diff --git a/examples/catalog_gallery/lib/samples_view.dart b/examples/catalog_gallery/lib/samples_view.dart
index 4328ba2a9..09b8b8d8d 100644
--- a/examples/catalog_gallery/lib/samples_view.dart
+++ b/examples/catalog_gallery/lib/samples_view.dart
@@ -40,7 +40,7 @@ class _SamplesViewState extends State<SamplesView> {
   @override
   void initState() {
     super.initState();
-    _genUiManager = GenUiManager(catalog: widget.catalog);
+    _genUiManager = GenUiManager(catalogs: [widget.catalog]);
     _loadSamples();
     _setupSurfaceListener();
   }
@@ -109,7 +109,7 @@ class _SamplesViewState extends State<SamplesView> {
     });
     // Re-create GenUiManager to ensure a clean state for the new sample.
     _genUiManager.dispose();
-    _genUiManager = GenUiManager(catalog: widget.catalog);
+    _genUiManager = GenUiManager(catalogs: [widget.catalog]);
     _setupSurfaceListener();
 
     try {
diff --git a/examples/custom_backend/lib/main.dart b/examples/custom_backend/lib/main.dart
index f68386932..59bab3b3c 100644
--- a/examples/custom_backend/lib/main.dart
+++ b/examples/custom_backend/lib/main.dart
@@ -77,7 +77,7 @@ class _IntegrationTesterState extends State<_IntegrationTester> {
   final _controller = TextEditingController(text: requestText);
 
   final _protocol = Backend(uiSchema);
-  late final GenUiManager _genUi = GenUiManager(catalog: _catalog);
+  late final GenUiManager _genUi = GenUiManager(catalogs: [_catalog]);
   String? _selectedResponse;
   bool _isLoading = false;
   String? _errorMessage;
diff --git a/examples/simple_chat/lib/main.dart b/examples/simple_chat/lib/main.dart
index 616ae1669..b127c7469 100644
--- a/examples/simple_chat/lib/main.dart
+++ b/examples/simple_chat/lib/main.dart
@@ -76,7 +76,7 @@ class _ChatScreenState extends State<ChatScreen> {
   void initState() {
     super.initState();
     final Catalog catalog = CoreCatalogItems.asCatalog();
-    _genUiManager = GenUiManager(catalog: catalog);
+    _genUiManager = GenUiManager(catalogs: [catalog]);
 
     final systemInstruction =
         '''You are a helpful assistant who chats with a user,
diff --git a/examples/travel_app/lib/src/catalog.dart b/examples/travel_app/lib/src/catalog.dart
index 924b0bbd9..987ca7e13 100644
--- a/examples/travel_app/lib/src/catalog.dart
+++ b/examples/travel_app/lib/src/catalog.dart
@@ -24,35 +24,20 @@ import 'catalog/travel_carousel.dart';
 /// for a travel planning experience, such as [travelCarousel], [itinerary],
 /// and [inputGroup]. The AI selects from these components to build a dynamic
 /// and interactive UI in response to user prompts.
-final Catalog travelAppCatalog = CoreCatalogItems.asCatalog()
-    .copyWithout([
-      CoreCatalogItems.audioPlayer,
-      CoreCatalogItems.card,
-      CoreCatalogItems.checkBox,
-      CoreCatalogItems.dateTimeInput,
-      CoreCatalogItems.divider,
-      CoreCatalogItems.textField,
-      CoreCatalogItems.list,
-      CoreCatalogItems.modal,
-      CoreCatalogItems.multipleChoice,
-      CoreCatalogItems.slider,
-      CoreCatalogItems.tabs,
-      CoreCatalogItems.video,
-      CoreCatalogItems.icon,
-      CoreCatalogItems.row,
-      CoreCatalogItems.image,
-    ])
-    .copyWith([
-      CoreCatalogItems.imageFixedSize,
-      checkboxFilterChipsInput,
-      dateInputChip,
-      informationCard,
-      inputGroup,
-      itinerary,
-      listingsBooker,
-      optionsFilterChipInput,
-      tabbedSections,
-      textInputChip,
-      trailhead,
-      travelCarousel,
-    ]);
+final Catalog travelAppCatalog = Catalog([
+  CoreCatalogItems.button,
+  CoreCatalogItems.column,
+  CoreCatalogItems.text,
+  CoreCatalogItems.imageFixedSize,
+  checkboxFilterChipsInput,
+  dateInputChip,
+  informationCard,
+  inputGroup,
+  itinerary,
+  listingsBooker,
+  optionsFilterChipInput,
+  tabbedSections,
+  textInputChip,
+  trailhead,
+  travelCarousel,
+], catalogId: 'example.com:travel_v0');
diff --git a/examples/travel_app/lib/src/travel_planner_page.dart b/examples/travel_app/lib/src/travel_planner_page.dart
index 2995c4a14..e8b86fdda 100644
--- a/examples/travel_app/lib/src/travel_planner_page.dart
+++ b/examples/travel_app/lib/src/travel_planner_page.dart
@@ -63,16 +63,7 @@ class _TravelPlannerPageState extends State<TravelPlannerPage>
   @override
   void initState() {
     super.initState();
-    final genUiManager = GenUiManager(
-      catalog: travelAppCatalog,
-      configuration: const GenUiConfiguration(
-        actions: ActionsConfig(
-          allowCreate: true,
-          allowUpdate: true,
-          allowDelete: true,
-        ),
-      ),
-    );
+    final genUiManager = GenUiManager(catalogs: [travelAppCatalog]);
     _userMessageSubscription = genUiManager.onSubmit.listen(
       _handleUserMessageFromUi,
     );
diff --git a/examples/travel_app/test/widgets/conversation_test.dart b/examples/travel_app/test/widgets/conversation_test.dart
index 6ca04aca7..c0d554c17 100644
--- a/examples/travel_app/test/widgets/conversation_test.dart
+++ b/examples/travel_app/test/widgets/conversation_test.dart
@@ -12,7 +12,7 @@ void main() {
     late GenUiManager manager;
 
     setUp(() {
-      manager = GenUiManager(catalog: CoreCatalogItems.asCatalog());
+      manager = GenUiManager(catalogs: [CoreCatalogItems.asCatalog()]);
     });
 
     testWidgets('renders a list of messages', (WidgetTester tester) async {
diff --git a/examples/verdure/client/lib/features/ai/ai_provider.dart b/examples/verdure/client/lib/features/ai/ai_provider.dart
index 067e16898..dddbdba16 100644
--- a/examples/verdure/client/lib/features/ai/ai_provider.dart
+++ b/examples/verdure/client/lib/features/ai/ai_provider.dart
@@ -67,7 +67,7 @@ class AiClientState {
 class Ai extends _$Ai {
   @override
   Future<AiClientState> build() async {
-    final genUiManager = GenUiManager(catalog: CoreCatalogItems.asCatalog());
+    final genUiManager = GenUiManager(catalogs: [CoreCatalogItems.asCatalog()]);
     final A2uiAgentConnector connector = await ref.watch(
       a2uiAgentConnectorProvider.future,
     );
diff --git a/packages/genui/CHANGELOG.md b/packages/genui/CHANGELOG.md
index 1c34efe8e..cacb066d1 100644
--- a/packages/genui/CHANGELOG.md
+++ b/packages/genui/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## 0.5.2 (in progress)
 
+- **Feature**: `GenUiManager` now supports multiple catalogs by accepting an `Iterable<Catalog>` in its constructor.
+- **Feature**: `catalogId` property added to `UiDefinition` to specify which catalog a UI surface should use.
+- **Refactor**: Moved `standardCatalogId` constant from `core_catalog.dart` to `primitives/constants.dart` for better organization and accessibility.
+
 ## 0.5.1
 
 - Homepage URL was updated.
diff --git a/packages/genui/README.md b/packages/genui/README.md
index cecee14d1..260d90021 100644
--- a/packages/genui/README.md
+++ b/packages/genui/README.md
@@ -164,7 +164,7 @@ provider.
 
        // Create a GenUiManager with a widget catalog.
        // The CoreCatalogItems contain basic widgets for text, markdown, and images.
-       _genUiManager = GenUiManager(catalog: CoreCatalogItems.asCatalog());
+       _genUiManager = GenUiManager(catalogs: [CoreCatalogItems.asCatalog()]);
 
        // Create a ContentGenerator to communicate with the LLM.
        // Provide system instructions and the tools from the GenUiManager.
@@ -378,7 +378,7 @@ Include your catalog items when instantiating `GenUiManager`.
 
 ```dart
 _genUiManager = GenUiManager(
-  catalog: CoreCatalogItems.asCatalog().copyWith([riddleCard]),
+  catalogs: [CoreCatalogItems.asCatalog().copyWith([riddleCard])],
 );
 ```
 
diff --git a/packages/genui/lib/genui.dart b/packages/genui/lib/genui.dart
index c027eee7e..85cf9d43e 100644
--- a/packages/genui/lib/genui.dart
+++ b/packages/genui/lib/genui.dart
@@ -22,6 +22,7 @@ export 'src/core/widgets/chat_primitives.dart';
 export 'src/development_utilities/catalog_view.dart';
 export 'src/facade/direct_call_integration/model.dart';
 export 'src/facade/direct_call_integration/utils.dart';
+export 'src/model/a2ui_client_capabilities.dart';
 export 'src/model/a2ui_message.dart';
 export 'src/model/a2ui_schemas.dart';
 export 'src/model/catalog.dart';
diff --git a/packages/genui/lib/src/catalog/core_catalog.dart b/packages/genui/lib/src/catalog/core_catalog.dart
index 5ad80a80a..66481ad64 100644
--- a/packages/genui/lib/src/catalog/core_catalog.dart
+++ b/packages/genui/lib/src/catalog/core_catalog.dart
@@ -4,6 +4,7 @@
 
 import '../model/catalog.dart';
 import '../model/catalog_item.dart';
+import '../primitives/constants.dart';
 import 'core_widgets/audio_player.dart' as audio_player_item;
 import 'core_widgets/button.dart' as button_item;
 import 'core_widgets/card.dart' as card_item;
@@ -129,6 +130,6 @@ class CoreCatalogItems {
       text,
       textField,
       video,
-    ]);
+    ], catalogId: standardCatalogId);
   }
 }
diff --git a/packages/genui/lib/src/content_generator.dart b/packages/genui/lib/src/content_generator.dart
index e69946bf0..c46728ae6 100644
--- a/packages/genui/lib/src/content_generator.dart
+++ b/packages/genui/lib/src/content_generator.dart
@@ -6,6 +6,7 @@ import 'dart:async';
 
 import 'package:flutter/foundation.dart';
 
+import 'model/a2ui_client_capabilities.dart';
 import 'model/a2ui_message.dart';
 import 'model/chat_message.dart';
 
@@ -49,6 +50,7 @@ abstract interface class ContentGenerator {
   Future<void> sendRequest(
     ChatMessage message, {
     Iterable<ChatMessage>? history,
+    A2UiClientCapabilities? clientCapabilities,
   });
 
   /// Disposes of the resources used by this generator.
diff --git a/packages/genui/lib/src/conversation/gen_ui_conversation.dart b/packages/genui/lib/src/conversation/gen_ui_conversation.dart
index 50deac99c..a0ab320df 100644
--- a/packages/genui/lib/src/conversation/gen_ui_conversation.dart
+++ b/packages/genui/lib/src/conversation/gen_ui_conversation.dart
@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 
 import '../content_generator.dart';
 import '../core/genui_manager.dart';
+import '../model/a2ui_client_capabilities.dart';
 import '../model/a2ui_message.dart';
 import '../model/chat_message.dart';
 import '../model/ui_models.dart';
@@ -149,7 +150,18 @@ class GenUiConversation {
     if (message is! UserUiInteractionMessage) {
       _conversation.value = [...history, message];
     }
-    return contentGenerator.sendRequest(message, history: history);
+    final clientCapabilities = A2UiClientCapabilities(
+      supportedCatalogIds: genUiManager.catalogs
+          .map((c) => c.catalogId)
+          .where((id) => id != null)
+          .cast<String>()
+          .toList(),
+    );
+    return contentGenerator.sendRequest(
+      message,
+      history: history,
+      clientCapabilities: clientCapabilities,
+    );
   }
 
   void _handleTextResponse(String text) {
diff --git a/packages/genui/lib/src/core/genui_manager.dart b/packages/genui/lib/src/core/genui_manager.dart
index daabf84b9..9a0adcd07 100644
--- a/packages/genui/lib/src/core/genui_manager.dart
+++ b/packages/genui/lib/src/core/genui_manager.dart
@@ -64,8 +64,8 @@ abstract interface class GenUiHost {
   /// Returns a [ValueNotifier] for the surface with the given [surfaceId].
   ValueNotifier<UiDefinition?> getSurfaceNotifier(String surfaceId);
 
-  /// The catalog of UI components available to the AI.
-  Catalog get catalog;
+  /// The catalogs of UI components available to the AI.
+  Iterable<Catalog> get catalogs;
 
   /// A map of data models for storing the UI state of each surface.
   Map<String, DataModel> get dataModels;
@@ -85,16 +85,17 @@ abstract interface class GenUiHost {
 /// `beginRendering`) that the AI uses to manipulate the UI. It exposes a stream
 /// of `GenUiUpdate` events so that the application can react to changes.
 class GenUiManager implements GenUiHost {
-  /// Creates a new [GenUiManager].
-  ///
-  /// The [catalog] defines the set of widgets available to the AI.
+  /// Creates a new [GenUiManager] with a list of supported widget catalogs.
   GenUiManager({
-    required this.catalog,
+    required this.catalogs,
     this.configuration = const GenUiConfiguration(),
   });
 
   final GenUiConfiguration configuration;
 
+  @override
+  final Iterable<Catalog> catalogs;
+
   final _surfaces = <String, ValueNotifier<UiDefinition?>>{};
   final _surfaceUpdates = StreamController<GenUiUpdate>.broadcast();
   final _onSubmit = StreamController<UserUiInteractionMessage>.broadcast();
@@ -129,9 +130,6 @@ class GenUiManager implements GenUiHost {
     _onSubmit.add(UserUiInteractionMessage.text(eventJsonString));
   }
 
-  @override
-  final Catalog catalog;
-
   @override
   ValueNotifier<UiDefinition?> getSurfaceNotifier(String surfaceId) {
     if (!_surfaces.containsKey(surfaceId)) {
@@ -192,6 +190,7 @@ class GenUiManager implements GenUiHost {
             notifier.value ?? UiDefinition(surfaceId: message.surfaceId);
         final UiDefinition newUiDefinition = uiDefinition.copyWith(
           rootComponentId: message.root,
+          catalogId: message.catalogId,
         );
         notifier.value = newUiDefinition;
         genUiLogger.info('Started rendering ${message.surfaceId}');
diff --git a/packages/genui/lib/src/core/genui_surface.dart b/packages/genui/lib/src/core/genui_surface.dart
index 10984c726..8d8cdc566 100644
--- a/packages/genui/lib/src/core/genui_surface.dart
+++ b/packages/genui/lib/src/core/genui_surface.dart
@@ -2,13 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:collection/collection.dart';
 import 'package:flutter/material.dart';
 
 import '../core/genui_manager.dart';
+import '../model/catalog.dart';
 import '../model/catalog_item.dart';
 import '../model/data_model.dart';
 import '../model/tools.dart';
 import '../model/ui_models.dart';
+import '../primitives/constants.dart';
 import '../primitives/logging.dart';
 import '../primitives/simple_items.dart';
 
@@ -58,8 +61,15 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
           genUiLogger.warning('Surface ${widget.surfaceId} has no widgets.');
           return const SizedBox.shrink();
         }
+
+        final Catalog? catalog = _findCatalogForDefinition(definition);
+        if (catalog == null) {
+          return Container();
+        }
+
         return _buildWidget(
           definition,
+          catalog,
           rootId,
           DataContext(widget.host.dataModelForSurface(widget.surfaceId), '/'),
         );
@@ -73,6 +83,7 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
   /// and constructs the corresponding Flutter widget.
   Widget _buildWidget(
     UiDefinition definition,
+    Catalog catalog,
     String widgetId,
     DataContext dataContext,
   ) {
@@ -84,12 +95,17 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
 
     final JsonMap widgetData = data.componentProperties;
     genUiLogger.finest('Building widget $widgetId');
-    return widget.host.catalog.buildWidget(
+    return catalog.buildWidget(
       CatalogItemContext(
         id: widgetId,
         data: widgetData,
         buildChild: (String childId, [DataContext? childDataContext]) =>
-            _buildWidget(definition, childId, childDataContext ?? dataContext),
+            _buildWidget(
+              definition,
+              catalog,
+              childId,
+              childDataContext ?? dataContext,
+            ),
         dispatchEvent: _dispatchEvent,
         buildContext: context,
         dataContext: dataContext,
@@ -106,6 +122,16 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
           .getSurfaceNotifier(widget.surfaceId)
           .value;
       if (definition == null) return;
+
+      final Catalog? catalog = _findCatalogForDefinition(definition);
+      if (catalog == null) {
+        genUiLogger.severe(
+          'Cannot show modal for surface "${widget.surfaceId}" because '
+          'a catalog was not found.',
+        );
+        return;
+      }
+
       final modalId = event.context['modalId'] as String;
       final Component? modalComponent = definition.components[modalId];
       if (modalComponent == null) return;
@@ -116,6 +142,7 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
         context: context,
         builder: (context) => _buildWidget(
           definition,
+          catalog,
           contentChildId,
           DataContext(widget.host.dataModelForSurface(widget.surfaceId), '/'),
         ),
@@ -133,4 +160,20 @@ class _GenUiSurfaceState extends State<GenUiSurface> {
         : UiEvent.fromMap(eventMap);
     widget.host.handleUiEvent(newEvent);
   }
+
+  Catalog? _findCatalogForDefinition(UiDefinition definition) {
+    final String catalogId = definition.catalogId ?? standardCatalogId;
+    final Catalog? catalog = widget.host.catalogs.firstWhereOrNull(
+      (c) => c.catalogId == catalogId,
+    );
+
+    if (catalog == null) {
+      genUiLogger.severe(
+        'Catalog with id "$catalogId" not found for surface '
+        '"${widget.surfaceId}". Ensure the catalog is provided to '
+        'GenUiManager.',
+      );
+    }
+    return catalog;
+  }
 }
diff --git a/packages/genui/lib/src/core/ui_tools.dart b/packages/genui/lib/src/core/ui_tools.dart
index 69740f365..25560e0a5 100644
--- a/packages/genui/lib/src/core/ui_tools.dart
+++ b/packages/genui/lib/src/core/ui_tools.dart
@@ -88,36 +88,28 @@ class DeleteSurfaceTool extends AiTool<JsonMap> {
 /// This tool allows the AI to specify the root component of a UI surface.
 class BeginRenderingTool extends AiTool<JsonMap> {
   /// Creates a [BeginRenderingTool].
-  BeginRenderingTool({required this.handleMessage})
+  BeginRenderingTool({required this.handleMessage, this.catalogId})
     : super(
         name: 'beginRendering',
         description:
             'Signals the client to begin rendering a surface with a '
             'root component.',
-        parameters: S.object(
-          properties: {
-            surfaceIdKey: S.string(
-              description:
-                  'The unique identifier for the UI surface to render.',
-            ),
-            'root': S.string(
-              description:
-                  'The ID of the root widget. This ID must correspond to '
-                  'the ID of one of the widgets in the `components` list.',
-            ),
-          },
-          required: [surfaceIdKey, 'root'],
-        ),
+        parameters: A2uiSchemas.beginRenderingSchemaNoCatalogId(),
       );
 
   /// The callback to invoke when signaling to begin rendering.
   final void Function(A2uiMessage message) handleMessage;
 
+  /// The ID of the catalog to use for rendering this surface.
+  final String? catalogId;
+
   @override
   Future<JsonMap> invoke(JsonMap args) async {
     final surfaceId = args[surfaceIdKey] as String;
     final root = args['root'] as String;
-    handleMessage(BeginRendering(surfaceId: surfaceId, root: root));
+    handleMessage(
+      BeginRendering(surfaceId: surfaceId, root: root, catalogId: catalogId),
+    );
     return {
       'status': 'Surface $surfaceId rendered and waiting for user input.',
     };
diff --git a/packages/genui/lib/src/development_utilities/catalog_view.dart b/packages/genui/lib/src/development_utilities/catalog_view.dart
index 8ad9d9508..e96486469 100644
--- a/packages/genui/lib/src/development_utilities/catalog_view.dart
+++ b/packages/genui/lib/src/development_utilities/catalog_view.dart
@@ -53,7 +53,7 @@ class _DebugCatalogViewState extends State<DebugCatalogView> {
   void initState() {
     super.initState();
 
-    _genUi = GenUiManager(catalog: widget.catalog);
+    _genUi = GenUiManager(catalogs: [widget.catalog]);
     if (widget.onSubmit != null) {
       _subscription = _genUi.onSubmit.listen(widget.onSubmit);
     } else {
diff --git a/packages/genui/lib/src/model/a2ui_client_capabilities.dart b/packages/genui/lib/src/model/a2ui_client_capabilities.dart
new file mode 100644
index 000000000..8ec6c5ed6
--- /dev/null
+++ b/packages/genui/lib/src/model/a2ui_client_capabilities.dart
@@ -0,0 +1,40 @@
+// Copyright 2025 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import '../primitives/simple_items.dart';
+
+/// Describes the client's UI rendering capabilities to the server.
+///
+/// This class represents the `a2uiClientCapabilities` object that is sent
+/// from the client to the server with each message to inform the server about
+/// the component catalogs the client supports.
+class A2UiClientCapabilities {
+  /// Creates a new [A2UiClientCapabilities] instance.
+  const A2UiClientCapabilities({
+    required this.supportedCatalogIds,
+    this.inlineCatalogs,
+  });
+
+  /// A list of identifiers for all pre-defined catalogs the client supports.
+  ///
+  /// The client MUST always include the standard catalog ID here if it
+  /// supports it.
+  final List<String> supportedCatalogIds;
+
+  /// An array of full Catalog Definition Documents.
+  ///
+  /// This allows a client to provide custom, on-the-fly catalogs. This should
+  /// only be provided if the server has advertised
+  /// `acceptsInlineCatalogs: true`. This is not yet implemented.
+  final List<JsonMap>? inlineCatalogs;
+
+  /// Serializes this object to a JSON-compatible map.
+  JsonMap toJson() {
+    final JsonMap json = {'supportedCatalogIds': supportedCatalogIds};
+    if (inlineCatalogs != null) {
+      json['inlineCatalogs'] = inlineCatalogs;
+    }
+    return json;
+  }
+}
diff --git a/packages/genui/lib/src/model/a2ui_message.dart b/packages/genui/lib/src/model/a2ui_message.dart
index a9208bbb7..c026fc532 100644
--- a/packages/genui/lib/src/model/a2ui_message.dart
+++ b/packages/genui/lib/src/model/a2ui_message.dart
@@ -113,6 +113,7 @@ final class BeginRendering extends A2uiMessage {
     required this.surfaceId,
     required this.root,
     this.styles,
+    this.catalogId,
   });
 
   /// Creates a [BeginRendering] message from a JSON map.
@@ -121,6 +122,7 @@ final class BeginRendering extends A2uiMessage {
       surfaceId: json[surfaceIdKey] as String,
       root: json['root'] as String,
       styles: json['styles'] as JsonMap?,
+      catalogId: json['catalogId'] as String?,
     );
   }
 
@@ -132,6 +134,9 @@ final class BeginRendering extends A2uiMessage {
 
   /// The styles to apply to the UI.
   final JsonMap? styles;
+
+  /// The ID of the catalog to use for rendering this surface.
+  final String? catalogId;
 }
 
 /// An A2UI message that deletes a surface.
diff --git a/packages/genui/lib/src/model/a2ui_schemas.dart b/packages/genui/lib/src/model/a2ui_schemas.dart
index f5571d954..6e35030d0 100644
--- a/packages/genui/lib/src/model/a2ui_schemas.dart
+++ b/packages/genui/lib/src/model/a2ui_schemas.dart
@@ -134,6 +134,35 @@ class A2uiSchemas {
   /// Schema for a beginRendering message, which provides the root widget ID for
   /// the given surface so that the surface can be rendered.
   static Schema beginRenderingSchema() => S.object(
+    properties: {
+      surfaceIdKey: S.string(
+        description: 'The surface ID of the surface to render.',
+      ),
+      'root': S.string(
+        description:
+            'The root widget ID for the surface. '
+            'All components must be descendents of this root in order to be '
+            'displayed.',
+      ),
+      'catalogId': S.string(
+        description:
+            'The identifier of the component catalog to use for this surface.',
+      ),
+      'styles': S.object(
+        properties: {
+          'font': S.string(description: 'The base font for this surface'),
+          'primaryColor': S.string(
+            description: 'The seed color for the theme of this surface.',
+          ),
+        },
+      ),
+    },
+    required: [surfaceIdKey, 'root'],
+  );
+
+  /// Schema for a beginRendering message, which provides the root widget ID for
+  /// the given surface so that the surface can be rendered.
+  static Schema beginRenderingSchemaNoCatalogId() => S.object(
     properties: {
       surfaceIdKey: S.string(
         description: 'The surface ID of the surface to render.',
diff --git a/packages/genui/lib/src/model/catalog.dart b/packages/genui/lib/src/model/catalog.dart
index 8838d75b0..d16c88de2 100644
--- a/packages/genui/lib/src/model/catalog.dart
+++ b/packages/genui/lib/src/model/catalog.dart
@@ -23,34 +23,38 @@ import 'data_model.dart';
 @immutable
 class Catalog {
   /// Creates a new catalog with the given list of items.
-  const Catalog(this.items);
+  const Catalog(this.items, {this.catalogId});
 
   /// The list of [CatalogItem]s available in this catalog.
   final Iterable<CatalogItem> items;
 
+  /// A string that uniquely identifies this catalog. It is recommended to use
+  /// a reverse-domain name notation, e.g. 'com.example.my_catalog'.
+  final String? catalogId;
+
   /// Returns a new [Catalog] containing the items from both this catalog and
   /// the provided [items].
   ///
   /// If an item with the same name already exists in the catalog, it will be
   /// replaced with the new item.
-  Catalog copyWith(List<CatalogItem> newItems) {
+  Catalog copyWith(List<CatalogItem> newItems, {String? catalogId}) {
     final Map<String, CatalogItem> itemsByName = {
       for (final item in items) item.name: item,
     };
     itemsByName.addAll({for (final item in newItems) item.name: item});
-    return Catalog(itemsByName.values);
+    return Catalog(itemsByName.values, catalogId: catalogId ?? this.catalogId);
   }
 
   /// Returns a new [Catalog] instance containing the items from this catalog
   /// with the specified items removed.
-  Catalog copyWithout(Iterable<CatalogItem> itemNames) {
+  Catalog copyWithout(Iterable<CatalogItem> itemNames, {String? catalogId}) {
     final Set<String> namesToRemove = itemNames
         .map<String>((item) => item.name)
         .toSet();
     final List<CatalogItem> updatedItems = items
         .where((item) => !namesToRemove.contains(item.name))
         .toList();
-    return Catalog(updatedItems);
+    return Catalog(updatedItems, catalogId: catalogId ?? this.catalogId);
   }
 
   /// Builds a Flutter widget from a JSON-like data structure.
diff --git a/packages/genui/lib/src/model/ui_models.dart b/packages/genui/lib/src/model/ui_models.dart
index 16c2cca25..8c5e4c137 100644
--- a/packages/genui/lib/src/model/ui_models.dart
+++ b/packages/genui/lib/src/model/ui_models.dart
@@ -84,6 +84,9 @@ class UiDefinition {
   /// The ID of the root widget in the UI tree.
   final String? rootComponentId;
 
+  /// The ID of the catalog to use for rendering this surface.
+  final String? catalogId;
+
   /// A map of all widget definitions in the UI, keyed by their ID.
   Map<String, Component> get components => UnmodifiableMapView(_components);
   final Map<String, Component> _components;
@@ -95,6 +98,7 @@ class UiDefinition {
   UiDefinition({
     required this.surfaceId,
     this.rootComponentId,
+    this.catalogId,
     Map<String, Component> components = const {},
     this.styles,
   }) : _components = components;
@@ -102,12 +106,14 @@ class UiDefinition {
   /// Creates a copy of this [UiDefinition] with the given fields replaced.
   UiDefinition copyWith({
     String? rootComponentId,
+    String? catalogId,
     Map<String, Component>? components,
     JsonMap? styles,
   }) {
     return UiDefinition(
       surfaceId: surfaceId,
       rootComponentId: rootComponentId ?? this.rootComponentId,
+      catalogId: catalogId ?? this.catalogId,
       components: components ?? _components,
       styles: styles ?? this.styles,
     );
diff --git a/packages/genui/lib/src/primitives/constants.dart b/packages/genui/lib/src/primitives/constants.dart
new file mode 100644
index 000000000..c826615b6
--- /dev/null
+++ b/packages/genui/lib/src/primitives/constants.dart
@@ -0,0 +1,6 @@
+// Copyright 2025 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// The catalog ID for the standard catalog.
+const String standardCatalogId = 'a2ui.org:standard_catalog_0_8_0';
diff --git a/packages/genui/lib/test/fake_content_generator.dart b/packages/genui/lib/test/fake_content_generator.dart
index 97b4e1b03..bde6dc8f3 100644
--- a/packages/genui/lib/test/fake_content_generator.dart
+++ b/packages/genui/lib/test/fake_content_generator.dart
@@ -31,6 +31,9 @@ class FakeContentGenerator implements ContentGenerator {
   /// The last history passed to [sendRequest].
   Iterable<ChatMessage>? lastHistory;
 
+  /// The last client capabilities passed to [sendRequest].
+  A2UiClientCapabilities? lastClientCapabilities;
+
   @override
   Stream<A2uiMessage> get a2uiMessageStream => _a2uiMessageController.stream;
 
@@ -55,12 +58,14 @@ class FakeContentGenerator implements ContentGenerator {
   Future<void> sendRequest(
     ChatMessage message, {
     Iterable<ChatMessage>? history,
+    A2UiClientCapabilities? clientCapabilities,
   }) async {
     _isProcessing.value = true;
     try {
       sendRequestCallCount++;
       lastMessage = message;
       lastHistory = history;
+      lastClientCapabilities = clientCapabilities;
       if (sendRequestCompleter != null) {
         await sendRequestCompleter!.future;
       }
diff --git a/packages/genui/test/catalog/core_widgets/button_test.dart b/packages/genui/test/catalog/core_widgets/button_test.dart
index 26cff1ba0..db072e80d 100644
--- a/packages/genui/test/catalog/core_widgets/button_test.dart
+++ b/packages/genui/test/catalog/core_widgets/button_test.dart
@@ -12,7 +12,12 @@ void main() {
   ) async {
     ChatMessage? message;
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.button, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.button,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     manager.onSubmit.listen((event) => message = event);
@@ -40,7 +45,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'button'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'button',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/card_test.dart b/packages/genui/test/catalog/core_widgets/card_test.dart
index 2c3eca5bc..6ad0c916c 100644
--- a/packages/genui/test/catalog/core_widgets/card_test.dart
+++ b/packages/genui/test/catalog/core_widgets/card_test.dart
@@ -9,7 +9,12 @@ import 'package:genui/genui.dart';
 void main() {
   testWidgets('Card widget renders child', (WidgetTester tester) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.card, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.card,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -33,7 +38,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'card'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'card',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/check_box_test.dart b/packages/genui/test/catalog/core_widgets/check_box_test.dart
index e891d0bfe..4bfb07ec5 100644
--- a/packages/genui/test/catalog/core_widgets/check_box_test.dart
+++ b/packages/genui/test/catalog/core_widgets/check_box_test.dart
@@ -11,7 +11,9 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.checkBox]),
+      catalogs: [
+        Catalog([CoreCatalogItems.checkBox], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -30,7 +32,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'checkbox'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'checkbox',
+        catalogId: 'test_catalog',
+      ),
     );
     manager.dataModelForSurface(surfaceId).update(DataPath('/myValue'), true);
 
diff --git a/packages/genui/test/catalog/core_widgets/column_test.dart b/packages/genui/test/catalog/core_widgets/column_test.dart
index a23e54cd8..0c0a2b938 100644
--- a/packages/genui/test/catalog/core_widgets/column_test.dart
+++ b/packages/genui/test/catalog/core_widgets/column_test.dart
@@ -9,7 +9,12 @@ import 'package:genui/genui.dart';
 void main() {
   testWidgets('Column widget renders children', (WidgetTester tester) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.column, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.column,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -45,7 +50,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'column'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'column',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
@@ -64,7 +73,12 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.column, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.column,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -110,7 +124,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'column'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'column',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/date_time_input_test.dart b/packages/genui/test/catalog/core_widgets/date_time_input_test.dart
index 05e3e1e08..5ae2294ec 100644
--- a/packages/genui/test/catalog/core_widgets/date_time_input_test.dart
+++ b/packages/genui/test/catalog/core_widgets/date_time_input_test.dart
@@ -11,7 +11,12 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.dateTimeInput]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.dateTimeInput,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -29,7 +34,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'datetime'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'datetime',
+        catalogId: 'test_catalog',
+      ),
     );
     manager
         .dataModelForSurface(surfaceId)
diff --git a/packages/genui/test/catalog/core_widgets/divider_test.dart b/packages/genui/test/catalog/core_widgets/divider_test.dart
index 78ede0e94..0a2d381fb 100644
--- a/packages/genui/test/catalog/core_widgets/divider_test.dart
+++ b/packages/genui/test/catalog/core_widgets/divider_test.dart
@@ -9,7 +9,9 @@ import 'package:genui/genui.dart';
 void main() {
   testWidgets('Divider widget renders', (WidgetTester tester) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.divider]),
+      catalogs: [
+        Catalog([CoreCatalogItems.divider], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -23,7 +25,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'divider'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'divider',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/icon_test.dart b/packages/genui/test/catalog/core_widgets/icon_test.dart
index 8e6673619..41d60e2b4 100644
--- a/packages/genui/test/catalog/core_widgets/icon_test.dart
+++ b/packages/genui/test/catalog/core_widgets/icon_test.dart
@@ -11,7 +11,9 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.icon]),
+      catalogs: [
+        Catalog([CoreCatalogItems.icon], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -29,7 +31,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'icon'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'icon',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
@@ -47,7 +53,9 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.icon]),
+      catalogs: [
+        Catalog([CoreCatalogItems.icon], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -72,7 +80,11 @@ void main() {
       ),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'icon'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'icon',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/list_test.dart b/packages/genui/test/catalog/core_widgets/list_test.dart
index 5b7fa650c..93fea445f 100644
--- a/packages/genui/test/catalog/core_widgets/list_test.dart
+++ b/packages/genui/test/catalog/core_widgets/list_test.dart
@@ -9,7 +9,12 @@ import 'package:genui/genui.dart';
 void main() {
   testWidgets('List widget renders children', (WidgetTester tester) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.list, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.list,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -45,7 +50,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'list'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'list',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/modal_test.dart b/packages/genui/test/catalog/core_widgets/modal_test.dart
index dfe1b4254..de6d44d87 100644
--- a/packages/genui/test/catalog/core_widgets/modal_test.dart
+++ b/packages/genui/test/catalog/core_widgets/modal_test.dart
@@ -11,11 +11,13 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([
-        CoreCatalogItems.modal,
-        CoreCatalogItems.button,
-        CoreCatalogItems.text,
-      ]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.modal,
+          CoreCatalogItems.button,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -64,7 +66,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'modal'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'modal',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/multiple_choice_test.dart b/packages/genui/test/catalog/core_widgets/multiple_choice_test.dart
index e5f1d1170..823116a77 100644
--- a/packages/genui/test/catalog/core_widgets/multiple_choice_test.dart
+++ b/packages/genui/test/catalog/core_widgets/multiple_choice_test.dart
@@ -11,7 +11,12 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.multipleChoice]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.multipleChoice,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -39,7 +44,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'multiple_choice'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'multiple_choice',
+        catalogId: 'test_catalog',
+      ),
     );
     manager.dataModelForSurface(surfaceId).update(DataPath('/mySelections'), [
       '1',
diff --git a/packages/genui/test/catalog/core_widgets/row_test.dart b/packages/genui/test/catalog/core_widgets/row_test.dart
index a5543506e..cc5a05fc8 100644
--- a/packages/genui/test/catalog/core_widgets/row_test.dart
+++ b/packages/genui/test/catalog/core_widgets/row_test.dart
@@ -9,7 +9,12 @@ import 'package:genui/genui.dart';
 void main() {
   testWidgets('Row widget renders children', (WidgetTester tester) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.row, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.row,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -45,7 +50,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'row'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'row',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
@@ -64,7 +73,12 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.row, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.row,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -110,7 +124,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'row'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'row',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets/slider_test.dart b/packages/genui/test/catalog/core_widgets/slider_test.dart
index 5a28fc0ab..d0a03f9d9 100644
--- a/packages/genui/test/catalog/core_widgets/slider_test.dart
+++ b/packages/genui/test/catalog/core_widgets/slider_test.dart
@@ -11,7 +11,9 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.slider]),
+      catalogs: [
+        Catalog([CoreCatalogItems.slider], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -29,7 +31,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'slider'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'slider',
+        catalogId: 'test_catalog',
+      ),
     );
     manager.dataModelForSurface(surfaceId).update(DataPath('/myValue'), 0.5);
 
diff --git a/packages/genui/test/catalog/core_widgets/tabs_test.dart b/packages/genui/test/catalog/core_widgets/tabs_test.dart
index 966ef4f2a..c1c6a77a0 100644
--- a/packages/genui/test/catalog/core_widgets/tabs_test.dart
+++ b/packages/genui/test/catalog/core_widgets/tabs_test.dart
@@ -11,7 +11,12 @@ void main() {
     WidgetTester tester,
   ) async {
     final manager = GenUiManager(
-      catalog: Catalog([CoreCatalogItems.tabs, CoreCatalogItems.text]),
+      catalogs: [
+        Catalog([
+          CoreCatalogItems.tabs,
+          CoreCatalogItems.text,
+        ], catalogId: 'test_catalog'),
+      ],
       configuration: const GenUiConfiguration(),
     );
     const surfaceId = 'testSurface';
@@ -54,7 +59,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'tabs'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'tabs',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
diff --git a/packages/genui/test/catalog/core_widgets_test.dart b/packages/genui/test/catalog/core_widgets_test.dart
index 7b4041977..8b8a06cac 100644
--- a/packages/genui/test/catalog/core_widgets_test.dart
+++ b/packages/genui/test/catalog/core_widgets_test.dart
@@ -21,7 +21,7 @@ void main() {
       message = null;
       manager?.dispose();
       manager = GenUiManager(
-        catalog: testCatalog,
+        catalogs: [testCatalog],
         configuration: const GenUiConfiguration(),
       );
       manager!.onSubmit.listen((event) => message = event);
@@ -30,7 +30,11 @@ void main() {
         SurfaceUpdate(surfaceId: surfaceId, components: components),
       );
       manager!.handleMessage(
-        BeginRendering(surfaceId: surfaceId, root: rootId),
+        BeginRendering(
+          surfaceId: surfaceId,
+          root: rootId,
+          catalogId: testCatalog.catalogId,
+        ),
       );
       await tester.pumpWidget(
         MaterialApp(
diff --git a/packages/genui/test/catalog_test.dart b/packages/genui/test/catalog_test.dart
index be04c9c29..05469ad6a 100644
--- a/packages/genui/test/catalog_test.dart
+++ b/packages/genui/test/catalog_test.dart
@@ -10,6 +10,13 @@ import 'package:logging/logging.dart';
 
 void main() {
   group('Catalog', () {
+    test('has a catalogId', () {
+      final catalog = Catalog([
+        CoreCatalogItems.text,
+      ], catalogId: 'test_catalog');
+      expect(catalog.catalogId, 'test_catalog');
+    });
+
     testWidgets('buildWidget finds and builds the correct widget', (
       WidgetTester tester,
     ) async {
diff --git a/packages/genui/test/core/genui_manager_test.dart b/packages/genui/test/core/genui_manager_test.dart
index 2e8c078c3..aaac785be 100644
--- a/packages/genui/test/core/genui_manager_test.dart
+++ b/packages/genui/test/core/genui_manager_test.dart
@@ -4,7 +4,7 @@
 
 import 'dart:convert';
 
-import 'package:flutter/src/foundation/change_notifier.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:genui/genui.dart';
 
@@ -14,7 +14,7 @@ void main() {
 
     setUp(() {
       manager = GenUiManager(
-        catalog: CoreCatalogItems.asCatalog(),
+        catalogs: [CoreCatalogItems.asCatalog()],
         configuration: const GenUiConfiguration(
           actions: ActionsConfig(
             allowCreate: true,
@@ -29,6 +29,15 @@ void main() {
       manager.dispose();
     });
 
+    test('can be initialized with multiple catalogs', () {
+      final catalog1 = const Catalog([], catalogId: 'cat1');
+      final catalog2 = const Catalog([], catalogId: 'cat2');
+      final multiManager = GenUiManager(catalogs: [catalog1, catalog2]);
+      expect(multiManager.catalogs, contains(catalog1));
+      expect(multiManager.catalogs, contains(catalog2));
+      expect(multiManager.catalogs.length, 2);
+    });
+
     test('handleMessage adds a new surface and fires SurfaceAdded with '
         'definition', () async {
       const surfaceId = 's1';
@@ -51,7 +60,11 @@ void main() {
 
       final Future<GenUiUpdate> futureUpdated = manager.surfaceUpdates.first;
       manager.handleMessage(
-        const BeginRendering(surfaceId: surfaceId, root: 'root'),
+        const BeginRendering(
+          surfaceId: surfaceId,
+          root: 'root',
+          catalogId: 'test_catalog',
+        ),
       );
       final GenUiUpdate updatedUpdate = await futureUpdated;
 
@@ -61,8 +74,10 @@ void main() {
           (updatedUpdate as SurfaceUpdated).definition;
       expect(definition, isNotNull);
       expect(definition.rootComponentId, 'root');
+      expect(definition.catalogId, 'test_catalog');
       expect(manager.surfaces[surfaceId]!.value, isNotNull);
       expect(manager.surfaces[surfaceId]!.value!.rootComponentId, 'root');
+      expect(manager.surfaces[surfaceId]!.value!.catalogId, 'test_catalog');
     });
 
     test(
diff --git a/packages/genui/test/core/ui_tools_test.dart b/packages/genui/test/core/ui_tools_test.dart
index 9a9ca8d9a..910387e6a 100644
--- a/packages/genui/test/core/ui_tools_test.dart
+++ b/packages/genui/test/core/ui_tools_test.dart
@@ -31,7 +31,7 @@ void main() {
             },
             dataSchema: Schema.object(properties: {}),
           ),
-        ]),
+        ], catalogId: 'test_catalog'),
         configuration: const GenUiConfiguration(),
       );
 
@@ -90,7 +90,10 @@ void main() {
         messages.add(message);
       }
 
-      final tool = BeginRenderingTool(handleMessage: fakeHandleMessage);
+      final tool = BeginRenderingTool(
+        handleMessage: fakeHandleMessage,
+        catalogId: 'test_catalog',
+      );
 
       final Map<String, String> args = {
         surfaceIdKey: 'testSurface',
@@ -104,6 +107,7 @@ void main() {
       final beginRendering = messages[0] as BeginRendering;
       expect(beginRendering.surfaceId, 'testSurface');
       expect(beginRendering.root, 'rootWidget');
+      expect(beginRendering.catalogId, 'test_catalog');
     });
   });
 }
diff --git a/packages/genui/test/genui_surface_test.dart b/packages/genui/test/genui_surface_test.dart
index 6efad683f..41ec58acb 100644
--- a/packages/genui/test/genui_surface_test.dart
+++ b/packages/genui/test/genui_surface_test.dart
@@ -5,17 +5,25 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:genui/genui.dart';
+import 'package:logging/logging.dart';
 
 void main() {
-  final testCatalog = Catalog([CoreCatalogItems.button, CoreCatalogItems.text]);
+  late GenUiManager manager;
+  final testCatalog = Catalog([
+    CoreCatalogItems.button,
+    CoreCatalogItems.text,
+  ], catalogId: 'test_catalog');
+
+  setUp(() {
+    manager = GenUiManager(
+      catalogs: [testCatalog],
+      configuration: const GenUiConfiguration(),
+    );
+  });
 
   testWidgets('SurfaceWidget builds a widget from a definition', (
     WidgetTester tester,
   ) async {
-    final manager = GenUiManager(
-      catalog: testCatalog,
-      configuration: const GenUiConfiguration(),
-    );
     const surfaceId = 'testSurface';
     final components = [
       const Component(
@@ -40,7 +48,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'root'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'root',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
@@ -54,10 +66,6 @@ void main() {
   });
 
   testWidgets('SurfaceWidget handles events', (WidgetTester tester) async {
-    final manager = GenUiManager(
-      catalog: testCatalog,
-      configuration: const GenUiConfiguration(),
-    );
     const surfaceId = 'testSurface';
     final components = [
       const Component(
@@ -82,7 +90,11 @@ void main() {
       SurfaceUpdate(surfaceId: surfaceId, components: components),
     );
     manager.handleMessage(
-      const BeginRendering(surfaceId: surfaceId, root: 'root'),
+      const BeginRendering(
+        surfaceId: surfaceId,
+        root: 'root',
+        catalogId: 'test_catalog',
+      ),
     );
 
     await tester.pumpWidget(
@@ -93,4 +105,57 @@ void main() {
 
     await tester.tap(find.byType(ElevatedButton));
   });
+
+  testWidgets(
+    'SurfaceWidget renders container and logs error on catalog miss',
+    (WidgetTester tester) async {
+      const surfaceId = 'testSurface';
+      final components = [
+        const Component(
+          id: 'root',
+          componentProperties: {
+            'Text': {
+              'text': {'literalString': 'Hello'},
+            },
+          },
+        ),
+      ];
+      manager.handleMessage(
+        SurfaceUpdate(surfaceId: surfaceId, components: components),
+      );
+      // Request a catalogId that doesn't exist in the manager.
+      manager.handleMessage(
+        const BeginRendering(
+          surfaceId: surfaceId,
+          root: 'root',
+          catalogId: 'non_existent_catalog',
+        ),
+      );
+
+      final logs = <LogRecord>[];
+      genUiLogger.onRecord.listen(logs.add);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: GenUiSurface(host: manager, surfaceId: surfaceId),
+        ),
+      );
+
+      // Should build an empty container instead of the widget tree.
+      expect(find.byType(Container), findsOneWidget);
+      expect(find.byType(Text), findsNothing);
+
+      // Should log a severe error.
+      expect(
+        logs.any(
+          (r) =>
+              r.level == Level.SEVERE &&
+              r.message.contains(
+                'Catalog with id "non_existent_catalog" not found',
+              ),
+        ),
+        isTrue,
+      );
+    },
+  );
 }
diff --git a/packages/genui/test/model/ui_definition_test.dart b/packages/genui/test/model/ui_definition_test.dart
index 11d119ba2..c7603f23a 100644
--- a/packages/genui/test/model/ui_definition_test.dart
+++ b/packages/genui/test/model/ui_definition_test.dart
@@ -11,6 +11,7 @@ void main() {
       final definition = UiDefinition(
         surfaceId: 'testSurface',
         rootComponentId: 'root',
+        catalogId: 'test_catalog',
         components: {
           'root': const Component(
             id: 'root',
diff --git a/packages/genui/test/ui_tools_test.dart b/packages/genui/test/ui_tools_test.dart
index 754acd90e..76e05e468 100644
--- a/packages/genui/test/ui_tools_test.dart
+++ b/packages/genui/test/ui_tools_test.dart
@@ -13,7 +13,7 @@ void main() {
     setUp(() {
       catalog = CoreCatalogItems.asCatalog();
       genUiManager = GenUiManager(
-        catalog: catalog,
+        catalogs: [catalog],
         configuration: const GenUiConfiguration(
           actions: ActionsConfig(
             allowCreate: true,
@@ -71,6 +71,7 @@ void main() {
     test('BeginRenderingTool sends BeginRendering message', () async {
       final tool = BeginRenderingTool(
         handleMessage: genUiManager.handleMessage,
+        catalogId: 'test_catalog',
       );
 
       final Map<String, String> args = {
@@ -105,6 +106,11 @@ void main() {
                 (e) => e.definition.rootComponentId,
                 'rootComponentId',
                 'root',
+              )
+              .having(
+                (e) => e.definition.catalogId,
+                'catalogId',
+                'test_catalog',
               ),
         ),
       );
diff --git a/packages/genui_a2ui/example/lib/main.dart b/packages/genui_a2ui/example/lib/main.dart
index cc1fe0e98..2fadaee8b 100644
--- a/packages/genui_a2ui/example/lib/main.dart
+++ b/packages/genui_a2ui/example/lib/main.dart
@@ -42,7 +42,7 @@ class ChatScreen extends StatefulWidget {
 class _ChatScreenState extends State<ChatScreen> {
   final TextEditingController _textController = TextEditingController();
   final GenUiManager _genUiManager = GenUiManager(
-    catalog: CoreCatalogItems.asCatalog(),
+    catalogs: [CoreCatalogItems.asCatalog()],
   );
   late final A2uiContentGenerator _contentGenerator;
   late final GenUiConversation _genUiConversation;
diff --git a/packages/genui_a2ui/lib/src/a2ui_agent_connector.dart b/packages/genui_a2ui/lib/src/a2ui_agent_connector.dart
index 48754846a..68a4b779b 100644
--- a/packages/genui_a2ui/lib/src/a2ui_agent_connector.dart
+++ b/packages/genui_a2ui/lib/src/a2ui_agent_connector.dart
@@ -80,7 +80,10 @@ class A2uiAgentConnector {
   /// Connects to the agent and sends a message.
   ///
   /// Returns the text response from the agent, if any.
-  Future<String?> connectAndSend(genui.ChatMessage chatMessage) async {
+  Future<String?> connectAndSend(
+    genui.ChatMessage chatMessage, {
+    genui.A2UiClientCapabilities? clientCapabilities,
+  }) async {
     final List<Object> parts = (chatMessage is genui.UserMessage)
         ? chatMessage.parts
         : (chatMessage is genui.UserUiInteractionMessage)
@@ -128,7 +131,11 @@ class A2uiAgentConnector {
     if (contextId != null) {
       message.contextId = contextId;
     }
-
+    if (clientCapabilities != null) {
+      message.metadata = {
+        'a2uiClientCapabilities': clientCapabilities.toJson(),
+      };
+    }
     final payload = A2AMessageSendParams()..message = message;
     payload.extensions = ['https://a2ui.org/ext/a2a-ui/v0.1'];
 
diff --git a/packages/genui_a2ui/lib/src/a2ui_content_generator.dart b/packages/genui_a2ui/lib/src/a2ui_content_generator.dart
index 02cc8bc0c..fddbaa3de 100644
--- a/packages/genui_a2ui/lib/src/a2ui_content_generator.dart
+++ b/packages/genui_a2ui/lib/src/a2ui_content_generator.dart
@@ -54,6 +54,7 @@ class A2uiContentGenerator implements ContentGenerator {
   Future<void> sendRequest(
     ChatMessage message, {
     Iterable<ChatMessage>? history,
+    A2UiClientCapabilities? clientCapabilities,
   }) async {
     _isProcessing.value = true;
     try {
@@ -62,7 +63,10 @@ class A2uiContentGenerator implements ContentGenerator {
           'A2uiContentGenerator is stateful and ignores history.',
         );
       }
-      final String? responseText = await connector.connectAndSend(message);
+      final String? responseText = await connector.connectAndSend(
+        message,
+        clientCapabilities: clientCapabilities,
+      );
       if (responseText != null && responseText.isNotEmpty) {
         _textResponseController.add(responseText);
       }
diff --git a/packages/genui_a2ui/test/a2ui_agent_connector_test.dart b/packages/genui_a2ui/test/a2ui_agent_connector_test.dart
index a1380bc92..0ece08435 100644
--- a/packages/genui_a2ui/test/a2ui_agent_connector_test.dart
+++ b/packages/genui_a2ui/test/a2ui_agent_connector_test.dart
@@ -42,6 +42,26 @@ void main() {
       expect(fakeClient.getAgentCardCalled, 1);
     });
 
+    test('connectAndSend includes clientCapabilities in metadata', () async {
+      const capabilities = genui.A2UiClientCapabilities(
+        supportedCatalogIds: ['cat1', 'cat2'],
+      );
+      fakeClient.sendMessageStreamHandler = (_) => const Stream.empty();
+
+      await connector.connectAndSend(
+        genui.UserMessage.text('Hi'),
+        clientCapabilities: capabilities,
+      );
+
+      expect(fakeClient.sendMessageStreamCalled, 1);
+      final a2a.A2AMessage sentMessage =
+          fakeClient.lastSendMessageParams!.message;
+      expect(sentMessage.metadata, isNotNull);
+      expect(sentMessage.metadata!['a2uiClientCapabilities'], {
+        'supportedCatalogIds': ['cat1', 'cat2'],
+      });
+    });
+
     test('connectAndSend processes stream and returns text response', () async {
       final responses = [
         a2a.A2ASendStreamMessageSuccessResponse()
diff --git a/packages/genui_a2ui/test/a2ui_content_generator_test.dart b/packages/genui_a2ui/test/a2ui_content_generator_test.dart
index 90966355e..e531d8ddb 100644
--- a/packages/genui_a2ui/test/a2ui_content_generator_test.dart
+++ b/packages/genui_a2ui/test/a2ui_content_generator_test.dart
@@ -42,7 +42,12 @@ void main() {
       final userMessage = UserMessage([const TextPart('Hello')]);
 
       expect(contentGenerator.isProcessing.value, isFalse);
-      final Future<void> future = contentGenerator.sendRequest(userMessage);
+      final Future<void> future = contentGenerator.sendRequest(
+        userMessage,
+        clientCapabilities: const A2UiClientCapabilities(
+          supportedCatalogIds: ['test_catalog'],
+        ),
+      );
       expect(contentGenerator.isProcessing.value, isTrue);
 
       await future;
@@ -51,12 +56,31 @@ void main() {
       expect(fakeConnector.lastConnectAndSendChatMessage, userMessage);
     });
 
+    test('sendRequest passes clientCapabilities to connector', () async {
+      final userMessage = UserMessage([const TextPart('Test')]);
+      const capabilities = A2UiClientCapabilities(
+        supportedCatalogIds: ['test_catalog'],
+      );
+
+      await contentGenerator.sendRequest(
+        userMessage,
+        clientCapabilities: capabilities,
+      );
+
+      expect(fakeConnector.lastClientCapabilities, capabilities);
+    });
+
     test('sendRequest adds response to textResponseStream', () async {
       final userMessage = UserMessage([const TextPart('Test')]);
       final completer = Completer<String>();
       contentGenerator.textResponseStream.listen(completer.complete);
 
-      await contentGenerator.sendRequest(userMessage);
+      await contentGenerator.sendRequest(
+        userMessage,
+        clientCapabilities: const A2UiClientCapabilities(
+          supportedCatalogIds: ['test_catalog'],
+        ),
+      );
 
       expect(await completer.future, 'Fake AI Response');
     });
diff --git a/packages/genui_a2ui/test/fakes.dart b/packages/genui_a2ui/test/fakes.dart
index db002a1db..4e93a5d07 100644
--- a/packages/genui_a2ui/test/fakes.dart
+++ b/packages/genui_a2ui/test/fakes.dart
@@ -148,10 +148,15 @@ class FakeA2uiAgentConnector implements A2uiAgentConnector {
   late a2a.A2AClient client;
 
   genui.ChatMessage? lastConnectAndSendChatMessage;
+  genui.A2UiClientCapabilities? lastClientCapabilities;
 
   @override
-  Future<String?> connectAndSend(genui.ChatMessage chatMessage) async {
+  Future<String?> connectAndSend(
+    genui.ChatMessage chatMessage, {
+    genui.A2UiClientCapabilities? clientCapabilities,
+  }) async {
     lastConnectAndSendChatMessage = chatMessage;
+    lastClientCapabilities = clientCapabilities;
     // Simulate sending a message and receiving a response
     return Future.value('Fake AI Response');
   }
diff --git a/packages/genui_firebase_ai/README.md b/packages/genui_firebase_ai/README.md
index 4d59d81bc..4186b235d 100644
--- a/packages/genui_firebase_ai/README.md
+++ b/packages/genui_firebase_ai/README.md
@@ -19,7 +19,7 @@ Then, you can create an instance of `FirebaseAiContentGenerator` and pass it to
 
 ```dart
 final catalog = CoreCatalogItems.asCatalog();
-final genUiManager = GenUiManager(catalog: catalog);
+final genUiManager = GenUiManager(catalogs: [catalog]);
 // Example of a custom tool
 final myCustomTool = DynamicAiTool<Map<String, Object?>>(
   name: 'my_custom_action',
diff --git a/packages/genui_firebase_ai/lib/src/firebase_ai_content_generator.dart b/packages/genui_firebase_ai/lib/src/firebase_ai_content_generator.dart
index 09463db31..acf02db3f 100644
--- a/packages/genui_firebase_ai/lib/src/firebase_ai_content_generator.dart
+++ b/packages/genui_firebase_ai/lib/src/firebase_ai_content_generator.dart
@@ -111,6 +111,7 @@ class FirebaseAiContentGenerator implements ContentGenerator {
   Future<void> sendRequest(
     ChatMessage message, {
     Iterable<ChatMessage>? history,
+    A2UiClientCapabilities? clientCapabilities,
   }) async {
     _isProcessing.value = true;
     try {
@@ -350,7 +351,10 @@ class FirebaseAiContentGenerator implements ContentGenerator {
           catalog: catalog,
           configuration: configuration,
         ),
-        BeginRenderingTool(handleMessage: _a2uiMessageController.add),
+        BeginRenderingTool(
+          handleMessage: _a2uiMessageController.add,
+          catalogId: catalog.catalogId,
+        ),
       ],
       if (configuration.actions.allowDelete)
         DeleteSurfaceTool(handleMessage: _a2uiMessageController.add),
diff --git a/packages/genui_google_generative_ai/lib/src/google_generative_ai_content_generator.dart b/packages/genui_google_generative_ai/lib/src/google_generative_ai_content_generator.dart
index 5a5683746..f6b177f36 100644
--- a/packages/genui_google_generative_ai/lib/src/google_generative_ai_content_generator.dart
+++ b/packages/genui_google_generative_ai/lib/src/google_generative_ai_content_generator.dart
@@ -113,6 +113,7 @@ class GoogleGenerativeAiContentGenerator implements ContentGenerator {
   Future<void> sendRequest(
     ChatMessage message, {
     Iterable<ChatMessage>? history,
+    A2UiClientCapabilities? clientCapabilities,
   }) async {
     _isProcessing.value = true;
     try {
@@ -351,7 +352,10 @@ class GoogleGenerativeAiContentGenerator implements ContentGenerator {
             catalog: catalog,
             configuration: configuration,
           ),
-          BeginRenderingTool(handleMessage: _a2uiMessageController.add),
+          BeginRenderingTool(
+            handleMessage: _a2uiMessageController.add,
+            catalogId: catalog.catalogId,
+          ),
         ],
         if (configuration.actions.allowDelete)
           DeleteSurfaceTool(handleMessage: _a2uiMessageController.add),