diff --git a/lib/shared/messages/text-message-spec.js b/lib/shared/messages/text-message-spec.js
--- a/lib/shared/messages/text-message-spec.js
+++ b/lib/shared/messages/text-message-spec.js
@@ -38,7 +38,10 @@
   stripSpoilersFromNotifications,
   stripSpoilersFromMarkdownAST,
 } from '../markdown.js';
-import { threadIsGroupChat, extractMentionedMembers } from '../thread-utils.js';
+import {
+  threadIsGroupChat,
+  extractNewMentionedParentMembers,
+} from '../thread-utils.js';
 
 /**
  * most of the markdown leaves contain `content` field
@@ -231,28 +234,20 @@
       }
       invariant(parentThreadInfo, 'all sidebars should have a parent thread');
 
-      const mentionedMembersOfParent = extractMentionedMembers(
+      const mentionedNewMembers = extractNewMentionedParentMembers(
         messageInfo.text,
+        threadInfo,
         parentThreadInfo,
       );
-      if (mentionedMembersOfParent.size === 0) {
-        return;
-      }
-
-      for (const member of threadInfo.members) {
-        if (member.role) {
-          mentionedMembersOfParent.delete(member.id);
-        }
-      }
 
-      const mentionedToAdd = [...mentionedMembersOfParent.keys()];
-      if (mentionedToAdd.length === 0) {
+      if (mentionedNewMembers.length === 0) {
         return;
       }
 
+      const newMemberIDs = mentionedNewMembers.map(({ id }) => id);
       const addMembersPromise = callChangeThreadSettings({
         threadID: threadInfo.id,
-        changes: { newMemberIDs: mentionedToAdd },
+        changes: { newMemberIDs },
       });
 
       dispatchActionPromise(changeThreadSettingsActionTypes, addMembersPromise);
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -478,6 +478,25 @@
   return mentionedMembers;
 }
 
+// When a member of the parent is mentioned in a sidebar,
+// they will be automatically added to that sidebar
+function extractNewMentionedParentMembers(
+  messageText: string,
+  threadInfo: ThreadInfo,
+  parentThreadInfo: ThreadInfo,
+): AccountUserInfo[] {
+  const mentionedMembersOfParent = extractMentionedMembers(
+    messageText,
+    parentThreadInfo,
+  );
+  for (const member of threadInfo.members) {
+    if (member.role) {
+      mentionedMembersOfParent.delete(member.id);
+    }
+  }
+  return [...mentionedMembersOfParent.values()];
+}
+
 type SharedCreatePendingSidebarInput = {
   +sourceMessageInfo: ComposableMessageInfo | RobotextMessageInfo,
   +parentThreadInfo: ThreadInfo,
@@ -1554,7 +1573,7 @@
   parsePendingThreadID,
   createPendingThread,
   createUnresolvedPendingSidebar,
-  extractMentionedMembers,
+  extractNewMentionedParentMembers,
   createPendingSidebar,
   pendingThreadType,
   createRealThreadFromPendingThread,