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
@@ -1768,12 +1768,30 @@
   return chatMentionCandidatesObj;
 }
 
-const emptyMentionCandidates = {};
 function useThreadChatMentionCandidates(
-  // eslint-disable-next-line no-unused-vars
   threadInfo: ThreadInfo,
 ): ChatMentionCandidates {
-  return emptyMentionCandidates;
+  const { chatMentionCandidatesObj, communityThreadIDForGenesisThreads } =
+    useChatMentionCandidatesObjAndUtils();
+  return React.useMemo(() => {
+    let communityID,
+      result = {};
+    if (threadInfo.community === genesis.id) {
+      communityID = communityThreadIDForGenesisThreads[threadInfo.id];
+    } else {
+      communityID = threadInfo.community ?? threadInfo.id;
+    }
+    if (chatMentionCandidatesObj[communityID]) {
+      result = { ...chatMentionCandidatesObj[communityID] };
+    }
+    delete result[threadInfo.id];
+    return result;
+  }, [
+    chatMentionCandidatesObj,
+    communityThreadIDForGenesisThreads,
+    threadInfo.community,
+    threadInfo.id,
+  ]);
 }
 
 function useUserProfileThreadInfo(userInfo: ?UserInfo): ?UserProfileThreadInfo {
@@ -1834,7 +1852,6 @@
   ]);
 }
 
-// eslint-disable-next-line no-unused-vars
 function useChatMentionSearchIndex(): {
   +[id: string]: SentencePrefixSearchIndex,
 } {
@@ -1863,12 +1880,20 @@
   }, [chatMentionCandidatesObj]);
 }
 
-const emptySearchIndex = new SentencePrefixSearchIndex();
 function useThreadChatMentionSearchIndex(
-  // eslint-disable-next-line no-unused-vars
   threadInfo: ThreadInfo,
 ): SentencePrefixSearchIndex {
-  return emptySearchIndex;
+  const chatMentionCandidatesSearchIndex = useChatMentionSearchIndex();
+  const { communityThreadIDForGenesisThreads } =
+    useChatMentionCandidatesObjAndUtils();
+  if (threadInfo.community === genesis.id) {
+    return chatMentionCandidatesSearchIndex[
+      communityThreadIDForGenesisThreads[threadInfo.id]
+    ];
+  }
+  return chatMentionCandidatesSearchIndex[
+    threadInfo.community ?? threadInfo.id
+  ];
 }
 
 export {