diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
--- a/lib/shared/mention-utils.js
+++ b/lib/shared/mention-utils.js
@@ -1,10 +1,16 @@
 // @flow
 
+import invariant from 'invariant';
+
 import { oldValidUsernameRegexString } from './account-utils.js';
 import SearchIndex from './search-index.js';
 import { threadOtherMembers } from './thread-utils.js';
 import { stringForUserExplicit } from './user-utils.js';
-import type { RelativeMemberInfo } from '../types/thread-types.js';
+import {
+  threadTypes,
+  type ThreadInfo,
+  type RelativeMemberInfo,
+} from '../types/thread-types.js';
 
 export type TypeaheadMatchedStrings = {
   +textBeforeAtSymbol: string,
@@ -89,6 +95,17 @@
   return { newText, newSelectionStart };
 }
 
+function getMentionsCandidates(
+  threadInfo: ThreadInfo,
+  parentThreadInfo: ?ThreadInfo,
+): $ReadOnlyArray<RelativeMemberInfo> {
+  if (threadInfo.type !== threadTypes.SIDEBAR) {
+    return threadInfo.members;
+  }
+  invariant(parentThreadInfo, 'sidebars should have parents');
+  return parentThreadInfo.members;
+}
+
 export {
   markdownMentionRegex,
   isMentioned,
@@ -96,4 +113,5 @@
   getTypeaheadUserSuggestions,
   getNewTextAndSelection,
   getTypeaheadRegexMatches,
+  getMentionsCandidates,
 };
diff --git a/native/chat/chat-input-bar.react.js b/native/chat/chat-input-bar.react.js
--- a/native/chat/chat-input-bar.react.js
+++ b/native/chat/chat-input-bar.react.js
@@ -29,14 +29,12 @@
 } from 'lib/actions/thread-actions.js';
 import { createLoadingStatusSelector } from 'lib/selectors/loading-selectors.js';
 import { threadInfoSelector } from 'lib/selectors/thread-selectors.js';
-import {
-  relativeMemberInfoSelectorForMembersOfThread,
-  userStoreSearchIndex,
-} from 'lib/selectors/user-selectors.js';
+import { userStoreSearchIndex } from 'lib/selectors/user-selectors.js';
 import {
   getTypeaheadUserSuggestions,
   getTypeaheadRegexMatches,
   type Selection,
+  getMentionsCandidates,
 } from 'lib/shared/mention-utils.js';
 import { localIDPrefix, trimMessage } from 'lib/shared/message-utils.js';
 import SearchIndex from 'lib/shared/search-index.js';
@@ -939,15 +937,17 @@
   const callJoinThread = useServerCall(joinThread);
 
   const userSearchIndex = useSelector(userStoreSearchIndex);
-  const mentionsCandidates = useSelector(
-    relativeMemberInfoSelectorForMembersOfThread(props.threadInfo.id),
-  );
 
   const { parentThreadID } = props.threadInfo;
   const parentThreadInfo = useSelector(state =>
     parentThreadID ? threadInfoSelector(state)[parentThreadID] : null,
   );
 
+  const mentionsCandidates = getMentionsCandidates(
+    props.threadInfo,
+    parentThreadInfo,
+  );
+
   return (
     <ChatInputBar
       {...props}
diff --git a/web/chat/chat-input-bar.react.js b/web/chat/chat-input-bar.react.js
--- a/web/chat/chat-input-bar.react.js
+++ b/web/chat/chat-input-bar.react.js
@@ -12,13 +12,11 @@
 import SWMansionIcon from 'lib/components/SWMansionIcon.react.js';
 import { createLoadingStatusSelector } from 'lib/selectors/loading-selectors.js';
 import { threadInfoSelector } from 'lib/selectors/thread-selectors.js';
-import {
-  userStoreSearchIndex,
-  relativeMemberInfoSelectorForMembersOfThread,
-} from 'lib/selectors/user-selectors.js';
+import { userStoreSearchIndex } from 'lib/selectors/user-selectors.js';
 import {
   getTypeaheadUserSuggestions,
   getTypeaheadRegexMatches,
+  getMentionsCandidates,
 } from 'lib/shared/mention-utils.js';
 import type { TypeaheadMatchedStrings } from 'lib/shared/mention-utils.js';
 import { localIDPrefix, trimMessage } from 'lib/shared/message-utils.js';
@@ -549,8 +547,15 @@
     const dispatchActionPromise = useDispatchActionPromise();
     const callJoinThread = useServerCall(joinThread);
     const userSearchIndex = useSelector(userStoreSearchIndex);
-    const mentionsCandidates = useSelector(
-      relativeMemberInfoSelectorForMembersOfThread(props.threadInfo.id),
+
+    const { parentThreadID } = props.threadInfo;
+    const parentThreadInfo = useSelector(state =>
+      parentThreadID ? threadInfoSelector(state)[parentThreadID] : null,
+    );
+
+    const mentionsCandidates = getMentionsCandidates(
+      props.threadInfo,
+      parentThreadInfo,
     );
 
     const typeaheadRegexMatches = React.useMemo(
@@ -609,11 +614,6 @@
         typeaheadMatchedStrings,
       ]);
 
-    const { parentThreadID } = props.threadInfo;
-    const parentThreadInfo = useSelector(state =>
-      parentThreadID ? threadInfoSelector(state)[parentThreadID] : null,
-    );
-
     return (
       <ChatInputBar
         {...props}