feat(app): /model picker + subtle model·thinking header indicator

Consume the new session.meta event: SessionMeta/ModelInfo models, a
sessionMetaProvider (mirrors commandsProvider), and reducer handling that
stores meta without polluting the chat list. Adds:
- /model client command → bottom-sheet picker of selectable models (current
  marked) → session.action model {provider,id}.
- A dim second line under the session title showing '<model> · <thinking>'.

Tests: reducer test for session.meta; analyze --fatal-infos clean; 84/84.

Author: leduckhc

app/lib/store/models.dart

@@ -37,6 +37,48 @@ class SlashCmd {
   }
 }
 
+/// A model the agent can run, as pushed via `session.meta`. Also used for the
+/// currently-active model.
+class ModelInfo {
+  const ModelInfo({required this.provider, required this.id, required this.name});
+
+  final String provider;
+  final String id;
+  final String name;
+
+  static ModelInfo? fromJson(Map<String, dynamic> j) {
+    final provider = j['provider'] as String?;
+    final id = j['id'] as String?;
+    if (provider == null || id == null) return null;
+    return ModelInfo(provider: provider, id: id, name: (j['name'] as String?) ?? id);
+  }
+}
+
+/// Per-session model + thinking-level snapshot. Drives the subtle header
+/// indicator and the `/model` picker. Pushed via the `session.meta` event.
+class SessionMeta {
+  const SessionMeta({this.model, required this.thinking, required this.models});
+
+  final ModelInfo? model;
+  final String thinking;
+  final List<ModelInfo> models;
+
+  static SessionMeta fromJson(Map<String, dynamic> j) {
+    final rawModel = j['model'];
+    return SessionMeta(
+      model: rawModel is Map
+          ? ModelInfo.fromJson(Map<String, dynamic>.from(rawModel))
+          : null,
+      thinking: (j['thinking'] as String?) ?? '',
+      models: ((j['models'] as List?) ?? const [])
+          .whereType<Map<dynamic, dynamic>>()
+          .map((m) => ModelInfo.fromJson(Map<String, dynamic>.from(m)))
+          .whereType<ModelInfo>()
+          .toList(),
+    );
+  }
+}
+
 class Project {
   Project({
     required this.id,
@@ -421,6 +463,9 @@ List<ChatItem> foldEvents(Iterable<SessionEvent> events) {
       case EventKind.sessionCommands:
         // Handled by store, not as a chat item.
         break;
+      case EventKind.sessionMeta:
+        // Model/thinking snapshot — handled by store, not a chat item.
+        break;
     }
   }
   return items;

app/lib/store/store.dart

@@ -44,6 +44,7 @@ class StoreState {
     required this.events,
     required this.cursors,
     required this.commands,
+    required this.meta,
   });
 
   factory StoreState.empty() => StoreState(
@@ -52,6 +53,7 @@ class StoreState {
     events: const {},
     cursors: const {},
     commands: const {},
+    meta: const {},
   );
 
   final List<Project> projects;
@@ -62,18 +64,23 @@ class StoreState {
   /// Per-session list of slash commands advertised by the agent.
   final Map<String, List<SlashCmd>> commands;
 
+  /// Per-session model + thinking-level snapshot from `session.meta`.
+  final Map<String, SessionMeta> meta;
+
   StoreState copyWith({
     List<Project>? projects,
     List<Session>? sessions,
     Map<String, List<SessionEvent>>? events,
     Map<String, int>? cursors,
     Map<String, List<SlashCmd>>? commands,
+    Map<String, SessionMeta>? meta,
   }) => StoreState(
     projects: projects ?? this.projects,
     sessions: sessions ?? this.sessions,
     events: events ?? this.events,
     cursors: cursors ?? this.cursors,
     commands: commands ?? this.commands,
+    meta: meta ?? this.meta,
   );
 }
 
@@ -115,6 +122,13 @@ StoreState reduceEvent(StoreState state, SessionEvent ev) {
     return state.copyWith(commands: commands, cursors: cursors);
   }
 
+  // session.meta updates the model/thinking indicator + /model picker, not chat.
+  if (ev.kind == EventKind.sessionMeta) {
+    final meta = Map<String, SessionMeta>.from(state.meta);
+    meta[ev.sessionId] = SessionMeta.fromJson(Map<String, dynamic>.from(ev.payload));
+    return state.copyWith(meta: meta, cursors: cursors);
+  }
+
   final events = Map<String, List<SessionEvent>>.from(state.events);
   final list = List<SessionEvent>.from(events[ev.sessionId] ?? const []);
   list.add(ev);
@@ -392,3 +406,13 @@ final commandsProvider = Provider.family<List<SlashCmd>, String>((
   final s = ref.watch(storeControllerProvider);
   return s.commands[sessionId] ?? const [];
 });
+
+/// Current model + thinking level + selectable models for a session (or null
+/// until the host pushes `session.meta`).
+final sessionMetaProvider = Provider.family<SessionMeta?, String>((
+  ref,
+  sessionId,
+) {
+  final s = ref.watch(storeControllerProvider);
+  return s.meta[sessionId];
+});

app/lib/transport/protocol.dart

@@ -96,7 +96,8 @@ enum EventKind {
   toolCallEnd,
   sessionStatus,
   sessionError,
-  sessionCommands;
+  sessionCommands,
+  sessionMeta;
 
   String get wire => switch (this) {
     EventKind.userMessage => 'user.message',
@@ -109,6 +110,7 @@ enum EventKind {
     EventKind.sessionStatus => 'session.status',
     EventKind.sessionError => 'session.error',
     EventKind.sessionCommands => 'session.commands',
+    EventKind.sessionMeta => 'session.meta',
   };
 
   static EventKind? fromWire(String s) {

app/lib/ui/composer/client_commands.dart

@@ -175,6 +175,30 @@ final List<ClientCommand> clientCommands = <ClientCommand>[
       );
     },
   ),
+  ClientCommand(
+    name: 'model',
+    description: 'Switch the agent model',
+    handler: (context, ref, {required sessionId}) async {
+      final meta = ref.read(sessionMetaProvider(sessionId));
+      final models = meta?.models ?? const [];
+      if (models.isEmpty) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(content: Text('No models available for this session')),
+        );
+        return;
+      }
+      final picked = await _pickModel(context, models, meta?.model);
+      if (picked == null || !context.mounted) return;
+      ref.read(storeControllerProvider.notifier).sendSessionAction(
+        sessionId,
+        'model',
+        args: {'provider': picked.provider, 'id': picked.id},
+      );
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('Model: ${picked.name}')),
+      );
+    },
+  ),
 ];
 
 /// pi's thinking levels, low → high. `off` disables reasoning.
@@ -206,3 +230,39 @@ Future<String?> _pickThinkingLevel(BuildContext context) {
     ),
   );
 }
+
+
+/// Present the selectable models in a modal sheet, marking [current]. Resolves
+/// with the chosen model or null if dismissed.
+Future<ModelInfo?> _pickModel(
+  BuildContext context,
+  List<ModelInfo> models,
+  ModelInfo? current,
+) {
+  return showModalBottomSheet<ModelInfo>(
+    context: context,
+    isScrollControlled: true,
+    builder: (sheetContext) => SafeArea(
+      child: ListView(
+        shrinkWrap: true,
+        children: [
+          const ListTile(
+            dense: true,
+            title: Text('Model', style: TextStyle(fontWeight: FontWeight.bold)),
+          ),
+          for (final m in models)
+            ListTile(
+              title: Text(m.name),
+              subtitle: Text(m.provider),
+              trailing: (current != null &&
+                      current.provider == m.provider &&
+                      current.id == m.id)
+                  ? const Icon(Icons.check)
+                  : null,
+              onTap: () => Navigator.pop(sheetContext, m),
+            ),
+        ],
+      ),
+    ),
+  );
+}

app/lib/ui/session/session_screen.dart

@@ -41,6 +41,7 @@ class _SessionScreenState extends ConsumerState<SessionScreen> {
   Widget build(BuildContext context) {
     final session = ref.watch(sessionsProvider).byId(widget.sessionId);
     final items = ref.watch(chatItemsProvider(widget.sessionId));
+    final meta = ref.watch(sessionMetaProvider(widget.sessionId));
 
     if (items.isNotEmpty && items.last.seq != _lastSeq) {
       final firstLoad = _lastSeq == 0;
@@ -173,26 +174,41 @@ class _SessionScreenState extends ConsumerState<SessionScreen> {
                     ),
                     const SizedBox(width: 12),
                     Expanded(
-                      child: Text(
-                        label,
-                        maxLines: 1,
-                        overflow: TextOverflow.ellipsis,
-                        style: Theme.of(context)
-                            .textTheme
-                            .titleMedium
-                            ?.copyWith(
-                              fontWeight: FontWeight.w600,
-                              shadows: [
-                                Shadow(
-                                  color: cs.surface,
-                                  blurRadius: 6,
+                      child: Column(
+                        crossAxisAlignment: CrossAxisAlignment.start,
+                        mainAxisSize: MainAxisSize.min,
+                        children: [
+                          Text(
+                            label,
+                            maxLines: 1,
+                            overflow: TextOverflow.ellipsis,
+                            style: Theme.of(context)
+                                .textTheme
+                                .titleMedium
+                                ?.copyWith(
+                                  fontWeight: FontWeight.w600,
+                                  shadows: [
+                                    Shadow(color: cs.surface, blurRadius: 6),
+                                    Shadow(color: cs.surface, blurRadius: 12),
+                                  ],
                                 ),
-                                Shadow(
-                                  color: cs.surface,
-                                  blurRadius: 12,
-                                ),
-                              ],
+                          ),
+                          if (meta?.model != null)
+                            Text(
+                              '${meta!.model!.name} · ${meta.thinking}',
+                              maxLines: 1,
+                              overflow: TextOverflow.ellipsis,
+                              style: Theme.of(context)
+                                  .textTheme
+                                  .bodySmall
+                                  ?.copyWith(
+                                    color: cs.onSurface.withValues(alpha: 0.55),
+                                    shadows: [
+                                      Shadow(color: cs.surface, blurRadius: 6),
+                                    ],
+                                  ),
                             ),
+                        ],
                       ),
                     ),
                     const SizedBox(width: 8),

app/test/store_reducer_test.dart

@@ -71,6 +71,28 @@ void main() {
       expect(state.commands[_sid]!.single.name, 'fix');
     });
 
+    test('session.meta advances cursor, stores meta, adds no chat item', () {
+      var state = _seeded();
+      state = reduce(
+        state,
+        SessionEventFrame(_ev(1, EventKind.sessionMeta, {
+          'model': {'provider': 'anthropic', 'id': 'opus', 'name': 'Opus'},
+          'thinking': 'high',
+          'models': [
+            {'provider': 'anthropic', 'id': 'opus', 'name': 'Opus'},
+            {'provider': 'anthropic', 'id': 'sonnet', 'name': 'Sonnet'},
+          ],
+        })),
+      );
+
+      expect(state.cursors[_sid], 1);
+      expect(state.events[_sid] ?? const [], isEmpty);
+      final meta = state.meta[_sid]!;
+      expect(meta.model!.name, 'Opus');
+      expect(meta.thinking, 'high');
+      expect(meta.models.length, 2);
+    });
+
     test('session.status + message preview bubble up to the session', () {
       var state = _seeded();
       state = reduce(