feat(chat): one shared chat context for ChatInput.Root

src/agent/react/use-chat/types.ts

@@ -110,7 +110,8 @@ export interface UseChatResult {
   setInput: (input: string) => void;
   /** Change the model for subsequent requests */
   setModel: (model: string | undefined) => void;
-  sendMessage: (message: { text: string; files?: ChatFilePart[] }) => Promise<void>;
+  /** Send a message. A message model overrides the session model for this request only. */
+  sendMessage: (message: { text: string; files?: ChatFilePart[]; model?: string }) => Promise<void>;
   /** Edit a user message and resubmit — truncates history to that point */
   editMessage: (messageId: string, newText: string) => Promise<void>;
   /** Get branch info for a message (returns { current, total }; total=1 if no branches) */

src/agent/react/use-chat/use-chat.status.test.tsx

@@ -192,6 +192,39 @@ describe("react/agent/useChat status lifecycle", () => {
     }
   });
 
+  it("uses a per-message model override for the request and response metadata", async () => {
+    const restoreDom = installDom();
+    const originalFetch = globalThis.fetch;
+    let requestBody: { model?: string } | undefined;
+    globalThis.fetch = (_input, init) => {
+      requestBody = JSON.parse(String((init as { body?: unknown } | undefined)?.body));
+      return Promise.resolve(sseResponse());
+    };
+    let latest: UseChatResult | null = null;
+
+    function Capture(): null {
+      latest = useChat({ api: "/api/ag-ui", model: "session-model" });
+      return null;
+    }
+
+    const root = createRoot(document.getElementById("root")!);
+    try {
+      flushSync(() => root.render(<Capture />));
+      await latest!.sendMessage({ text: "Hello", model: "flat-model" });
+      await settle();
+
+      assertEquals(requestBody?.model, "flat-model");
+      const assistant = latest!.messages.find((message) => message.role === "assistant");
+      assert(assistant, "the streamed assistant response should be retained");
+      assertEquals(assistant.metadata?.model, "flat-model");
+    } finally {
+      flushSync(() => root.unmount());
+      await settle();
+      globalThis.fetch = originalFetch;
+      restoreDom();
+    }
+  });
+
   it("keeps a successful client tool output persistable", async () => {
     const restoreDom = installDom();
     let latest: UseChatResult | null = null;

src/agent/react/use-chat/use-chat.ts

@@ -215,6 +215,7 @@ function useChatState(options: UseChatOptions): ResettableUseChatResult {
       message: {
         text: string;
         files?: ChatFilePart[];
+        model?: string;
         baseMessages?: ChatMessage[];
         userMessageId?: string;
       },
@@ -241,6 +242,7 @@ function useChatState(options: UseChatOptions): ResettableUseChatResult {
       let didError = false;
       try {
         const allMessages = [...base, userMessage];
+        const requestModel = message.model ?? model;
 
         const abortController = new AbortController();
         abortControllerRef.current = abortController;
@@ -257,7 +259,7 @@ function useChatState(options: UseChatOptions): ResettableUseChatResult {
           credentials: options.credentials,
           body: JSON.stringify({
             messages: allMessages,
-            ...(model ? { model } : {}),
+            ...(requestModel ? { model: requestModel } : {}),
             ...options.body,
           }),
           signal: abortController.signal,
@@ -301,7 +303,7 @@ function useChatState(options: UseChatOptions): ResettableUseChatResult {
           setStreamingMessageId(currentMessageIdRef.current);
         };
         // Mutable local — updated by onData before onMessage/onUpdate use it.
-        let serverModel: string | undefined = model;
+        let serverModel: string | undefined = requestModel;
         setActiveModel((current) =>
           isLatestRequest(requestIdRef.current, requestId) ? undefined : current
         );