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
@@ -19,6 +19,12 @@
 import { getNameForThreadEntity } from '../utils/entity-text.js';
 import { useSelector } from '../utils/redux-utils.js';
 
+type BaseThreadInfo = {
+  +id: string,
+  +containingThreadID?: ?string,
+  ...
+};
+
 type Props = {
   +children: React.Node,
 };
@@ -26,7 +32,9 @@
   +getChatMentionSearchIndex: (
     threadInfo: ThreadInfo,
   ) => ?SentencePrefixSearchIndex,
-  +getCommunityThreadIDForGenesisThreads: (threadInfo: ThreadInfo) => string,
+  +getCommunityThreadIDForGenesisThreads: (
+    threadInfo: BaseThreadInfo,
+  ) => string,
   +chatMentionCandidatesObj: ChatMentionCandidatesObj,
 };
 
@@ -217,7 +225,7 @@
 
 function useChatMentionCandidatesObjAndUtils(): {
   chatMentionCandidatesObj: ChatMentionCandidatesObj,
-  getCommunityThreadIDForGenesisThreads: (threadInfo: ThreadInfo) => string,
+  getCommunityThreadIDForGenesisThreads: (threadInfo: BaseThreadInfo) => string,
 } {
   const threadInfos = useSelector(threadInfoSelector);
   const resolvedThreadInfos = useResolvedThreadInfosObj(
@@ -231,7 +239,7 @@
     );
 
   const getCommunityThreadIDForGenesisThreads = React.useCallback(
-    (threadInfo: ThreadInfo): string => {
+    (threadInfo: BaseThreadInfo): string => {
       const threadID =
         threadIsPending(threadInfo.id) && threadInfo.containingThreadID
           ? threadInfo.containingThreadID
diff --git a/lib/hooks/thread-hooks.js b/lib/hooks/thread-hooks.js
--- a/lib/hooks/thread-hooks.js
+++ b/lib/hooks/thread-hooks.js
@@ -3,8 +3,8 @@
 import * as React from 'react';
 
 import { useChatMentionContext } from './chat-mention-hooks.js';
+import genesis from '../facts/genesis.js';
 import { childThreadInfos } from '../selectors/thread-selectors.js';
-import { getCommunity } from '../shared/thread-utils.js';
 import type { ResolvedThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
 import { useSelector } from '../utils/redux-utils.js';
 
@@ -13,14 +13,20 @@
 } {
   const childThreadInfosMap = useSelector(childThreadInfos);
 
-  const { chatMentionCandidatesObj } = useChatMentionContext();
+  const { getCommunityThreadIDForGenesisThreads, chatMentionCandidatesObj } =
+    useChatMentionContext();
 
   return React.useMemo(() => {
     const result: { [id: string]: $ReadOnlyArray<ResolvedThreadInfo> } = {};
     for (const parentThreadID in childThreadInfosMap) {
       result[parentThreadID] = childThreadInfosMap[parentThreadID]
         .map(rawThreadInfo => {
-          const community = getCommunity(rawThreadInfo);
+          const communityThreadIDForGenesisThreads =
+            getCommunityThreadIDForGenesisThreads(rawThreadInfo);
+          const community =
+            rawThreadInfo.community === genesis().id
+              ? communityThreadIDForGenesisThreads
+              : rawThreadInfo.community ?? rawThreadInfo.id;
           if (!community) {
             return undefined;
           }
@@ -33,7 +39,11 @@
         .filter(Boolean);
     }
     return result;
-  }, [childThreadInfosMap, chatMentionCandidatesObj]);
+  }, [
+    childThreadInfosMap,
+    getCommunityThreadIDForGenesisThreads,
+    chatMentionCandidatesObj,
+  ]);
 }
 
 export { useChildThreadInfosMap };