fix(frontend): show new thread in sidebar immediately on creation (#3276) (#3283)

When a user starts a new conversation, the sidebar list did not display
it until the AI finished streaming and generated a title. This made it
impossible to switch back to an in-progress conversation when working
with multiple threads concurrently.

Optimistically insert the new thread into the TanStack Query cache
during the `onCreated` callback so the sidebar renders a placeholder
entry ("New chat") as soon as the backend acknowledges thread creation.
The existing `onUpdateEvent` title handler and `onFinish` query
invalidation then update the entry in-place with the real title.

frontend/src/core/threads/hooks.ts

@@ -1,7 +1,12 @@
 import type { AIMessage, Message, Run } from "@langchain/langgraph-sdk";
 import type { ThreadsClient } from "@langchain/langgraph-sdk/client";
 import { useStream } from "@langchain/langgraph-sdk/react";
-import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import {
+  type QueryClient,
+  useMutation,
+  useQuery,
+  useQueryClient,
+} from "@tanstack/react-query";
 import { useCallback, useEffect, useRef, useState } from "react";
 import { toast } from "sonner";
 
@@ -149,6 +154,48 @@ export function getVisibleOptimisticMessages(
   return optimisticMessages;
 }
 
+export function upsertThreadInSearchCache(
+  queryClient: QueryClient,
+  thread: AgentThread,
+) {
+  queryClient.setQueriesData(
+    {
+      queryKey: ["threads", "search"],
+      exact: false,
+    },
+    (oldData: Array<AgentThread> | undefined) => {
+      if (!oldData) {
+        return [thread];
+      }
+
+      const existingIndex = oldData.findIndex(
+        (t) => t.thread_id === thread.thread_id,
+      );
+      if (existingIndex === -1) {
+        return [thread, ...oldData];
+      }
+
+      return oldData.map((t, index) => {
+        if (index !== existingIndex) {
+          return t;
+        }
+        return {
+          ...thread,
+          ...t,
+          metadata: {
+            ...(thread.metadata ?? {}),
+            ...(t.metadata ?? {}),
+          },
+          values: {
+            ...thread.values,
+            ...t.values,
+          },
+        };
+      });
+    },
+  );
+}
+
 function getStreamErrorMessage(error: unknown): string {
   if (typeof error === "string" && error.trim()) {
     return error;
@@ -241,6 +288,20 @@ export function useThreadStream({
     fetchStateHistory: { limit: 1 },
     onCreated(meta) {
       handleStreamStart(meta.thread_id, meta.run_id);
+      const now = new Date().toISOString();
+      upsertThreadInSearchCache(queryClient, {
+        thread_id: meta.thread_id,
+        created_at: now,
+        updated_at: now,
+        metadata: context.agent_name ? { agent_name: context.agent_name } : {},
+        status: "busy",
+        values: {
+          title: t.pages.newChat,
+          messages: [],
+          artifacts: [],
+        },
+        interrupts: {},
+      });
       if (context.agent_name && !isMock) {
         void getAPIClient()
           .threads.update(meta.thread_id, {