diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -71,6 +71,7 @@
   localIDPrefix,
 } from '../shared/message-utils.js';
 import {
+  parsePendingThreadID,
   threadHasPermission,
   threadInChatList,
   threadIsPending,
@@ -103,6 +104,7 @@
   stateSyncPayloadTypes,
 } from '../types/socket-types.js';
 import { threadPermissions } from '../types/thread-permission-types.js';
+import { threadTypeIsThick } from '../types/thread-types-enum.js';
 import type {
   LegacyRawThreadInfo,
   RawThreadInfos,
@@ -618,16 +620,27 @@
       type: 'replace_threads',
       payload: {
         threads: Object.fromEntries(
-          Object.entries(updatedThreads).map(([threadID, thread]) => [
-            threadID,
-            threadInfos[threadID].thick
-              ? {
-                  ...thread,
-                  startReached:
-                    thread.messageIDs.length < defaultNumberPerThread,
-                }
-              : thread,
-          ]),
+          Object.entries(updatedThreads).map(([threadID, thread]) => {
+            let isThreadThick;
+            if (threadInfos[threadID]) {
+              isThreadThick = threadInfos[threadID].thick;
+            } else {
+              const threadIDParseResult = parsePendingThreadID(threadID);
+              isThreadThick = threadIDParseResult
+                ? threadTypeIsThick(threadIDParseResult.threadType)
+                : false;
+            }
+            return [
+              threadID,
+              isThreadThick
+                ? {
+                    ...thread,
+                    startReached:
+                      thread.messageIDs.length < defaultNumberPerThread,
+                  }
+                : thread,
+            ];
+          }),
         ),
       },
     },