diff --git a/lib/components/chat-mention-provider.react.js b/lib/components/chat-mention-provider.react.js
--- a/lib/components/chat-mention-provider.react.js
+++ b/lib/components/chat-mention-provider.react.js
@@ -5,6 +5,7 @@
 import genesis from '../facts/genesis.js';
 import { threadInfoSelector } from '../selectors/thread-selectors.js';
 import SentencePrefixSearchIndex from '../shared/sentence-prefix-search-index.js';
+import { threadIsPending } from '../shared/thread-utils.js';
 import type {
   ResolvedThreadInfo,
   ThreadInfo,
@@ -25,7 +26,7 @@
   +getChatMentionSearchIndex: (
     threadInfo: ThreadInfo,
   ) => SentencePrefixSearchIndex,
-  +communityThreadIDForGenesisThreads: { +[id: string]: string },
+  +getCommunityThreadIDForGenesisThreads: (threadInfo: ThreadInfo) => string,
   +chatMentionCandidatesObj: ChatMentionCandidatesObj,
 };
 
@@ -33,36 +34,38 @@
 const ChatMentionContext: React.Context<?ChatMentionContextType> =
   React.createContext<?ChatMentionContextType>({
     getChatMentionSearchIndex: () => emptySearchIndex,
-    communityThreadIDForGenesisThreads: {},
+    getCommunityThreadIDForGenesisThreads: () => '',
     chatMentionCandidatesObj: {},
   });
 
 function ChatMentionContextProvider(props: Props): React.Node {
   const { children } = props;
 
-  const { communityThreadIDForGenesisThreads, chatMentionCandidatesObj } =
+  const { chatMentionCandidatesObj, getCommunityThreadIDForGenesisThreads } =
     useChatMentionCandidatesObjAndUtils();
   const searchIndices = useChatMentionSearchIndex(chatMentionCandidatesObj);
 
   const getChatMentionSearchIndex = React.useCallback(
     (threadInfo: ThreadInfo) => {
       if (threadInfo.community === genesis.id) {
-        return searchIndices[communityThreadIDForGenesisThreads[threadInfo.id]];
+        const communityThreadID =
+          getCommunityThreadIDForGenesisThreads(threadInfo);
+        return searchIndices[communityThreadID];
       }
       return searchIndices[threadInfo.community ?? threadInfo.id];
     },
-    [communityThreadIDForGenesisThreads, searchIndices],
+    [getCommunityThreadIDForGenesisThreads, searchIndices],
   );
 
   const value = React.useMemo(
     () => ({
       getChatMentionSearchIndex,
-      communityThreadIDForGenesisThreads,
+      getCommunityThreadIDForGenesisThreads,
       chatMentionCandidatesObj,
     }),
     [
       getChatMentionSearchIndex,
-      communityThreadIDForGenesisThreads,
+      getCommunityThreadIDForGenesisThreads,
       chatMentionCandidatesObj,
     ],
   );
@@ -214,10 +217,7 @@
 
 function useChatMentionCandidatesObjAndUtils(): {
   chatMentionCandidatesObj: ChatMentionCandidatesObj,
-  resolvedThreadInfos: {
-    +[id: string]: ResolvedThreadInfo,
-  },
-  communityThreadIDForGenesisThreads: { +[id: string]: string },
+  getCommunityThreadIDForGenesisThreads: (threadInfo: ThreadInfo) => string,
 } {
   const threadInfos = useSelector(threadInfoSelector);
   const resolvedThreadInfos = useResolvedThreadInfosObj(
@@ -229,10 +229,22 @@
       () => getChatMentionCandidates(threadInfos, resolvedThreadInfos),
       [threadInfos, resolvedThreadInfos],
     );
+
+  const getCommunityThreadIDForGenesisThreads = React.useCallback(
+    (threadInfo: ThreadInfo): string => {
+      const threadID =
+        threadIsPending(threadInfo.id) && threadInfo.containingThreadID
+          ? threadInfo.containingThreadID
+          : threadInfo.id;
+
+      return communityThreadIDForGenesisThreads[threadID];
+    },
+    [communityThreadIDForGenesisThreads],
+  );
+
   return {
     chatMentionCandidatesObj,
-    resolvedThreadInfos,
-    communityThreadIDForGenesisThreads,
+    getCommunityThreadIDForGenesisThreads,
   };
 }
 
diff --git a/lib/hooks/chat-mention-hooks.js b/lib/hooks/chat-mention-hooks.js
--- a/lib/hooks/chat-mention-hooks.js
+++ b/lib/hooks/chat-mention-hooks.js
@@ -21,13 +21,18 @@
 function useThreadChatMentionCandidates(
   threadInfo: ThreadInfo,
 ): ChatMentionCandidates {
-  const { communityThreadIDForGenesisThreads, chatMentionCandidatesObj } =
+  const { getCommunityThreadIDForGenesisThreads, chatMentionCandidatesObj } =
     useChatMentionContext();
+
+  const communityThreadIDForGenesisThreads =
+    getCommunityThreadIDForGenesisThreads(threadInfo);
+
   return React.useMemo(() => {
     const communityID =
       threadInfo.community === genesis.id
-        ? communityThreadIDForGenesisThreads[threadInfo.id]
+        ? communityThreadIDForGenesisThreads
         : threadInfo.community ?? threadInfo.id;
+
     const allChatsWithinCommunity = chatMentionCandidatesObj[communityID];
     if (!allChatsWithinCommunity) {
       return {};