fix(mobile): start no threads in Solo chats (#9123)

Agents in a Solo chat always answer at the chat's top level. On mobile
you can still start a thread in Solo (Reply action, swipe-to-reply,
media reply) or reply inside an existing one, so the agent's answer
lands outside the thread you wrote in. Desktop already avoids this: Solo
shows existing threads but starts none and has no thread composer.

This matches desktop on mobile:

- No Reply quick action, swipe-to-reply or media reply in Solo chats.
- Existing Solo threads still open from their reply summary, read-only
(no composer).
- Every other channel is unchanged.

### Demo

Synthetic Solo chat "QA demo", iPhone 17 Pro simulator. Each clip: right
swipe on an agent message, then long-press for the actions.

Before (main): the swipe shows the reply circle and opens a thread with
a composer; Reply is in the actions.


https://github.com/user-attachments/assets/cc5025a3-b7ba-4ab2-bebb-06c7b54e7906

After (this branch): the swipe does nothing; the actions have no Reply.


https://github.com/user-attachments/assets/b31f9983-8c52-45c2-ae88-1661aa71210e

### Validation

Added tests: a Solo chat's message neither swipes to reply nor offers
Reply in its actions, and a Solo thread opens without a composer.
Existing swipe-reply and Solo channel tests pass, along with the full
mobile suite and analyzer.

Simulator run on an iPhone 17 Pro, signed in to a real community: in an
existing Solo chat, a right swipe on an agent message no longer moves or
opens a thread, and the long-press actions show no Reply (Copy link,
Remind me and the rest are unchanged). Team-channel Reply is covered by
the existing swipe-reply tests, not this run.

A second simulator run on an iPhone 17 Pro, in a synthetic Solo chat,
compared main and this branch: on main the swipe opens a thread with a
composer and the actions list Reply; on this branch neither does (see
Demo). The Demo's "Follow thread" action still appears in a Solo chat's
actions on both builds; this PR does not change it.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

mobile/lib/features/channels/channel_detail_page/message_bubble.dart

@@ -39,6 +39,9 @@ class _MessageBubble extends HookConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final startsThreads = ref.watch(
+      channelStartsThreadsProvider(currentChannelId),
+    );
     final messageSnapshotKey = useMemoized(GlobalKey.new, const []);
     final swipeContentKey = useMemoized(GlobalKey.new, const []);
     final mediaOffset = useMessageMediaOffset(message.content, message.tags);
@@ -379,7 +382,8 @@ class _MessageBubble extends HookConsumerWidget {
                                       scaleEmojiOnly: true,
                                       mediaCarouselTrailingOverflow:
                                           Grid.gutter,
-                                      onMediaReply: allMessages == null
+                                      onMediaReply:
+                                          allMessages == null || !startsThreads
                                           ? null
                                           : () {
                                               if (!context.mounted) return;

mobile/lib/features/channels/channels_provider.dart

@@ -1155,6 +1155,18 @@ final channelByIdProvider = Provider.autoDispose.family<Channel?, String>(
       ref.watch(_openedAheadProvider.select((opened) => opened[channelId])),
 );
 
+/// Whether this channel can start or continue threads. A Solo chat's agents
+/// answer at its top level, so it keeps existing threads readable but starts
+/// none and takes no thread replies, as desktop does. A channel not loaded yet
+/// keeps the default thread behavior.
+final channelStartsThreadsProvider = Provider.autoDispose.family<bool, String>(
+  (ref, channelId) => !(ref.watch(
+    channelByIdProvider(
+      channelId,
+    ).select((channel) => channel?.isSolo ?? false),
+  )),
+);
+
 /// Channels opened from their own metadata before the list had them, as a
 /// fast DM open does. Scoped to one relay and identity. Each is dropped once
 /// the list has it, so a DM later hidden or left is not found through it.

mobile/lib/features/channels/message_actions.dart

@@ -599,9 +599,10 @@ class _MessageQuickActionsRow extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final messages = allMessages;
     final canRemind = ref.watch(reminderServiceProvider) != null;
+    final startsThreads = ref.watch(channelStartsThreadsProvider(channelId));
 
     final tiles = <Widget>[
-      if (messages != null)
+      if (messages != null && startsThreads)
         _MessageQuickActionTile(
           icon: BuzzIcons.messageSquareReply,
           label: 'Reply',

mobile/lib/features/channels/message_swipe_reply.dart

@@ -9,6 +9,7 @@ import 'package:hooks_riverpod/hooks_riverpod.dart';
 
 import '../../shared/theme/buzz_icons.dart';
 import '../../shared/theme/theme.dart';
+import 'channels_provider.dart';
 import 'thread_detail_page.dart';
 import 'message_reply_drag_target.dart';
 import 'timeline_message.dart';
@@ -55,7 +56,8 @@ class MessageSwipeReply extends HookConsumerWidget {
     final cancelled = useRef(false);
     final reducedMotion = MediaQuery.disableAnimationsOf(context);
     final messages = allMessages;
-    if (messages == null || message.isSystem) return child;
+    final startsThreads = ref.watch(channelStartsThreadsProvider(channelId));
+    if (messages == null || message.isSystem || !startsThreads) return child;
 
     void reset() {
       pulse.stop();

mobile/lib/features/channels/thread_detail_page.dart

@@ -116,6 +116,11 @@ class ThreadDetailPage extends HookConsumerWidget {
     // signing identity instead of freezing that incomplete route snapshot.
     final currentPubkey =
         ref.watch(currentPubkeyProvider) ?? this.currentPubkey;
+    // A Solo chat's threads are read-only: its agents answer at top level.
+    final canReply =
+        isMember &&
+        !isArchived &&
+        ref.watch(channelStartsThreadsProvider(channelId));
     final appView = View.of(context);
     final composerDockHeight = useState(0.0);
     final composerFocusNode = useFocusNode();
@@ -348,8 +353,7 @@ class ThreadDetailPage extends HookConsumerWidget {
     );
 
     bool threadTailIsVisible() {
-      if (isMember &&
-          !isArchived &&
+      if (canReply &&
           (!viewportHeight.isFinite ||
               viewportHeight <= 0 ||
               composerDockHeight.value <= 0)) {
@@ -656,7 +660,7 @@ class ThreadDetailPage extends HookConsumerWidget {
         previousReplyCount.value = replies.length;
         return null;
       }
-      if (isMember && !isArchived && composerDockHeight.value <= 0) {
+      if (canReply && composerDockHeight.value <= 0) {
         return null;
       }
       if (!initialTailSettle.isComplete) {
@@ -1059,7 +1063,7 @@ class ThreadDetailPage extends HookConsumerWidget {
               ),
             ],
           ),
-          if (isMember && !isArchived)
+          if (canReply)
             AndroidImeLift(
               child: Align(
                 alignment: Alignment.bottomCenter,

mobile/lib/features/channels/thread_detail_page/thread_message.dart

@@ -40,6 +40,7 @@ class _ThreadMessage extends HookConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final startsThreads = ref.watch(channelStartsThreadsProvider(channelId));
     final messageSnapshotKey = useMemoized(GlobalKey.new, const []);
     final swipeContentKey = useMemoized(GlobalKey.new, const []);
     final mediaOffset = useMessageMediaOffset(message.content, message.tags);
@@ -344,7 +345,8 @@ class _ThreadMessage extends HookConsumerWidget {
                                       scaleEmojiOnly: true,
                                       mediaCarouselTrailingOverflow:
                                           Grid.gutter,
-                                      onMediaReply: allMessages == null
+                                      onMediaReply:
+                                          allMessages == null || !startsThreads
                                           ? null
                                           : () {
                                               if (!context.mounted) return;

mobile/test/features/channels/channel_detail_page_test/swipe_reply_tests.dart

@@ -1,6 +1,87 @@
 part of '../channel_detail_page_test.dart';
 
 void swipeReplyTests() {
+  // Solo agents answer at top level, so a Solo chat starts no threads.
+  Channel soloChannel() => Channel(
+    id: _channelId,
+    name: 'Solo',
+    channelType: 'stream',
+    visibility: 'private',
+    labels: const ['buzz.solo'],
+    description: '',
+    createdBy: 'self',
+    createdAt: DateTime(2026),
+    memberCount: 2,
+    isMember: true,
+  );
+
+  testWidgets('Solo chat offers no reply by swipe or message actions', (
+    tester,
+  ) async {
+    final channel = soloChannel();
+    final events = [
+      _textMsg(id: threadIdRoot, pubkey: 'agent', content: 'Agent answer'),
+    ];
+    await tester.pumpWidget(
+      _buildTestable(
+        signingPubkey: 'self',
+        messages: events,
+        channel: channel,
+        personalChat: true,
+        organization: Stream.value(const MeGroups()),
+        home: ChannelDetailPage(channel: channel),
+      ),
+    );
+    await tester.pumpAndSettle();
+    final row = find.byKey(const ValueKey('message-row-$threadIdRoot'));
+    final bubble = find
+        .descendant(of: row, matching: find.byType(LocalSendBubbleSurface))
+        .first;
+    final initial = tester.getTopLeft(row);
+    final gesture = await tester.startGesture(tester.getCenter(bubble));
+    await gesture.moveBy(const Offset(25, 0));
+    await gesture.moveBy(const Offset(150, 0));
+    await tester.pump();
+    expect(tester.getTopLeft(row), initial);
+    expect(find.byKey(const ValueKey('swipe-reply-circle')), findsNothing);
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(find.byType(ThreadDetailPage), findsNothing);
+
+    await tester.longPress(bubble);
+    await tester.pumpAndSettle();
+    expect(find.text('Copy link'), findsOneWidget);
+    expect(find.text('Reply'), findsNothing);
+  });
+
+  testWidgets('Solo thread is read-only', (tester) async {
+    final channel = soloChannel();
+    final events = [
+      _textMsg(id: threadIdRoot, pubkey: 'self', content: 'Old thread head'),
+    ];
+    final messages = formatTimeline(events);
+    await tester.pumpWidget(
+      _buildTestable(
+        signingPubkey: 'self',
+        messages: events,
+        channel: channel,
+        personalChat: true,
+        organization: Stream.value(const MeGroups()),
+        home: ThreadDetailPage(
+          threadHead: messages.first,
+          allMessages: messages,
+          channelId: _channelId,
+          currentPubkey: 'self',
+          isMember: true,
+          isArchived: false,
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('Old thread head'), findsOneWidget);
+    expect(find.byType(ComposeBar), findsNothing);
+  });
+
   for (final thread in [false, true]) {
     for (final own in [false, true]) {
       testWidgets(