diff --git a/frontend/src/core/threads/hooks.ts b/frontend/src/core/threads/hooks.ts
index b177c1c9..c05310b8 100644
--- a/frontend/src/core/threads/hooks.ts
+++ b/frontend/src/core/threads/hooks.ts
@@ -271,7 +271,7 @@ export function useThreadStream({
 
   const sendMessage = useCallback(
     async (
-      threadId: string,
+      threadId: string | undefined,
       message: PromptInputMessage,
       extraContext?: Record<string, unknown>,
     ) => {
@@ -281,6 +281,8 @@ export function useThreadStream({
       sendInFlightRef.current = true;
 
       const text = message.text.trim();
+      const resolvedThreadId =
+        threadId ?? threadIdRef.current ?? undefined;
 
       // Capture current count before showing optimistic messages
       prevMsgCountRef.current = thread.messages.length;
@@ -315,7 +317,9 @@ export function useThreadStream({
       }
       setOptimisticMessages(newOptimistic);
 
-      _handleOnStart(threadId);
+      if (resolvedThreadId) {
+        _handleOnStart(resolvedThreadId);
+      }
 
       let uploadedFileInfo: UploadedFileInfo[] = [];
 
@@ -359,12 +363,12 @@ export function useThreadStream({
               );
             }
 
-            if (!threadId) {
+            if (!resolvedThreadId) {
               throw new Error("Thread is not ready for file upload.");
             }
 
             if (files.length > 0) {
-              const uploadResponse = await uploadFiles(threadId, files);
+              const uploadResponse = await uploadFiles(resolvedThreadId, files);
               uploadedFileInfo = uploadResponse.files;
 
               // Update optimistic human message with uploaded status + paths
@@ -431,7 +435,7 @@ export function useThreadStream({
             ],
           },
           {
-            threadId: threadId,
+            threadId: resolvedThreadId,
             streamSubgraphs: true,
             streamResumable: true,
             config: {
@@ -452,7 +456,7 @@ export function useThreadStream({
                     : context.mode === "thinking"
                       ? "low"
                       : undefined),
-              thread_id: threadId,
+              thread_id: resolvedThreadId,
             },
           },
         );