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,
@@ -47,7 +48,12 @@
   const getChatMentionSearchIndex = React.useCallback(
     (threadInfo: ThreadInfo) => {
       if (threadInfo.community === genesis.id) {
-        return searchIndices[communityThreadIDForGenesisThreads[threadInfo.id]];
+        const threadID =
+          threadIsPending(threadInfo.id) && threadInfo.containingThreadID
+            ? threadInfo.containingThreadID
+            : threadInfo.id;
+
+        return searchIndices[communityThreadIDForGenesisThreads[threadID]];
       }
       return searchIndices[threadInfo.community ?? threadInfo.id];
     },
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
@@ -8,6 +8,7 @@
   type ChatMentionContextType,
 } from '../components/chat-mention-provider.react.js';
 import genesis from '../facts/genesis.js';
+import { threadIsPending } from '../shared/thread-utils.js';
 import type { ThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
 import type { ChatMentionCandidates } from '../types/thread-types.js';
 
@@ -23,11 +24,20 @@
 ): ChatMentionCandidates {
   const { communityThreadIDForGenesisThreads, chatMentionCandidatesObj } =
     useChatMentionContext();
+
   return React.useMemo(() => {
-    const communityID =
-      threadInfo.community === genesis.id
-        ? communityThreadIDForGenesisThreads[threadInfo.id]
-        : threadInfo.community ?? threadInfo.id;
+    let communityID;
+    if (threadInfo.community === genesis.id) {
+      const threadID =
+        threadIsPending(threadInfo.id) && threadInfo.containingThreadID
+          ? threadInfo.containingThreadID
+          : threadInfo.id;
+
+      communityID = communityThreadIDForGenesisThreads[threadID];
+    } else {
+      communityID = threadInfo.community ?? threadInfo.id;
+    }
+
     const allChatsWithinCommunity = chatMentionCandidatesObj[communityID];
     if (!allChatsWithinCommunity) {
       return {};
@@ -38,6 +48,7 @@
     chatMentionCandidatesObj,
     communityThreadIDForGenesisThreads,
     threadInfo.community,
+    threadInfo.containingThreadID,
     threadInfo.id,
   ]);
 }