diff --git a/__tests__/html2/part-grouping/folded.skip.html b/__tests__/html2/part-grouping/folded.skip.html
index 0a729fc598..a120b0c938 100644
--- a/__tests__/html2/part-grouping/folded.skip.html
+++ b/__tests__/html2/part-grouping/folded.skip.html
@@ -286,10 +286,14 @@
       // 2nd is sent with status not set to show the work is in
       directLine.emulateIncomingActivity(withCreativeWorkStatus(activities.at(1), undefined));
 
+      await pageConditions.numActivitiesShown(3);
+
+      // When (copilot): opening the group
+      await host.click(document.querySelector('.collapsible-grouping__toggle'));
+
       // Then: show both activities in the list:
       // 1st with the loading indicator
       // 2nd with an empty status indicator
-      await pageConditions.numActivitiesShown(3);
       await host.snapshot('local');
 
       // When folding the list:
diff --git a/__tests__/html2/part-grouping/index.html b/__tests__/html2/part-grouping/index.html
index 80cd5fc2dd..151b8b3d8b 100644
--- a/__tests__/html2/part-grouping/index.html
+++ b/__tests__/html2/part-grouping/index.html
@@ -347,6 +347,12 @@
       // 1st with the loading indicator
       // 2nd with an empty status indicator
       await pageConditions.numActivitiesShown(3);
+
+      // When (copilot): opening the group
+      if (variant === 'copilot') {
+        await host.click(document.querySelector('.collapsible-grouping__toggle'));
+      }
+
       await host.snapshot('local');
 
       // When marking the 1st activity as 'Published':
diff --git a/packages/api/src/StyleOptions.ts b/packages/api/src/StyleOptions.ts
index 109ae1e23a..6ca9f04333 100644
--- a/packages/api/src/StyleOptions.ts
+++ b/packages/api/src/StyleOptions.ts
@@ -966,6 +966,13 @@ type StyleOptions = {
    */
   speechRecognitionContinuous?: boolean | undefined;
 
+  /**
+   * Whether collapsible activity groupings ("part" groups) are open by default.
+   *
+   * @default true
+   */
+  collapsibleGroupingDefaultOpen?: boolean | undefined;
+
   /**
    * Defines how activities are being grouped by (in the order of appearance in the array). Default to `['sender', 'status', 'part']` or `sender,status` in CSS.
    *
diff --git a/packages/api/src/defaultStyleOptions.ts b/packages/api/src/defaultStyleOptions.ts
index a77ae7d48b..1f6fd50f0a 100644
--- a/packages/api/src/defaultStyleOptions.ts
+++ b/packages/api/src/defaultStyleOptions.ts
@@ -313,6 +313,7 @@ const DEFAULT_OPTIONS: Required<StyleOptions> = {
   // Speech recognition
   speechRecognitionContinuous: false,
 
+  collapsibleGroupingDefaultOpen: true,
   groupActivitiesBy: ['sender', 'status', 'part'],
 
   // Send box attachment bar
diff --git a/packages/component/src/Middleware/ActivityGrouping/ui/PartGrouping/private/PartGroupingActivity.tsx b/packages/component/src/Middleware/ActivityGrouping/ui/PartGrouping/private/PartGroupingActivity.tsx
index 98e8876a2a..ebb1e4cd5c 100644
--- a/packages/component/src/Middleware/ActivityGrouping/ui/PartGrouping/private/PartGroupingActivity.tsx
+++ b/packages/component/src/Middleware/ActivityGrouping/ui/PartGrouping/private/PartGroupingActivity.tsx
@@ -143,7 +143,8 @@ function PartGroupingActivity(props: PartGroupingActivityProps) {
 
   const classNames = useStyles(styles);
   const getKeyByActivity = useGetKeyByActivity();
-  const [isGroupOpen, setIsGroupOpen] = useState(true);
+  const [{ collapsibleGroupingDefaultOpen }] = useStyleOptions();
+  const [isGroupOpen, setIsGroupOpen] = useState(collapsibleGroupingDefaultOpen);
 
   const messages = useMemo(
     () => activities.map(activity => getOrgSchemaMessage(activity.entities)).filter(message => !!message),
diff --git a/packages/fluent-theme/src/private/FluentThemeProvider.tsx b/packages/fluent-theme/src/private/FluentThemeProvider.tsx
index fcb0613a43..60ed9f04bd 100644
--- a/packages/fluent-theme/src/private/FluentThemeProvider.tsx
+++ b/packages/fluent-theme/src/private/FluentThemeProvider.tsx
@@ -104,10 +104,11 @@ function FluentThemeProvider(props: FluentThemeProviderProps) {
   const fluentStyleOptions = useMemo(
     () =>
       Object.freeze({
+        ...(variant === 'copilot' && { collapsibleGroupingDefaultOpen: false }),
         feedbackActionsPlacement: 'activity-actions',
         stylesRoot
       }),
-    [stylesRoot]
+    [stylesRoot, variant]
   );
 
   return (