diff --git a/webui/src/components/thread/ThreadShell.tsx b/webui/src/components/thread/ThreadShell.tsx
index 65ffd1e0d..7dc2afaec 100644
--- a/webui/src/components/thread/ThreadShell.tsx
+++ b/webui/src/components/thread/ThreadShell.tsx
@@ -44,6 +44,7 @@ export function ThreadShell({
   const [booting, setBooting] = useState(false);
   const pendingFirstRef = useRef<string | null>(null);
   const messageCacheRef = useRef<Map<string, UIMessage[]>>(new Map());
+  const lastCachedChatIdRef = useRef<string | null>(null);
 
   const initial = useMemo(() => {
     if (!chatId) return historical;
@@ -91,6 +92,13 @@ export function ThreadShell({
 
   useEffect(() => {
     if (!chatId) return;
+    // Skip the first cache write after a chat switch. During that render,
+    // `messages` can still belong to the previous chat until the stream hook
+    // resets its local state for the new session.
+    if (lastCachedChatIdRef.current !== chatId) {
+      lastCachedChatIdRef.current = chatId;
+      return;
+    }
     messageCacheRef.current.set(chatId, messages);
   }, [chatId, messages]);
 
diff --git a/webui/src/tests/thread-shell.test.tsx b/webui/src/tests/thread-shell.test.tsx
index d134fcce2..f5dea5960 100644
--- a/webui/src/tests/thread-shell.test.tsx
+++ b/webui/src/tests/thread-shell.test.tsx
@@ -267,6 +267,93 @@ describe("ThreadShell", () => {
     expect(screen.queryByText("old answer")).not.toBeInTheDocument();
   });
 
+  it("does not cache optimistic messages under the next chat during a session switch", async () => {
+    const client = makeClient();
+    const onNewChat = vi.fn().mockResolvedValue("chat-b");
+
+    const { rerender } = render(
+      wrap(
+        client,
+        <ThreadShell
+          session={session("chat-a")}
+          title="Chat chat-a"
+          onToggleSidebar={() => {}}
+          onGoHome={() => {}}
+          onNewChat={onNewChat}
+        />,
+      ),
+    );
+
+    fireEvent.change(screen.getByLabelText("Message input"), {
+      target: { value: "only in chat a" },
+    });
+    fireEvent.click(screen.getByRole("button", { name: "Send message" }));
+
+    await waitFor(() =>
+      expect(client.sendMessage).toHaveBeenCalledWith(
+        "chat-a",
+        "only in chat a",
+        undefined,
+      ),
+    );
+    expect(screen.getByText("only in chat a")).toBeInTheDocument();
+
+    await act(async () => {
+      rerender(
+        wrap(
+          client,
+          <ThreadShell
+            session={session("chat-b")}
+            title="Chat chat-b"
+            onToggleSidebar={() => {}}
+            onGoHome={() => {}}
+            onNewChat={onNewChat}
+          />,
+        ),
+      );
+    });
+
+    await waitFor(() => {
+      expect(screen.queryByText("only in chat a")).not.toBeInTheDocument();
+    });
+
+    await act(async () => {
+      rerender(
+        wrap(
+          client,
+          <ThreadShell
+            session={session("chat-a")}
+            title="Chat chat-a"
+            onToggleSidebar={() => {}}
+            onGoHome={() => {}}
+            onNewChat={onNewChat}
+          />,
+        ),
+      );
+    });
+
+    expect(screen.getByText("only in chat a")).toBeInTheDocument();
+
+    await act(async () => {
+      rerender(
+        wrap(
+          client,
+          <ThreadShell
+            session={session("chat-b")}
+            title="Chat chat-b"
+            onToggleSidebar={() => {}}
+            onGoHome={() => {}}
+            onNewChat={onNewChat}
+          />,
+        ),
+      );
+    });
+
+    await waitFor(() => {
+      expect(screen.queryByText("only in chat a")).not.toBeInTheDocument();
+    });
+  });
+
   it("surfaces a dismissible banner when the stream reports message_too_big", async () => {
     const client = makeClient();
     const onNewChat = vi.fn().mockResolvedValue("chat-a");