Fix AppCheck concurrency infinite recursion bug in Auth

FirebaseAI/Sources/Types/Internal/AppCheck.swift

@@ -27,7 +27,7 @@ extension AppCheckInterop {
   ///   - domain: A string dictating where this method is being called from. Used in any thrown
   ///     errors, to avoid hard-to-parse traces.
   func fetchAppCheckToken(limitedUse: Bool,
-                          domain: String) async throws -> FIRAppCheckTokenResultInterop {
+                          domain: String) async throws -> (token: String, error: Error?) {
     if limitedUse {
       if let token = await getLimitedUseTokenAsync() {
         return token
@@ -47,14 +47,18 @@ extension AppCheckInterop {
       #endif
     }
 
-    return await getToken(forcingRefresh: false)
+    return await withCheckedContinuation { continuation in
+      self.getToken(forcingRefresh: false) { result in
+        continuation.resume(returning: (token: result.token, error: result.error))
+      }
+    }
   }
 
   private func getLimitedUseTokenAsync() async
-    -> FIRAppCheckTokenResultInterop? {
+    -> (token: String, error: Error?)? {
     // At the moment, `await` doesn’t get along with Objective-C’s optional protocol methods.
     await withCheckedContinuation { (continuation: CheckedContinuation<
-      FIRAppCheckTokenResultInterop?,
+      (token: String, error: Error?)?,
       Never
     >) in
       guard
@@ -67,7 +71,7 @@ extension AppCheckInterop {
 
       limitedUseTokenClosure { tokenResult in
         // The placeholder token should be used in the case of App Check error.
-        continuation.resume(returning: tokenResult)
+        continuation.resume(returning: (token: tokenResult.token, error: tokenResult.error))
       }
     }
   }

FirebaseAuth/CHANGELOG.md

@@ -1,4 +1,7 @@
 # Unreleased
+- [fixed] Fixed a persistent crash when interacting with App Check under some
+  Swift toolchains (like the Flutter SDK) by avoiding a compiler bug in Swift
+  concurrency thunk generation. (#16549)
 - [fixed] Fixed an issue on iOS 15+ where users were randomly logged out during
   prewarming while the device was locked. (#16498)
 

FirebaseAuth/Sources/Swift/Auth/AppCheckInterop+FirebaseAuth.swift

@@ -0,0 +1,27 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+
+@preconcurrency import FirebaseAppCheckInterop
+
+extension AppCheckInterop {
+  func getTokenResult(forcingRefresh: Bool) async -> (token: String, error: Error?) {
+    await withCheckedContinuation { continuation in
+      self.getToken(forcingRefresh: forcingRefresh) { result in
+        continuation.resume(returning: (token: result.token, error: result.error))
+      }
+    }
+  }
+}

FirebaseAuth/Sources/Swift/AuthProvider/OAuthProvider.swift

@@ -435,7 +435,7 @@ import Foundation
     }
 
     if let appCheck {
-      let tokenResult = await appCheck.getToken(forcingRefresh: false)
+      let tokenResult = await appCheck.getTokenResult(forcingRefresh: false)
       if let error = tokenResult.error {
         AuthLog.logWarning(code: "I-AUT000018",
                            message: "Error getting App Check token; using placeholder " +

FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift

@@ -641,7 +641,7 @@ import Foundation
       components?.queryItems = queryItems
 
       if let appCheck {
-        let tokenResult = await appCheck.getToken(forcingRefresh: false)
+        let tokenResult = await appCheck.getTokenResult(forcingRefresh: false)
         if let error = tokenResult.error {
           AuthLog.logWarning(code: "I-AUT000018",
                              message: "Error getting App Check token; using placeholder " +

FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift

@@ -70,7 +70,7 @@ final class AuthBackend: AuthBackendProtocol {
       await requestConfiguration.heartbeatLogger?.asyncHeaderValue()
     }
     let appCheckTokenHeaderValue = Task {
-      await requestConfiguration.appCheck?.getToken(forcingRefresh: false)
+      await requestConfiguration.appCheck?.getTokenResult(forcingRefresh: false)
     }
 
     return await withTaskCancellationHandler {

FirebaseFunctions/Sources/Internal/FunctionsContext.swift

@@ -66,12 +66,13 @@ struct FunctionsContextProvider: Sendable {
   }
 
   private func getAppCheckToken(options: HTTPSCallableOptions?) async -> String? {
-    guard
-      options?.requireLimitedUseAppCheckTokens != true,
-      let tokenResult = await appCheck?.getToken(forcingRefresh: false)
-    else { return nil }
-    // The placeholder token should be used in the case of App Check error.
-    return tokenResult.token
+    guard options?.requireLimitedUseAppCheckTokens != true, let appCheck else { return nil }
+
+    return await withCheckedContinuation { continuation in
+      appCheck.getToken(forcingRefresh: false) { result in
+        continuation.resume(returning: result.token)
+      }
+    }
   }
 
   private func getLimitedUseAppCheckToken(options: HTTPSCallableOptions?) async -> String? {