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
@@ -44,6 +44,7 @@
   usersWithPersonalThreadSelector,
 } from '../selectors/user-selectors.js';
 import { getUserAvatarForThread } from '../shared/avatar-utils.js';
+import SentencePrefixSearchIndex from '../shared/sentence-prefix-search-index.js';
 import type { CalendarQuery } from '../types/entry-types.js';
 import { messageTypes } from '../types/message-types-enum.js';
 import {
@@ -1771,6 +1772,45 @@
   ]);
 }
 
+function useChatMentionSearchIndex(): {
+  +[id: string]: SentencePrefixSearchIndex,
+} {
+  const { chatMentionCandidatesObj, resolvedThreadInfos } =
+    useChatMentionCandidatesObjAndResolvedThreadInfos();
+  return React.useMemo(() => {
+    const result = {};
+    const visitedThreads = new Set();
+    for (const threadID in chatMentionCandidatesObj) {
+      if (visitedThreads.has(threadID) || !resolvedThreadInfos[threadID]) {
+        continue;
+      }
+      visitedThreads.add(threadID);
+      const searchIndex = new SentencePrefixSearchIndex();
+      result[threadID] = searchIndex;
+      searchIndex.addEntry(
+        resolvedThreadInfos[threadID].id,
+        resolvedThreadInfos[threadID].uiName,
+      );
+      for (const key in chatMentionCandidatesObj[threadID]) {
+        result[key] = searchIndex;
+        searchIndex.addEntry(
+          chatMentionCandidatesObj[threadID][key].id,
+          chatMentionCandidatesObj[threadID][key].uiName,
+        );
+        visitedThreads.add(chatMentionCandidatesObj[threadID][key].id);
+      }
+    }
+    return result;
+  }, [chatMentionCandidatesObj, resolvedThreadInfos]);
+}
+
+function useThreadChatMentionSearchIndex(
+  threadInfo: ThreadInfo,
+): SentencePrefixSearchIndex {
+  const chatMentionCandidatesSearchIndex = useChatMentionSearchIndex();
+  return chatMentionCandidatesSearchIndex[threadInfo.id];
+}
+
 export {
   threadHasPermission,
   viewerIsMember,
@@ -1842,4 +1882,5 @@
   getThreadsToDeleteText,
   useChatMentionCandidatesObj,
   useThreadChatMentionCandidates,
+  useThreadChatMentionSearchIndex,
 };