When attempting to use Google Sign-In on a physical iOS device (both debug and release builds), the FirebaseAuth.instance.signInWithCredential call fails with a FirebaseAuthException whose code is internal-error. This occurs even though Firebase App Check appears to be perfectly configured on both the client and server. The error suggests a server-side failure during App Check token validation.

Steps to Reproduce

1. Run the app on a physical iOS device (fails on both debug and release builds).
2. Tap the "Sign In with Google" button.
3. The native Google Sign-In prompt appears and is completed successfully.
4. The FirebaseAuth.instance.signInWithCredential call is made.
   Expected behavior

The user should be successfully signed into Firebase.

Actual behavior

The signInWithCredential method throws a FirebaseAuthException with the code internal-error.

Exhaustive Troubleshooting Steps Taken

We have performed an exhaustive series of diagnostic steps that rule out all common configuration errors. The issue is not a user configuration error.

• Client-Side Configuration (iOS):

  • Verified Bundle ID and Team ID match exactly between Xcode and the Firebase Console.
  • Confirmed the App Attest capability is enabled in Xcode.
  • Verified the Runner.entitlements file contains <key>com.apple.developer.devicecheck.appattest-environment</key><string>production</string>.
  • Verified the GoogleService-Info.plist is up-to-date and is a member of the main Runner build target.
  • Confirmed AppDelegate.swift does not contain a conflicting manual FirebaseApp.configure() call.

• Server-Side Configuration:

  • Confirmed the "Play Integrity API" (for Android) is enabled in Google Cloud.
  • Confirmed the "Firebase App Check API" is enabled in Google Cloud.

• Build Environment & Dependencies:

  • Updated all firebase_* and google_sign_in packages to the latest stable versions.
  • Performed a full "deep clean" (flutter clean, deleting app from device, removing Pods and Podfile.lock, and running pod install --repo-update).

• Ultimate Isolation Test:

  • The error still occurs when using a GoogleService-Info.plist from a brand new, clean Firebase project. This proves the issue is not in the Firebase project's backend configuration but is local to the app code/dependencies.

This appears to be a deep incompatibility or bug within the FlutterFire SDKs.