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
@@ -1543,6 +1543,35 @@
   return result;
 }
 
+function patchThreadInfoToIncludeMentionedMembersOfParent(
+  threadInfo: ThreadInfo,
+  parentThreadInfo: ThreadInfo,
+  messageText: string,
+  viewerID: string,
+): ThreadInfo {
+  const members: UserIDAndUsername[] = threadInfo.members
+    .map(({ id, username }) => (username ? { id, username } : null))
+    .filter(Boolean);
+  const mentionedNewMembers = extractNewMentionedParentMembers(
+    messageText,
+    threadInfo,
+    parentThreadInfo,
+  );
+  if (mentionedNewMembers.length === 0) {
+    return threadInfo;
+  }
+  members.push(...mentionedNewMembers);
+  return createPendingThread({
+    viewerID,
+    threadType: threadTypes.SIDEBAR,
+    members,
+    parentThreadInfo,
+    threadColor: threadInfo.color,
+    name: threadInfo.name,
+    sourceMessageID: threadInfo.sourceMessageID,
+  });
+}
+
 export {
   colorIsDark,
   generateRandomColor,
@@ -1611,4 +1640,5 @@
   threadMembersWithoutAddedAshoat,
   validChatNameRegex,
   chatNameMaxLength,
+  patchThreadInfoToIncludeMentionedMembersOfParent,
 };
diff --git a/native/input/input-state-container.react.js b/native/input/input-state-container.react.js
--- a/native/input/input-state-container.react.js
+++ b/native/input/input-state-container.react.js
@@ -41,6 +41,7 @@
 import {
   createRealThreadFromPendingThread,
   threadIsPending,
+  patchThreadInfoToIncludeMentionedMembersOfParent,
 } from 'lib/shared/thread-utils.js';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import type {
@@ -69,10 +70,11 @@
   type MediaMissionReportCreationRequest,
   reportTypes,
 } from 'lib/types/report-types.js';
-import type {
-  ClientNewThreadRequest,
-  NewThreadResult,
-  ThreadInfo,
+import {
+  type ClientNewThreadRequest,
+  type NewThreadResult,
+  type ThreadInfo,
+  threadTypes,
 } from 'lib/types/thread-types.js';
 import {
   type DispatchActionPromise,
@@ -389,15 +391,19 @@
 
   sendTextMessage = async (
     messageInfo: RawTextMessageInfo,
-    threadInfo: ThreadInfo,
+    inputThreadInfo: ThreadInfo,
     parentThreadInfo: ?ThreadInfo,
   ) => {
     this.sendCallbacks.forEach(callback => callback());
 
-    if (!threadIsPending(threadInfo.id)) {
+    if (!threadIsPending(inputThreadInfo.id)) {
       this.props.dispatchActionPromise(
         sendTextMessageActionTypes,
-        this.sendTextMessageAction(messageInfo, threadInfo, parentThreadInfo),
+        this.sendTextMessageAction(
+          messageInfo,
+          inputThreadInfo,
+          parentThreadInfo,
+        ),
         undefined,
         messageInfo,
       );
@@ -409,6 +415,18 @@
       payload: messageInfo,
     });
 
+    let threadInfo = inputThreadInfo;
+    const { viewerID } = this.props;
+    if (viewerID && inputThreadInfo.type === threadTypes.SIDEBAR) {
+      invariant(parentThreadInfo, 'sidebar should have parent');
+      threadInfo = patchThreadInfoToIncludeMentionedMembersOfParent(
+        inputThreadInfo,
+        parentThreadInfo,
+        messageInfo.text,
+        viewerID,
+      );
+    }
+
     let newThreadID = null;
     try {
       newThreadID = await this.startThreadCreation(threadInfo);
@@ -447,9 +465,9 @@
     );
   };
 
-  async startThreadCreation(threadInfo: ThreadInfo): Promise<string> {
+  startThreadCreation(threadInfo: ThreadInfo): Promise<string> {
     if (!threadIsPending(threadInfo.id)) {
-      return threadInfo.id;
+      return Promise.resolve(threadInfo.id);
     }
     let threadCreationPromise = this.pendingThreadCreations.get(threadInfo.id);
     if (!threadCreationPromise) {
diff --git a/web/input/input-state-container.react.js b/web/input/input-state-container.react.js
--- a/web/input/input-state-container.react.js
+++ b/web/input/input-state-container.react.js
@@ -44,6 +44,7 @@
   createRealThreadFromPendingThread,
   draftKeyFromThreadID,
   threadIsPending,
+  patchThreadInfoToIncludeMentionedMembersOfParent,
 } from 'lib/shared/thread-utils.js';
 import type { CalendarQuery } from 'lib/types/entry-types.js';
 import type {
@@ -65,10 +66,11 @@
 import type { RawTextMessageInfo } from 'lib/types/messages/text.js';
 import type { Dispatch } from 'lib/types/redux-types.js';
 import { reportTypes } from 'lib/types/report-types.js';
-import type {
-  ClientNewThreadRequest,
-  NewThreadResult,
-  ThreadInfo,
+import {
+  type ClientNewThreadRequest,
+  type NewThreadResult,
+  type ThreadInfo,
+  threadTypes,
 } from 'lib/types/thread-types.js';
 import {
   type DispatchActionPromise,
@@ -457,9 +459,9 @@
     }
   }
 
-  async startThreadCreation(threadInfo: ThreadInfo): Promise<string> {
+  startThreadCreation(threadInfo: ThreadInfo): Promise<string> {
     if (!threadIsPending(threadInfo.id)) {
-      return threadInfo.id;
+      return Promise.resolve(threadInfo.id);
     }
     let threadCreationPromise = this.pendingThreadCreations.get(threadInfo.id);
     if (!threadCreationPromise) {
@@ -989,15 +991,19 @@
 
   async sendTextMessage(
     messageInfo: RawTextMessageInfo,
-    threadInfo: ThreadInfo,
+    inputThreadInfo: ThreadInfo,
     parentThreadInfo: ?ThreadInfo,
   ) {
     this.props.sendCallbacks.forEach(callback => callback());
 
-    if (!threadIsPending(threadInfo.id)) {
+    if (!threadIsPending(inputThreadInfo.id)) {
       this.props.dispatchActionPromise(
         sendTextMessageActionTypes,
-        this.sendTextMessageAction(messageInfo, threadInfo, parentThreadInfo),
+        this.sendTextMessageAction(
+          messageInfo,
+          inputThreadInfo,
+          parentThreadInfo,
+        ),
         undefined,
         messageInfo,
       );
@@ -1009,6 +1015,18 @@
       payload: messageInfo,
     });
 
+    let threadInfo = inputThreadInfo;
+    const { viewerID } = this.props;
+    if (viewerID && inputThreadInfo.type === threadTypes.SIDEBAR) {
+      invariant(parentThreadInfo, 'sidebar should have parent');
+      threadInfo = patchThreadInfoToIncludeMentionedMembersOfParent(
+        inputThreadInfo,
+        parentThreadInfo,
+        messageInfo.text,
+        viewerID,
+      );
+    }
+
     let newThreadID = null;
     try {
       newThreadID = await this.startThreadCreation(threadInfo);