From 8eced4df4e134590afca4a36e69142a7ea9c54f4 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
 <41898282+github-actions[bot]@users.noreply.github.com>
Date: Mon, 6 Jul 2026 20:07:50 +0000
Subject: [PATCH] [v6.0] fix(react): preserve chat when id is undefined
 (#16797)

Backport of #16484 to `release-v6.0`. Passing an explicitly-undefined
`id` to `useChat` recreated the internal `Chat` instance on every render
(because the `id` key exists in options while the generated chat ID
never equals `undefined`), wiping messages and stream state; the fix
changes the recreate guard to only apply when the provided `id` is
non-nullish. The `use-chat.ts` guard change and the changeset applied
cleanly; the regression test from the original commit conflicted only
because `main`'s test file wraps everything in an outer
`describe('use-chat')` block that does not exist on v6, so the new
`describe('undefined id')` block was re-indented to match v6's flat test
file structure with no semantic changes. Part of the v6.0 backport
tracking issue #16767.
---
 .changeset/use-chat-nullish-id.md       |  5 ++
 packages/react/src/use-chat.ts          |  4 +-
 packages/react/src/use-chat.ui.test.tsx | 76 +++++++++++++++++++++++++
 3 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/use-chat-nullish-id.md

diff --git a/.changeset/use-chat-nullish-id.md b/.changeset/use-chat-nullish-id.md
new file mode 100644
index 000000000000..ca361e14f158
--- /dev/null
+++ b/.changeset/use-chat-nullish-id.md
@@ -0,0 +1,5 @@
+---
+"@ai-sdk/react": patch
+---
+
+Treat nullish `useChat` IDs the same as omitted IDs so the chat instance is not recreated on every render.
diff --git a/packages/react/src/use-chat.ts b/packages/react/src/use-chat.ts
index 819d77ba51cc..71df9a02830d 100644
--- a/packages/react/src/use-chat.ts
+++ b/packages/react/src/use-chat.ts
@@ -60,7 +60,9 @@ export function useChat<UI_MESSAGE extends UIMessage = UIMessage>({
 
   const shouldRecreateChat =
     ('chat' in options && options.chat !== chatRef.current) ||
-    ('id' in options && chatRef.current.id !== options.id);
+    ('id' in options &&
+      options.id != null &&
+      chatRef.current.id !== options.id);
 
   if (shouldRecreateChat) {
     chatRef.current = 'chat' in options ? options.chat : new Chat(options);
diff --git a/packages/react/src/use-chat.ui.test.tsx b/packages/react/src/use-chat.ui.test.tsx
index 84f0775923f7..a1619fd3ffc1 100644
--- a/packages/react/src/use-chat.ui.test.tsx
+++ b/packages/react/src/use-chat.ui.test.tsx
@@ -2216,6 +2216,82 @@ describe('id changes', () => {
   });
 });
 
+describe('undefined id', () => {
+  setupTestComponent(
+    () => {
+      const [renderCount, setRenderCount] = React.useState(0);
+      const {
+        messages,
+        setMessages,
+        id: idKey,
+      } = useChat({
+        id: undefined,
+        generateId: mockId(),
+      });
+
+      return (
+        <div>
+          <div data-testid="id">{idKey}</div>
+          <div data-testid="render-count">{renderCount}</div>
+          <div data-testid="messages">{JSON.stringify(messages, null, 2)}</div>
+          <button
+            data-testid="set-message"
+            onClick={() => {
+              setMessages([
+                {
+                  id: 'message-0',
+                  role: 'user',
+                  parts: [{ text: 'hi', type: 'text' }],
+                },
+              ]);
+            }}
+          />
+          <button
+            data-testid="rerender"
+            onClick={() => {
+              setRenderCount(count => count + 1);
+            }}
+          />
+        </div>
+      );
+    },
+    {
+      init: TestComponent => <TestComponent />,
+    },
+  );
+
+  it('should not recreate chat when id is explicitly undefined', async () => {
+    const initialId = screen.getByTestId('id').textContent;
+
+    await userEvent.click(screen.getByTestId('set-message'));
+
+    await waitFor(() => {
+      expect(
+        JSON.parse(screen.getByTestId('messages').textContent ?? ''),
+      ).toStrictEqual([
+        {
+          id: 'message-0',
+          role: 'user',
+          parts: [{ text: 'hi', type: 'text' }],
+        },
+      ]);
+    });
+
+    await userEvent.click(screen.getByTestId('rerender'));
+
+    expect(screen.getByTestId('id').textContent).toBe(initialId);
+    expect(
+      JSON.parse(screen.getByTestId('messages').textContent ?? ''),
+    ).toStrictEqual([
+      {
+        id: 'message-0',
+        role: 'user',
+        parts: [{ text: 'hi', type: 'text' }],
+      },
+    ]);
+  });
+});
+
 describe('chat instance changes', () => {
   setupTestComponent(
     () => {