diff --git a/lib/api/model/events.dart b/lib/api/model/events.dart
index fd806a4ff1..e1e8d92bbd 100644
--- a/lib/api/model/events.dart
+++ b/lib/api/model/events.dart
@@ -40,6 +40,12 @@ sealed class Event {
       case 'message': return MessageEvent.fromJson(json);
       case 'update_message': return UpdateMessageEvent.fromJson(json);
       case 'delete_message': return DeleteMessageEvent.fromJson(json);
+      case 'update_message_flags':
+        switch (json['op'] as String) {
+          case 'add': return UpdateMessageFlagsAddEvent.fromJson(json);
+          case 'remove': return UpdateMessageFlagsRemoveEvent.fromJson(json);
+          default: return UnexpectedEvent.fromJson(json);
+        }
       case 'reaction': return ReactionEvent.fromJson(json);
       case 'heartbeat': return HeartbeatEvent.fromJson(json);
       // TODO add many more event types
@@ -439,13 +445,122 @@ class DeleteMessageEvent extends Event {
   Map<String, dynamic> toJson() => _$DeleteMessageEventToJson(this);
 }
 
-/// As in [DeleteMessageEvent.messageType].
+/// As in [DeleteMessageEvent.messageType]
+/// or [UpdateMessageFlagsMessageDetail.type].
 @JsonEnum(fieldRename: FieldRename.snake)
 enum MessageType {
   stream,
   private;
 }
 
+/// A Zulip event of type `update_message_flags`.
+///
+/// For the corresponding API docs, see subclasses.
+sealed class UpdateMessageFlagsEvent extends Event {
+  @override
+  @JsonKey(includeToJson: true)
+  String get type => 'update_message_flags';
+
+  String get op;
+
+  @JsonKey(unknownEnumValue: MessageFlag.unknown)
+  final MessageFlag flag;
+  final List<int> messages;
+
+  UpdateMessageFlagsEvent({
+    required super.id,
+    required this.flag,
+    required this.messages,
+  });
+}
+
+/// An [UpdateMessageFlagsEvent] with op `add`: https://zulip.com/api/get-events#update_message_flags-add
+@JsonSerializable(fieldRename: FieldRename.snake)
+class UpdateMessageFlagsAddEvent extends UpdateMessageFlagsEvent {
+  @override
+  String get op => 'add';
+
+  final bool all;
+
+  UpdateMessageFlagsAddEvent({
+    required super.id,
+    required super.flag,
+    required super.messages,
+    required this.all,
+  });
+
+  factory UpdateMessageFlagsAddEvent.fromJson(Map<String, dynamic> json) =>
+    _$UpdateMessageFlagsAddEventFromJson(json);
+
+  @override
+  Map<String, dynamic> toJson() => _$UpdateMessageFlagsAddEventToJson(this);
+}
+
+/// An [UpdateMessageFlagsEvent] with op `remove`: https://zulip.com/api/get-events#update_message_flags-remove
+@JsonSerializable(fieldRename: FieldRename.snake)
+class UpdateMessageFlagsRemoveEvent extends UpdateMessageFlagsEvent {
+  @override
+  String get op => 'remove';
+
+  // final bool all; // deprecated, ignore
+  final Map<int, UpdateMessageFlagsMessageDetail>? messageDetails;
+
+  UpdateMessageFlagsRemoveEvent({
+    required super.id,
+    required super.flag,
+    required super.messages,
+    required this.messageDetails,
+  });
+
+  factory UpdateMessageFlagsRemoveEvent.fromJson(Map<String, dynamic> json) {
+    final result = _$UpdateMessageFlagsRemoveEventFromJson(json);
+    // Crunchy-shell validation
+    if (
+      result.flag == MessageFlag.read
+      && true // (we assume `event_types` has `message` and `update_message_flags`)
+    ) {
+      result.messageDetails as Map<int, UpdateMessageFlagsMessageDetail>;
+    }
+    return result;
+  }
+
+  @override
+  Map<String, dynamic> toJson() => _$UpdateMessageFlagsRemoveEventToJson(this);
+}
+
+/// As in [UpdateMessageFlagsRemoveEvent.messageDetails].
+@JsonSerializable(fieldRename: FieldRename.snake)
+class UpdateMessageFlagsMessageDetail {
+  final MessageType type;
+  final bool? mentioned;
+  final List<int>? userIds;
+  final int? streamId;
+  final String? topic;
+
+  UpdateMessageFlagsMessageDetail({
+    required this.type,
+    required this.mentioned,
+    required this.userIds,
+    required this.streamId,
+    required this.topic,
+  });
+
+  factory UpdateMessageFlagsMessageDetail.fromJson(Map<String, dynamic> json) {
+    final result = _$UpdateMessageFlagsMessageDetailFromJson(json);
+    // Crunchy-shell validation
+    switch (result.type) {
+      case MessageType.stream:
+        result.streamId as int;
+        result.topic as String;
+      case MessageType.private:
+        result.userIds as List<int>;
+    }
+    return result;
+  }
+
+  Map<String, dynamic> toJson() => _$UpdateMessageFlagsMessageDetailToJson(this);
+}
+
 /// A Zulip event of type `reaction`, with op `add` or `remove`.
 ///
 /// See:
diff --git a/lib/api/model/events.g.dart b/lib/api/model/events.g.dart
index b84f6991db..8424ac3315 100644
--- a/lib/api/model/events.g.dart
+++ b/lib/api/model/events.g.dart
@@ -275,6 +275,75 @@ const _$MessageTypeEnumMap = {
   MessageType.private: 'private',
 };
 
+UpdateMessageFlagsAddEvent _$UpdateMessageFlagsAddEventFromJson(
+        Map<String, dynamic> json) =>
+    UpdateMessageFlagsAddEvent(
+      id: json['id'] as int,
+      flag: $enumDecode(_$MessageFlagEnumMap, json['flag'],
+          unknownValue: MessageFlag.unknown),
+      messages:
+          (json['messages'] as List<dynamic>).map((e) => e as int).toList(),
+      all: json['all'] as bool,
+    );
+
+Map<String, dynamic> _$UpdateMessageFlagsAddEventToJson(
+        UpdateMessageFlagsAddEvent instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'type': instance.type,
+      'flag': instance.flag,
+      'messages': instance.messages,
+      'all': instance.all,
+    };
+
+UpdateMessageFlagsRemoveEvent _$UpdateMessageFlagsRemoveEventFromJson(
+        Map<String, dynamic> json) =>
+    UpdateMessageFlagsRemoveEvent(
+      id: json['id'] as int,
+      flag: $enumDecode(_$MessageFlagEnumMap, json['flag'],
+          unknownValue: MessageFlag.unknown),
+      messages:
+          (json['messages'] as List<dynamic>).map((e) => e as int).toList(),
+      messageDetails: (json['message_details'] as Map<String, dynamic>?)?.map(
+        (k, e) => MapEntry(
+            int.parse(k),
+            UpdateMessageFlagsMessageDetail.fromJson(
+                e as Map<String, dynamic>)),
+      ),
+    );
+
+Map<String, dynamic> _$UpdateMessageFlagsRemoveEventToJson(
+        UpdateMessageFlagsRemoveEvent instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'type': instance.type,
+      'flag': instance.flag,
+      'messages': instance.messages,
+      'message_details':
+          instance.messageDetails?.map((k, e) => MapEntry(k.toString(), e)),
+    };
+
+UpdateMessageFlagsMessageDetail _$UpdateMessageFlagsMessageDetailFromJson(
+        Map<String, dynamic> json) =>
+    UpdateMessageFlagsMessageDetail(
+      type: $enumDecode(_$MessageTypeEnumMap, json['type']),
+      mentioned: json['mentioned'] as bool?,
+      userIds:
+          (json['user_ids'] as List<dynamic>?)?.map((e) => e as int).toList(),
+      streamId: json['stream_id'] as int?,
+      topic: json['topic'] as String?,
+    );
+
+Map<String, dynamic> _$UpdateMessageFlagsMessageDetailToJson(
+        UpdateMessageFlagsMessageDetail instance) =>
+    <String, dynamic>{
+      'type': _$MessageTypeEnumMap[instance.type]!,
+      'mentioned': instance.mentioned,
+      'user_ids': instance.userIds,
+      'stream_id': instance.streamId,
+      'topic': instance.topic,
+    };
+
 ReactionEvent _$ReactionEventFromJson(Map<String, dynamic> json) =>
     ReactionEvent(
       id: json['id'] as int,
diff --git a/lib/model/message_list.dart b/lib/model/message_list.dart
index a08de48185..1b93fc99c7 100644
--- a/lib/model/message_list.dart
+++ b/lib/model/message_list.dart
@@ -420,6 +420,35 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     notifyListeners();
   }
 
+  void maybeUpdateMessageFlags(UpdateMessageFlagsEvent event) {
+    final isAdd = switch (event) {
+      UpdateMessageFlagsAddEvent()    => true,
+      UpdateMessageFlagsRemoveEvent() => false,
+    };
+
+    bool didUpdateAny = false;
+    if (isAdd && (event as UpdateMessageFlagsAddEvent).all) {
+      for (final message in messages) {
+        message.flags.add(event.flag);
+        didUpdateAny = true;
+      }
+    } else {
+      for (final messageId in event.messages) {
+        final index = _findMessageWithId(messageId);
+        if (index != -1) {
+          final message = messages[index];
+          isAdd ? message.flags.add(event.flag) : message.flags.remove(event.flag);
+          didUpdateAny = true;
+        }
+      }
+    }
+    if (!didUpdateAny) {
+      return;
+    }
+
+    notifyListeners();
+  }
+
   void maybeUpdateMessageReactions(ReactionEvent event) {
     final index = _findMessageWithId(event.messageId);
     if (index == -1) {
diff --git a/lib/model/store.dart b/lib/model/store.dart
index 630b6dce00..8860d815f3 100644
--- a/lib/model/store.dart
+++ b/lib/model/store.dart
@@ -309,6 +309,11 @@ class PerAccountStore extends ChangeNotifier {
     } else if (event is DeleteMessageEvent) {
       assert(debugLog("server event: delete_message ${event.messageIds}"));
       // TODO handle
+    } else if (event is UpdateMessageFlagsEvent) {
+      assert(debugLog("server event: update_message_flags/${event.op} ${event.flag.toJson()}"));
+      for (final view in _messageListViews) {
+        view.maybeUpdateMessageFlags(event);
+      }
     } else if (event is ReactionEvent) {
       assert(debugLog("server event: reaction/${event.op}"));
       for (final view in _messageListViews) {
diff --git a/test/api/model/events_test.dart b/test/api/model/events_test.dart
index 7e0a404ca6..337fa99b94 100644
--- a/test/api/model/events_test.dart
+++ b/test/api/model/events_test.dart
@@ -10,19 +10,6 @@ import 'events_checks.dart';
 import 'model_checks.dart';
 
 void main() {
-  test('message: move flags into message object', () {
-    final message = eg.streamMessage();
-    MessageEvent mkEvent(List<MessageFlag> flags) => Event.fromJson({
-      'type': 'message',
-      'id': 1,
-      'message': (deepToJson(message) as Map<String, dynamic>)..remove('flags'),
-      'flags': flags.map((f) => f.toJson()).toList(),
-    }) as MessageEvent;
-    check(mkEvent(message.flags)).message.jsonEquals(message);
-    check(mkEvent([])).message.flags.deepEquals([]);
-    check(mkEvent([MessageFlag.read])).message.flags.deepEquals([MessageFlag.read]);
-  });
-
   test('user_settings: all known settings have event handling', () {
     final dataClassFieldNames = UserSettings.debugKnownNames;
     final enumNames = UserSettingName.values.map((n) => n.name);
@@ -42,4 +29,35 @@ void main() {
         '      on the pattern of the existing cases.'
     ).isEmpty();
   });
+
+  test('message: move flags into message object', () {
+    final message = eg.streamMessage();
+    MessageEvent mkEvent(List<MessageFlag> flags) => Event.fromJson({
+      'type': 'message',
+      'id': 1,
+      'message': (deepToJson(message) as Map<String, dynamic>)..remove('flags'),
+      'flags': flags.map((f) => f.toJson()).toList(),
+    }) as MessageEvent;
+    check(mkEvent(message.flags)).message.jsonEquals(message);
+    check(mkEvent([])).message.flags.deepEquals([]);
+    check(mkEvent([MessageFlag.read])).message.flags.deepEquals([MessageFlag.read]);
+  });
+
+  test('update_message_flags/remove: require messageDetails in mark-as-unread', () {
+    final baseJson = {
+      'id': 1,
+      'type': 'update_message_flags',
+      'op': 'remove',
+      'flag': 'starred',
+      'messages': [123],
+      'all': false,
+    };
+    check(() => UpdateMessageFlagsRemoveEvent.fromJson(baseJson)).returnsNormally();
+    check(() => UpdateMessageFlagsRemoveEvent.fromJson({
+      ...baseJson, 'flag': 'read',
+    })).throws();
+    check(() => UpdateMessageFlagsRemoveEvent.fromJson({
+      ...baseJson, 'message_details': {'123': {'type': 'private', 'mentioned': false, 'user_ids': [2]}},
+    })).returnsNormally();
+  });
 }
diff --git a/test/example_data.dart b/test/example_data.dart
index cc6472a3a2..7f8206c735 100644
--- a/test/example_data.dart
+++ b/test/example_data.dart
@@ -155,7 +155,7 @@ StreamMessage streamMessage({
   int? lastEditTimestamp,
   List<Reaction>? reactions,
   int? timestamp,
-  List<String>? flags,
+  List<MessageFlag>? flags,
 }) {
   final effectiveStream = stream ?? _stream();
   // The use of JSON here is convenient in order to delegate parts of the data
@@ -191,7 +191,7 @@ DmMessage dmMessage({
   String? contentMarkdown,
   int? lastEditTimestamp,
   int? timestamp,
-  List<String>? flags,
+  List<MessageFlag>? flags,
 }) {
   assert(!to.any((user) => user.userId == from.userId));
   return DmMessage.fromJson({
diff --git a/test/model/message_list_test.dart b/test/model/message_list_test.dart
index de2288a441..d93af2258d 100644
--- a/test/model/message_list_test.dart
+++ b/test/model/message_list_test.dart
@@ -439,6 +439,137 @@ void main() async {
     });
   });
 
+  group('maybeUpdateMessageFlags', () {
+    UpdateMessageFlagsAddEvent mkAddEvent(
+      MessageFlag flag,
+      List<int> messageIds, {
+      bool all = false,
+    }) {
+      return UpdateMessageFlagsAddEvent(
+        id: 1,
+        flag: flag,
+        messages: messageIds,
+        all: all,
+      );
+    }
+
+    UpdateMessageFlagsRemoveEvent mkRemoveEvent(MessageFlag flag, List<Message> messages) {
+      final messageDetails = Map.fromEntries(messages.map((message) {
+        final mentioned = message.flags.contains(MessageFlag.mentioned)
+          || message.flags.contains(MessageFlag.wildcardMentioned);
+        return MapEntry(
+          message.id,
+          switch (message) {
+            StreamMessage() => UpdateMessageFlagsMessageDetail(
+              type: MessageType.stream,
+              mentioned: mentioned,
+              streamId: message.streamId,
+              topic: message.subject,
+              userIds: null,
+            ),
+            DmMessage() => UpdateMessageFlagsMessageDetail(
+              type: MessageType.private,
+              mentioned: mentioned,
+              streamId: null,
+              topic: null,
+              userIds: DmNarrow.ofMessage(message, selfUserId: eg.selfUser.userId).otherRecipientIds,
+            ),
+          });
+      }));
+
+      return UpdateMessageFlagsRemoveEvent(
+        id: 1,
+        flag: flag,
+        messages: messages.map((m) => m.id).toList(),
+        messageDetails: messageDetails,
+      );
+    }
+
+    group('add flag', () {
+      test('not in list', () async {
+        prepare();
+        final message = eg.streamMessage(id: 1, flags: []);
+        await prepareMessages(foundOldest: true, messages: [message]);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [2]));
+        checkNotNotified();
+        check(model).messages.single.flags.deepEquals([]);
+      });
+
+      test('affected message, unaffected message, absent message', () async {
+        prepare();
+        final message1 = eg.streamMessage(id: 1, flags: []);
+        final message2 = eg.streamMessage(id: 2, flags: []);
+        await prepareMessages(foundOldest: true, messages: [message1, message2]);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [message2.id, 3]));
+        checkNotifiedOnce();
+        check(model).messages
+          ..[0].flags.deepEquals([])
+          ..[1].flags.deepEquals([MessageFlag.read]);
+      });
+
+      test('all: true, list non-empty', () async {
+        prepare();
+        final message1 = eg.streamMessage(id: 1, flags: []);
+        final message2 = eg.streamMessage(id: 2, flags: []);
+        await prepareMessages(foundOldest: true, messages: [message1, message2]);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [], all: true));
+        checkNotifiedOnce();
+        check(model).messages
+          ..[0].flags.deepEquals([MessageFlag.read])
+          ..[1].flags.deepEquals([MessageFlag.read]);
+      });
+
+      test('all: true, list empty', () async {
+        prepare();
+        await prepareMessages(foundOldest: true, messages: []);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [], all: true));
+        checkNotNotified();
+      });
+
+      test('other flags not clobbered', () async {
+        final message = eg.streamMessage(flags: [MessageFlag.starred]);
+        prepare();
+        await prepareMessages(foundOldest: true, messages: [message]);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [message.id]));
+        checkNotifiedOnce();
+        check(model).messages.single.flags.deepEquals([MessageFlag.starred, MessageFlag.read]);
+      });
+    });
+
+    group('remove flag', () {
+      test('not in list', () async {
+        prepare();
+        final message = eg.streamMessage(id: 1, flags: [MessageFlag.read]);
+        await prepareMessages(foundOldest: true, messages: [message]);
+        model.maybeUpdateMessageFlags(mkAddEvent(MessageFlag.read, [2]));
+        checkNotNotified();
+        check(model).messages.single.flags.deepEquals([MessageFlag.read]);
+      });
+
+      test('affected message, unaffected message, absent message', () async {
+        prepare();
+        final message1 = eg.streamMessage(id: 1, flags: [MessageFlag.read]);
+        final message2 = eg.streamMessage(id: 2, flags: [MessageFlag.read]);
+        final message3 = eg.streamMessage(id: 3, flags: [MessageFlag.read]);
+        await prepareMessages(foundOldest: true, messages: [message1, message2]);
+        model.maybeUpdateMessageFlags(mkRemoveEvent(MessageFlag.read, [message2, message3]));
+        checkNotifiedOnce();
+        check(model).messages
+          ..[0].flags.deepEquals([MessageFlag.read])
+          ..[1].flags.deepEquals([]);
+      });
+
+      test('other flags not affected', () async {
+        final message = eg.streamMessage(flags: [MessageFlag.starred, MessageFlag.read]);
+        prepare();
+        await prepareMessages(foundOldest: true, messages: [message]);
+        model.maybeUpdateMessageFlags(mkRemoveEvent(MessageFlag.read, [message]));
+        checkNotifiedOnce();
+        check(model).messages.single.flags.deepEquals([MessageFlag.starred]);
+      });
+    });
+  });
+
   test('reassemble', () async {
     final stream = eg.stream();
     prepare(narrow: StreamNarrow(stream.streamId));
diff --git a/test/stdlib_checks.dart b/test/stdlib_checks.dart
index 2829c92ec5..87b3026288 100644
--- a/test/stdlib_checks.dart
+++ b/test/stdlib_checks.dart
@@ -9,6 +9,10 @@ import 'dart:convert';
 import 'package:checks/checks.dart';
 import 'package:http/http.dart' as http;
 
+extension ListChecks<T> on Subject<List<T>> {
+  Subject<T> operator [](int index) => has((l) => l[index], '[$index]');
+}
+
 extension NullableMapChecks<K, V> on Subject<Map<K, V>?> {
   void deepEquals(Map<Object?, Object?>? expected) {
     if (expected == null) {