feat(core): Add Auth and AppCheck as App's registered service.

packages/firebase_ai/firebase_ai/example/lib/main.dart

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 import 'package:firebase_ai/firebase_ai.dart';
+
 import 'package:firebase_auth/firebase_auth.dart';
 import 'package:firebase_core/firebase_core.dart';
 import 'package:flutter/material.dart';
@@ -70,10 +71,10 @@ class _GenerativeAISampleState extends State<GenerativeAISample> {
 
   void _initializeModel(bool useVertexBackend) {
     if (useVertexBackend) {
-      final vertexInstance = FirebaseAI.vertexAI(auth: FirebaseAuth.instance);
+      final vertexInstance = FirebaseAI.vertexAI();
       _currentModel = vertexInstance.generativeModel(model: 'gemini-2.5-flash');
     } else {
-      final googleAI = FirebaseAI.googleAI(auth: FirebaseAuth.instance);
+      final googleAI = FirebaseAI.googleAI();
       _currentModel = googleAI.generativeModel(model: 'gemini-2.5-flash');
     }
   }

packages/firebase_ai/firebase_ai/example/lib/pages/chat_page.dart

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import 'package:firebase_auth/firebase_auth.dart';
 import 'package:flutter/material.dart';
 import 'package:firebase_ai/firebase_ai.dart';
 import '../widgets/message_widget.dart';
@@ -57,12 +56,12 @@ class _ChatPageState extends State<ChatPage> {
           : null,
     );
     if (widget.useVertexBackend) {
-      _model = FirebaseAI.vertexAI(auth: FirebaseAuth.instance).generativeModel(
+      _model = FirebaseAI.vertexAI().generativeModel(
         model: 'gemini-2.5-flash',
         generationConfig: generationConfig,
       );
     } else {
-      _model = FirebaseAI.googleAI(auth: FirebaseAuth.instance).generativeModel(
+      _model = FirebaseAI.googleAI().generativeModel(
         model: 'gemini-2.5-flash',
         generationConfig: generationConfig,
       );

packages/firebase_ai/firebase_ai/example/lib/pages/function_calling_page.dart

@@ -14,7 +14,7 @@
 
 import 'package:flutter/material.dart';
 import 'package:firebase_ai/firebase_ai.dart';
-import 'package:firebase_auth/firebase_auth.dart';
+
 import '../utils/function_call_utils.dart';
 import '../widgets/message_widget.dart';
 
@@ -235,9 +235,8 @@ class _FunctionCallingPageState extends State<FunctionCallingPage> {
           : null,
     );
 
-    final aiClient = widget.useVertexBackend
-        ? FirebaseAI.vertexAI(auth: FirebaseAuth.instance)
-        : FirebaseAI.googleAI(auth: FirebaseAuth.instance);
+    final aiClient =
+        widget.useVertexBackend ? FirebaseAI.vertexAI() : FirebaseAI.googleAI();
 
     _functionCallModel = aiClient.generativeModel(
       model: 'gemini-2.5-flash',

packages/firebase_ai/firebase_ai/example/lib/pages/grounding_page.dart

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-import 'package:firebase_auth/firebase_auth.dart';
 import 'package:flutter/material.dart';
 import 'package:firebase_ai/firebase_ai.dart';
 import '../widgets/message_widget.dart';
@@ -74,9 +73,8 @@ class _GroundingPageState extends State<GroundingPage> {
       }
     }
 
-    final aiProvider = widget.useVertexBackend
-        ? FirebaseAI.vertexAI(auth: FirebaseAuth.instance)
-        : FirebaseAI.googleAI(auth: FirebaseAuth.instance);
+    final aiProvider =
+        widget.useVertexBackend ? FirebaseAI.vertexAI() : FirebaseAI.googleAI();
 
     _model = aiProvider.generativeModel(
       model: 'gemini-2.5-flash',

packages/firebase_ai/firebase_ai/example/lib/pages/image_generation_page.dart

@@ -15,7 +15,7 @@
 import 'dart:typed_data';
 import 'package:flutter/material.dart';
 import 'package:firebase_ai/firebase_ai.dart';
-import 'package:firebase_auth/firebase_auth.dart';
+
 import '../widgets/message_widget.dart';
 
 class ImageGenerationPage extends StatefulWidget {
@@ -47,9 +47,8 @@ class _ImageGenerationPageState extends State<ImageGenerationPage> {
   }
 
   void _initializeModel() {
-    final aiClient = widget.useVertexBackend
-        ? FirebaseAI.vertexAI(auth: FirebaseAuth.instance)
-        : FirebaseAI.googleAI(auth: FirebaseAuth.instance);
+    final aiClient =
+        widget.useVertexBackend ? FirebaseAI.vertexAI() : FirebaseAI.googleAI();
 
     _model = aiClient.generativeModel(
       model: 'gemini-2.5-flash-image',

packages/firebase_ai/firebase_ai/lib/src/base_model.dart

@@ -282,19 +282,23 @@ abstract class BaseModel {
   ) {
     return () async {
       Map<String, String> headers = {};
+
+      final effectiveAppCheck = appCheck ?? app?.getService<FirebaseAppCheck>();
+      final effectiveAuth = auth ?? app?.getService<FirebaseAuth>();
+
       // Override the client name in Google AI SDK
       headers['x-goog-api-client'] =
           'gl-dart/$packageVersion fire/$packageVersion';
-      if (appCheck != null) {
+      if (effectiveAppCheck != null) {
         final appCheckToken = useLimitedUseAppCheckTokens == true
-            ? await appCheck.getLimitedUseToken()
-            : await appCheck.getToken();
+            ? await effectiveAppCheck.getLimitedUseToken()
+            : await effectiveAppCheck.getToken();
         if (appCheckToken != null) {
           headers['X-Firebase-AppCheck'] = appCheckToken;
         }
       }
-      if (auth != null) {
-        final idToken = await auth.currentUser?.getIdToken();
+      if (effectiveAuth != null) {
+        final idToken = await effectiveAuth.currentUser?.getIdToken();
         if (idToken != null) {
           headers['Authorization'] = 'Firebase $idToken';
         }

packages/firebase_ai/firebase_ai/lib/src/firebase_ai.dart

@@ -62,12 +62,18 @@ class FirebaseAI extends FirebasePluginPlatform {
   /// If pass in [appCheck], request session will get protected from abusing.
   static FirebaseAI vertexAI({
     FirebaseApp? app,
+    @Deprecated(
+        'Passing an explicit instance is deprecated, internal handling is now automatic.')
     FirebaseAppCheck? appCheck,
+    @Deprecated(
+        'Passing an explicit instance is deprecated, internal handling is now automatic.')
     FirebaseAuth? auth,
     String? location,
     bool? useLimitedUseAppCheckTokens,
   }) {
     app ??= Firebase.app();
+    appCheck ??= app.getService<FirebaseAppCheck>();
+    auth ??= app.getService<FirebaseAuth>();
     var instanceKey = '${app.name}::vertexai::$location';
 
     if (_cachedInstances.containsKey(instanceKey)) {
@@ -95,11 +101,17 @@ class FirebaseAI extends FirebasePluginPlatform {
   /// If pass in [appCheck], request session will get protected from abusing.
   static FirebaseAI googleAI({
     FirebaseApp? app,
+    @Deprecated(
+        'Passing an explicit instance is deprecated, internal handling is now automatic.')
     FirebaseAppCheck? appCheck,
+    @Deprecated(
+        'Passing an explicit instance is deprecated, internal handling is now automatic.')
     FirebaseAuth? auth,
     bool? useLimitedUseAppCheckTokens,
   }) {
     app ??= Firebase.app();
+    appCheck ??= app.getService<FirebaseAppCheck>();
+    auth ??= app.getService<FirebaseAuth>();
     var instanceKey = '${app.name}::googleai';
 
     if (_cachedInstances.containsKey(instanceKey)) {

packages/firebase_ai/firebase_ai/test/firebase_vertexai_test.dart

@@ -14,6 +14,7 @@
 
 import 'package:firebase_ai/firebase_ai.dart';
 import 'package:firebase_app_check/firebase_app_check.dart';
+import 'package:firebase_auth/firebase_auth.dart';
 import 'package:firebase_core/firebase_core.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -28,6 +29,7 @@ void main() {
   late FirebaseApp customApp;
   late FirebaseApp limitTokenApp;
   late FirebaseAppCheck customAppCheck;
+  late FirebaseAuth customAuth;
   late FirebaseAppCheck limitTokenAppCheck;
 
   group('FirebaseAI Tests', () {
@@ -47,6 +49,7 @@ void main() {
       appCheck = FirebaseAppCheck.instance;
       customAppCheck = FirebaseAppCheck.instanceFor(app: customApp);
       limitTokenAppCheck = FirebaseAppCheck.instanceFor(app: limitTokenApp);
+      customAuth = FirebaseAuth.instanceFor(app: customApp);
     });
 
     test('Singleton behavior', () {
@@ -96,6 +99,20 @@ void main() {
       expect(vertexAIAppCheck.useLimitedUseAppCheckTokens, true);
     });
 
+    test('Instance creation with auto-injected AppCheck', () {
+      final vertexAI = FirebaseAI.vertexAI(app: customApp);
+
+      expect(vertexAI.app, equals(customApp));
+      expect(vertexAI.appCheck, equals(customAppCheck));
+    });
+
+    test('Instance creation with auto-injected Auth', () {
+      final vertexAI = FirebaseAI.vertexAI(app: customApp);
+
+      expect(vertexAI.app, equals(customApp));
+      expect(vertexAI.auth, equals(customAuth));
+    });
+
     test('generativeModel creation with Grounding tools', () {
       final ai = FirebaseAI.googleAI();
 

packages/firebase_app_check/firebase_app_check/lib/src/firebase_app_check.dart

@@ -5,7 +5,8 @@
 
 part of '../firebase_app_check.dart';
 
-class FirebaseAppCheck extends FirebasePluginPlatform {
+class FirebaseAppCheck extends FirebasePluginPlatform
+    implements FirebaseService {
   static Map<String, FirebaseAppCheck> _firebaseAppCheckInstances = {};
 
   FirebaseAppCheck._({required this.app})
@@ -41,7 +42,9 @@ class FirebaseAppCheck extends FirebasePluginPlatform {
   /// Returns an instance using a specified [FirebaseApp].
   static FirebaseAppCheck instanceFor({required FirebaseApp app}) {
     return _firebaseAppCheckInstances.putIfAbsent(app.name, () {
-      return FirebaseAppCheck._(app: app);
+      final instance = FirebaseAppCheck._(app: app);
+      app.registerService<FirebaseAppCheck>(instance);
+      return instance;
     });
   }
 

packages/firebase_auth/firebase_auth/lib/src/firebase_auth.dart

@@ -6,7 +6,7 @@
 part of '../firebase_auth.dart';
 
 /// The entry point of the Firebase Authentication SDK.
-class FirebaseAuth extends FirebasePluginPlatform {
+class FirebaseAuth extends FirebasePluginPlatform implements FirebaseService {
   // Cached instances of [FirebaseAuth].
   static Map<String, FirebaseAuth> _firebaseAuthInstances = {};
 
@@ -45,7 +45,9 @@ class FirebaseAuth extends FirebasePluginPlatform {
     required FirebaseApp app,
   }) {
     return _firebaseAuthInstances.putIfAbsent(app.name, () {
-      return FirebaseAuth._(app: app);
+      final instance = FirebaseAuth._(app: app);
+      app.registerService<FirebaseAuth>(instance);
+      return instance;
     });
   }
 

packages/firebase_core/firebase_core/lib/src/firebase_app.dart

@@ -28,6 +28,7 @@ class FirebaseApp {
   /// Deleting the default app is not possible and throws an exception.
   Future<void> delete() async {
     await _delegate.delete();
+    _registries.remove(name);
   }
 
   /// The name of this [FirebaseApp].
@@ -71,4 +72,24 @@ class FirebaseApp {
 
   @override
   String toString() => '$FirebaseApp($name)';
+
+  static final Map<String, Map<Type, dynamic>> _registries = {};
+
+  /// Registers a service instance for this app.
+  void registerService<T extends FirebaseService>(T service) {
+    final registry = _registries.putIfAbsent(name, () => {});
+    registry[T] = service;
+  }
+
+  /// Returns a registered service instance for this app.
+  T? getService<T extends FirebaseService>() {
+    final registry = _registries[name];
+    if (registry != null) {
+      return registry[T] as T?;
+    }
+    return null;
+  }
 }
+
+/// A marker interface for Firebase services that can be registered in [FirebaseApp].
+abstract class FirebaseService {}

packages/firebase_core/firebase_core/test/firebase_core_test.dart

@@ -64,6 +64,15 @@ void main() {
         mock.app(testAppName),
       ]);
     });
+
+    test('.registerService() and .getService()', () {
+      FirebaseApp app = Firebase.app(testAppName);
+
+      final testService = TestService();
+      app.registerService<TestService>(testService);
+
+      expect(app.getService<TestService>(), testService);
+    });
   });
 
   test('.initializeApp() with demoProjectId', () async {
@@ -152,3 +161,5 @@ class MockFirebaseCore extends Mock
 
 // ignore: avoid_implementing_value_types
 class FakeFirebaseAppPlatform extends Fake implements FirebaseAppPlatform {}
+
+class TestService implements FirebaseService {}