RC_SIGN_IN)
{
    var task = GoogleSignIn.GetSignedInAccountFromIntent(data);

    if (!task.IsSuccessful)
    {
        Console.WriteLine($"Google Sign-In failed. Exception: {task.Exception}");
        return;
    }

    var account = task.Result as GoogleSignInAccount;

    // Check if the IdToken is available
    if (!string.IsNullOrEmpty(account?.IdToken))
    {
        string idToken = account.IdToken;

        Console.WriteLine($"AccessToken: {account?.ServerAuthCode}"); // Access token
        Console.WriteLine($"IdToken: {account?.IdToken}"); // ID token


        Console.WriteLine($"IdToken: {idToken}");
        //OnGoogleSignInCompleted?.Invoke(idToken);
        SignInWithFirebase(idToken);
    }
    else
    {
        Console.WriteLine("IdToken is missing. Check your GoogleSignInOptions configuration.");
        OnGoogleSignInCompleted?.Invoke(null);
    }
}

The failure occurs at the Firebase sign-in step:

CSHARPCopied
var userCredential = await authClient.SignInWithCredentialAsync(credential);

Here is the complete Firebase sign-in method:

CSHARPCopied
private async void SignInWithFirebase(string idToken)
{
    try
    {
        Console.WriteLine("Starting Firebase Sign-In...");

        // Create the credential with the IdToken
        var credential = GoogleProvider.GetCredential(idToken);
        Console.WriteLine("Firebase Credential created.");

        // Initialize Firebase Auth Client
        var authClient = new FirebaseAuthClient(new FirebaseAuthConfig
        {
            ApiKey = "MY_KEY",
            AuthDomain = "MY_DOMAIN",
            Providers = new FirebaseAuthProvider[]
            {
            new GoogleProvider()
            }
        });
        Console.WriteLine("Firebase Auth Client initialized.");

        // Sign in with the credential
        var userCredential = await authClient.SignInWithCredentialAsync(credential);
        Console.WriteLine($"User signed in: {userCredential.User.Info.DisplayName} ({userCredential.User.Info.Email})");
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Firebase Sign-In failed: {ex.Message}");
        Console.WriteLine($"Stack Trace: {ex.StackTrace}");
    }
}

The error message received is:

DOTNETCopied
[DOTNET] Firebase Sign-In failed: Exception occured during Firebase Http request.
[DOTNET] Url: https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyAssertion?key=MY_KEY
[DOTNET] Request Data: {"requestUri":"https://mydomain,"postBody":"access_token=eyJhbGciOiJSUzI1NiIsImtpZCI6IjY... ",
[DOTNET] "message": "INVALID_IDP_RESPONSE : Unsuccessful check authorization response from Google: {\"error_description\": \"Invalid Value\"}",
[DOTNET] "errors": [
[DOTNET] {
[DOTNET] "message": "INVALID_IDP_RESPONSE : Unsuccessful check authorization response from Google: {\"error_description\": \"Invalid Value\"}",
[DOTNET] "domain": "global",
[DOTNET] "reason": "invalid"
[DOTNET] }
[DOTNET] ]

I suspect that the issue relates to the token type being passed (an access token instead of an ID token) when creating the Firebase credential. Any help or guidance is greatly appreciated, as I've spent an excessive amount of time troubleshooting this problem.