diff --git a/frontend/src/components/workspace/chats/use-thread-chat.ts b/frontend/src/components/workspace/chats/use-thread-chat.ts
index 85e7db1a8..6913e3b76 100644
--- a/frontend/src/components/workspace/chats/use-thread-chat.ts
+++ b/frontend/src/components/workspace/chats/use-thread-chat.ts
@@ -24,6 +24,14 @@ export function useThreadChat() {
       setThreadId(uuid());
       return;
     }
+    // Guard: after history.replaceState updates the URL from /chats/new to
+    // /chats/{UUID}, Next.js useParams may still return the stale "new" value
+    // because replaceState does not trigger router updates.  Avoid propagating
+    // this invalid thread ID to downstream hooks (e.g. useStream), which would
+    // cause a 422 from LangGraph Server.
+    if (threadIdFromPath === "new") {
+      return;
+    }
     setIsNewThread(false);
     setThreadId(threadIdFromPath);
   }, [pathname, threadIdFromPath]);