diff --git a/lib/widgets/compose_box.dart b/lib/widgets/compose_box.dart
index 64d867891a..bd31bdd015 100644
--- a/lib/widgets/compose_box.dart
+++ b/lib/widgets/compose_box.dart
@@ -102,16 +102,24 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
   // TODO(#307) use `max_topic_length` instead of hardcoded limit
   @override final maxLengthUnicodeCodePoints = kMaxTopicLengthCodePoints;
 
+  // TODO(server-10): simplify
+  String get noTopicTopic => store.connection.zulipFeatureLevel! >= 334
+    ? '' : kNoTopicTopic;
+
   @override
   String _computeTextNormalized() {
     String trimmed = text.trim();
-    return trimmed.isEmpty ? kNoTopicTopic : trimmed;
+    return trimmed.isEmpty ? noTopicTopic : trimmed;
   }
 
   @override
   List<TopicValidationError> _computeValidationErrors() {
     return [
-      if (mandatory && textNormalized == kNoTopicTopic)
+      if (mandatory && (
+        // While this may appear repetitive, the latter is not guarded behind
+        // a `zulipFeatureLevel` check. "(no topic)" still gets rejected
+        // by newer servers when topics are mandatory.
+        textNormalized == noTopicTopic || textNormalized == kNoTopicTopic))
         TopicValidationError.mandatoryButEmpty,
 
       if (
diff --git a/test/widgets/compose_box_test.dart b/test/widgets/compose_box_test.dart
index 46d5fbe635..550fff8d0e 100644
--- a/test/widgets/compose_box_test.dart
+++ b/test/widgets/compose_box_test.dart
@@ -680,6 +680,7 @@ void main() {
     late ZulipStream channel;
 
     Future<void> setupAndTapSend(WidgetTester tester, {
+      Account? account,
       bool? mandatoryTopics,
       required String topicInputText,
     }) async {
@@ -688,8 +689,10 @@ void main() {
       addTearDown(testBinding.reset);
 
       channel = eg.stream();
-      final account = eg.account(user: eg.selfUser);
+      account ??= eg.account(
+        user: eg.selfUser, zulipFeatureLevel: eg.futureZulipFeatureLevel);
       final initialSnapshot = eg.initialSnapshot(
+        zulipFeatureLevel: account.zulipFeatureLevel,
         realmMandatoryTopics: mandatoryTopics);
       await testBinding.globalStore.add(account, initialSnapshot);
 
@@ -714,8 +717,23 @@ void main() {
         expectedMessage: 'Topics are required in this organization.');
     }
 
-    testWidgets('empty topic -> (no topic)', (tester) async {
+    testWidgets('empty topic -> general chat', (tester) async {
       await setupAndTapSend(tester, topicInputText: '');
+      check(connection.lastRequest).isA<http.Request>()
+        ..method.equals('POST')
+        ..url.path.equals('/api/v1/messages')
+        ..bodyFields.deepEquals({
+          'type': 'stream',
+          'to': channel.streamId.toString(),
+          'topic': eg.defaultRealmEmptyTopicDisplayName,
+          'content': 'test content',
+          'read_by_sender': 'true',
+        });
+    });
+
+    testWidgets('legacy: empty topic -> (no topic)', (tester) async {
+      await setupAndTapSend(tester, topicInputText: '',
+        account: eg.account(user: eg.selfUser, zulipFeatureLevel: 333));
       check(connection.lastRequest).isA<http.Request>()
         ..method.equals('POST')
         ..url.path.equals('/api/v1/messages')
@@ -734,11 +752,24 @@ void main() {
       checkMessageNotSent(tester);
     });
 
+    testWidgets('if topics are mandatory, reject `realmEmptyTopicDisplayName`', (tester) async {
+      await setupAndTapSend(tester, topicInputText: eg.defaultRealmEmptyTopicDisplayName,
+        mandatoryTopics: true);
+      checkMessageNotSent(tester);
+    });
+
     testWidgets('if topics are mandatory, reject (no topic)', (tester) async {
       await setupAndTapSend(tester, topicInputText: '(no topic)',
         mandatoryTopics: true);
       checkMessageNotSent(tester);
     });
+
+    testWidgets('legacy: if topics are mandatory, reject (no topic)', (tester) async {
+      await setupAndTapSend(tester, topicInputText: '(no topic)',
+        account: eg.account(user: eg.selfUser, zulipFeatureLevel: 333),
+        mandatoryTopics: true);
+      checkMessageNotSent(tester);
+    });
   });
 
   group('uploads', () {