diff --git a/test/features/composer/presentation/extensions/get_subject_composer_test.dart b/test/features/composer/presentation/extensions/get_subject_composer_test.dart
new file mode 100644
index 000000000..7a321d1d7
--- /dev/null
+++ b/test/features/composer/presentation/extensions/get_subject_composer_test.dart
@@ -0,0 +1,210 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:model/email/email_action_type.dart';
+import 'package:tmail_ui_user/features/composer/presentation/extensions/email_action_type_extension.dart';
+import 'package:tmail_ui_user/main/localizations/app_localizations.dart';
+import 'package:tmail_ui_user/main/localizations/app_localizations_delegate.dart';
+import 'package:tmail_ui_user/main/localizations/localization_service.dart';
+
+void main() {
+  Future<BuildContext> pumpLocalizedWidget(
+    WidgetTester tester,
+    Locale locale,
+  ) async {
+    late BuildContext context;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        locale: locale,
+        supportedLocales: LocalizationService.supportedLocales,
+        localizationsDelegates: const [
+          AppLocalizationsDelegate(),
+          GlobalMaterialLocalizations.delegate,
+          GlobalWidgetsLocalizations.delegate,
+          GlobalCupertinoLocalizations.delegate,
+        ],
+        home: Builder(
+          builder: (ctx) {
+            context = ctx;
+            return const SizedBox();
+          },
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    return context;
+  }
+
+  group('EmailActionTypeExtension.getSubjectComposer', () {
+    const subject = 'Hello world';
+
+    group('reply actions', () {
+      test('should add default Re prefix when context is null', () {
+        final result = EmailActionType.reply.getSubjectComposer(null, subject);
+
+        expect(result, 'Re: Hello world');
+      });
+
+      test('should not duplicate Re prefix', () {
+        const subjectWithPrefix = 'Re: Hello world';
+
+        final result =
+            EmailActionType.reply.getSubjectComposer(null, subjectWithPrefix);
+
+        expect(result, subjectWithPrefix);
+      });
+
+      test('should ignore case when checking prefix', () {
+        const subjectWithPrefix = 're: Hello world';
+
+        final result =
+            EmailActionType.reply.getSubjectComposer(null, subjectWithPrefix);
+
+        expect(result, subjectWithPrefix);
+      });
+    });
+
+    group('forward actions', () {
+      test('should add default Fwd prefix when context is null', () {
+        final result =
+            EmailActionType.forward.getSubjectComposer(null, subject);
+
+        expect(result, 'Fwd: Hello world');
+      });
+
+      test('should not duplicate Fwd prefix', () {
+        const subjectWithPrefix = 'Fwd: Hello world';
+
+        final result =
+            EmailActionType.forward.getSubjectComposer(null, subjectWithPrefix);
+
+        expect(result, subjectWithPrefix);
+      });
+
+      test('should ignore case when checking prefix', () {
+        const subjectWithPrefix = 'fwd: Hello world';
+
+        final result =
+            EmailActionType.forward.getSubjectComposer(null, subjectWithPrefix);
+
+        expect(result, subjectWithPrefix);
+      });
+    });
+
+    group('edit actions', () {
+      test('editDraft should return original subject', () {
+        final result =
+            EmailActionType.editDraft.getSubjectComposer(null, subject);
+
+        expect(result, subject);
+      });
+
+      test('editSendingEmail should return original subject', () {
+        final result =
+            EmailActionType.editSendingEmail.getSubjectComposer(null, subject);
+
+        expect(result, subject);
+      });
+
+      test('editAsNewEmail should return original subject', () {
+        final result =
+            EmailActionType.editAsNewEmail.getSubjectComposer(null, subject);
+
+        expect(result, subject);
+      });
+    });
+
+    group('default case', () {
+      test('should return empty string for unsupported action', () {
+        final result =
+            EmailActionType.markAsRead.getSubjectComposer(null, subject);
+
+        expect(result, '');
+      });
+    });
+
+    group('localized prefix', () {
+      testWidgets('should use localized reply prefix (vi)', (tester) async {
+        final context = await pumpLocalizedWidget(
+          tester,
+          const Locale('vi'),
+        );
+
+        const subject = 'Hello world';
+
+        final result =
+            EmailActionType.reply.getSubjectComposer(context, subject);
+
+        final prefix = AppLocalizations.of(context).prefix_reply_email;
+
+        expect(result, '$prefix Hello world');
+      });
+
+      testWidgets('should not duplicate localized reply prefix',
+          (tester) async {
+        final context = await pumpLocalizedWidget(
+          tester,
+          const Locale('vi'),
+        );
+
+        final prefix = AppLocalizations.of(context).prefix_reply_email;
+
+        final subject = '$prefix Hello world';
+
+        final result =
+            EmailActionType.reply.getSubjectComposer(context, subject);
+
+        expect(result, subject);
+      });
+
+      testWidgets('should use localized forward prefix', (tester) async {
+        final context = await pumpLocalizedWidget(
+          tester,
+          const Locale('fr'),
+        );
+
+        const subject = 'Hello world';
+
+        final result =
+            EmailActionType.forward.getSubjectComposer(context, subject);
+
+        final prefix = AppLocalizations.of(context).prefix_forward_email;
+
+        expect(result, '$prefix Hello world');
+      });
+
+      testWidgets('should not duplicate localized forward prefix',
+          (tester) async {
+        final context = await pumpLocalizedWidget(
+          tester,
+          const Locale('fr'),
+        );
+
+        final prefix = AppLocalizations.of(context).prefix_forward_email;
+
+        final subject = '$prefix Hello world';
+
+        final result =
+            EmailActionType.forward.getSubjectComposer(context, subject);
+
+        expect(result, subject);
+      });
+
+      for (final locale in LocalizationService.supportedLocales) {
+        testWidgets('reply prefix works for $locale', (tester) async {
+          final context = await pumpLocalizedWidget(tester, locale);
+
+          final result =
+              EmailActionType.reply.getSubjectComposer(context, 'Hello');
+
+          final prefix = AppLocalizations.of(context).prefix_reply_email;
+
+          expect(result, '$prefix Hello');
+        });
+      }
+    });
+  });
+}