diff --git a/pubspec.lock b/pubspec.lock
index 8aacf7c..a9f336b 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -17,6 +17,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.3.0"
+  antlr4:
+    dependency: transitive
+    description:
+      name: antlr4
+      sha256: "752b4a6e4ad97953652a2b2bbf5377f46c94b579d3372b50080c7e5858234a05"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.13.2"
   args:
     dependency: transitive
     description:
@@ -49,6 +57,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.2"
+  cel:
+    dependency: transitive
+    description:
+      name: cel
+      sha256: "51d77e16424d41b5fdb0a239be4c8a0550d4dd3f952801d35375ddd90cfb49da"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.4+1"
   characters:
     dependency: transitive
     description:
@@ -121,6 +137,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  convert:
+    dependency: transitive
+    description:
+      name: convert
+      sha256: b30acd5944035672bc15c6b7a8b47d773e41e2f17de064350988c5d02adb1c68
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.1.2"
   cross_file:
     dependency: transitive
     description:
@@ -145,6 +169,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.8"
+  dart_jsonwebtoken:
+    dependency: transitive
+    description:
+      name: dart_jsonwebtoken
+      sha256: cb79ed79baa02b4f59a597bf365873cbd83f9bb15273d63f7803802d21717c7d
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.4.0"
   equatable:
     dependency: "direct main"
     description:
@@ -161,6 +193,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.3.3"
+  fake_cloud_firestore:
+    dependency: "direct dev"
+    description:
+      name: fake_cloud_firestore
+      sha256: "95e01c95f956b31c62cb9fc54b8d51f32f1ae2909e6c0a5ce316da997e83a5c4"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.0+1"
+  fake_firebase_security_rules:
+    dependency: transitive
+    description:
+      name: fake_firebase_security_rules
+      sha256: "6af54bedfd6985451a9735f2cfac91ffe0128bfc92bf15e8544cd56c6941d6cc"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.4"
   ffi:
     dependency: transitive
     description:
@@ -273,6 +321,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.3.0"
+  firebase_auth_mocks:
+    dependency: "direct dev"
+    description:
+      name: firebase_auth_mocks
+      sha256: ee1076150ea51d181ae24d5f8e2686a1926b7cc54e25c0deaa8ac58705025551
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.15.1"
   firebase_auth_platform_interface:
     dependency: transitive
     description:
@@ -298,7 +354,7 @@ packages:
     source: hosted
     version: "4.6.0"
   firebase_core_platform_interface:
-    dependency: transitive
+    dependency: "direct dev"
     description:
       name: firebase_core_platform_interface
       sha256: "0ecda14c1bfc9ed8cac303dd0f8d04a320811b479362a9a4efb14fd331a473ce"
@@ -608,6 +664,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.1"
+  logger:
+    dependency: transitive
+    description:
+      name: logger
+      sha256: "25aee487596a6257655a1e091ec2ae66bc30e7af663592cc3a27e6591e05035c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.7.0"
+  logging:
+    dependency: transitive
+    description:
+      name: logging
+      sha256: c8245ada5f1717ed44271ed1c26b8ce85ca3228fd2ffdb75468ab01979309d61
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.3.0"
   matcher:
     dependency: transitive
     description:
@@ -640,6 +712,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.0.0"
+  mock_exceptions:
+    dependency: transitive
+    description:
+      name: mock_exceptions
+      sha256: "6e3e623712d2c6106ffe9e14732912522b565ddaa82a8dcee6cd4441b5984056"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.8.2"
+  more:
+    dependency: transitive
+    description:
+      name: more
+      sha256: e252628d2183cc09539b686abfbd9d8302675959b89a2a8146f5f4baca6ac5ba
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.7.0"
   nested:
     dependency: transitive
     description:
@@ -736,6 +824,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.8"
+  pointycastle:
+    dependency: transitive
+    description:
+      name: pointycastle
+      sha256: "92aa3841d083cc4b0f4709b5c74fd6409a3e6ba833ffc7dc6a8fee096366acf5"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.0.0"
   provider:
     dependency: transitive
     description:
@@ -744,14 +840,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.1.5+1"
+  rx:
+    dependency: transitive
+    description:
+      name: rx
+      sha256: "7f54bd39cc63a01c770c1de4b6ce8e135eb13119614cba2216bd9a93ccd29e56"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.4.0"
   rxdart:
     dependency: "direct main"
     description:
       name: rxdart
-      sha256: "0c7c0cedd93788d996e33041ffecda924cc54389199cde4e6a34b440f50044cb"
+      sha256: "5c3004a4a8dbb94bd4bf5412a4def4acdaa12e12f269737a5751369e12d1a962"
       url: "https://pub.dev"
     source: hosted
-    version: "0.27.7"
+    version: "0.28.0"
   shared_preferences:
     dependency: "direct main"
     description:
diff --git a/pubspec.yaml b/pubspec.yaml
index d6c8ab2..3ff181a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -39,7 +39,7 @@ dependencies:
   equatable: ^2.0.5
   meta: ^1.8.0
   font_awesome_flutter: ^10.2.1
-  rxdart: ^0.27.5
+  rxdart: ^0.28.0
   animated_text_kit: ^4.2.2
   shared_preferences: ^2.2.2
   google_fonts: ^6.2.1
@@ -48,6 +48,9 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  firebase_core_platform_interface: ^6.0.0
+  fake_cloud_firestore: ^4.0.0
+  firebase_auth_mocks: ^0.15.0
 
 
 # For information on the generic Dart part of this file, see the
diff --git a/test/widget_test.dart b/test/widget_test.dart
index b452b2a..ac74e14 100644
--- a/test/widget_test.dart
+++ b/test/widget_test.dart
@@ -1,7 +1,48 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:firebase_auth_mocks/firebase_auth_mocks.dart';
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart';
+import 'package:firebase_core_platform_interface/test.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:postbox_game/main.dart';
 import 'package:postbox_game/theme.dart';
+import 'package:postbox_game/user_repository.dart';
+
+// ---------------------------------------------------------------------------
+// Firebase mock setup
+// ---------------------------------------------------------------------------
+
+/// Call once per test run (or in setUp) to initialise the Firebase platform
+/// mock so that Firebase.initializeApp() succeeds without hitting real servers.
+Future<void> setupFirebaseMocks() async {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  setupFirebaseCoreMocks();
+  await Firebase.initializeApp();
+}
+
+// ---------------------------------------------------------------------------
+// Widget-level smoke tests
+// ---------------------------------------------------------------------------
 
 void main() {
+  setUpAll(setupFirebaseMocks);
+
+  group('App smoke tests', () {
+    testWidgets('PostboxGame widget tree renders without crashing', (tester) async {
+      await tester.pumpWidget(PostboxGame());
+      await tester.pump(); // process one frame
+      // Should show SplashScreen or LoginScreen (Firebase is mocked)
+      expect(find.byType(MaterialApp), findsOneWidget);
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // AppSpacing unit tests (from phase4-polish)
+  // ---------------------------------------------------------------------------
+
   group('AppSpacing', () {
     test('spacing scale is strictly increasing', () {
       expect(AppSpacing.xs, lessThan(AppSpacing.sm));
@@ -15,4 +56,70 @@ void main() {
       expect(AppSpacing.xs, greaterThan(0));
     });
   });
+
+  // ---------------------------------------------------------------------------
+  // UserRepository unit tests
+  // ---------------------------------------------------------------------------
+
+  group('UserRepository', () {
+    late FakeFirebaseFirestore fakeFirestore;
+    late MockFirebaseAuth mockAuth;
+    late UserRepository repo;
+
+    setUp(() {
+      fakeFirestore = FakeFirebaseFirestore();
+      mockAuth = MockFirebaseAuth();
+      repo = UserRepository(
+        firebaseAuth: mockAuth,
+        firestore: fakeFirestore,
+      );
+    });
+
+    test('signUp writes displayName to Firestore', () async {
+      // MockFirebaseAuth creates a user automatically on signUp.
+      await repo.signUp(email: 'test@example.com', password: 'password123');
+
+      final uid = mockAuth.currentUser?.uid;
+      expect(uid, isNotNull);
+
+      final doc = await fakeFirestore.collection('users').doc(uid).get();
+      expect(doc.data()?['displayName'], equals('test'));
+    });
+
+    test('getDisplayName returns stored name', () async {
+      const uid = 'user-abc';
+      await fakeFirestore.collection('users').doc(uid).set({'displayName': 'Postbox Pete'});
+
+      final name = await repo.getDisplayName(uid);
+      expect(name, equals('Postbox Pete'));
+    });
+
+    test('getDisplayName returns null for unknown uid', () async {
+      final name = await repo.getDisplayName('nonexistent-uid');
+      expect(name, isNull);
+    });
+
+    test('isSignedIn returns false when no user', () async {
+      final signedIn = await repo.isSignedIn();
+      expect(signedIn, isFalse);
+    });
+
+    test('isSignedIn returns true when a user is signed in', () async {
+      final auth = MockFirebaseAuth(signedIn: true);
+      final repo = UserRepository(firebaseAuth: auth, firestore: fakeFirestore);
+      expect(await repo.isSignedIn(), isTrue);
+    });
+
+    test('signUp writes email to Firestore', () async {
+      await repo.signUp(email: 'alice@example.com', password: 'password');
+      final uid = mockAuth.currentUser?.uid;
+      expect(uid, isNotNull);
+      final doc = await fakeFirestore
+          .collection('users')
+          .doc(uid)
+          .get();
+      expect(doc.data()?['email'], equals('alice@example.com'));
+    });
+  });
+
 }