fix(webui): finalize turn-ending slash commands

maintainer edit: keep /new and manually submitted /stop from leaving stale WebUI streaming state after they cancel or reset the active turn.

webui/src/tests/thread-composer.test.tsx

@@ -1345,6 +1345,47 @@ describe("ThreadComposer", () => {
     expect(onSend).toHaveBeenCalledWith("/status", undefined, { sideChannel: true });
   });
 
+  it("marks new chat commands as side-channel sends that finalize the active turn", () => {
+    const onSend = vi.fn();
+    render(
+      <ThreadComposer
+        onSend={onSend}
+        placeholder="Type your message..."
+      />,
+    );
+
+    const input = screen.getByLabelText("Message input");
+    fireEvent.change(input, { target: { value: "/new" } });
+    fireEvent.click(screen.getByRole("button", { name: "Send message" }));
+
+    expect(onSend).toHaveBeenCalledWith(
+      "/new",
+      undefined,
+      { sideChannel: true, finalizeActiveTurn: true },
+    );
+  });
+
+  it("routes a manually submitted stop command through the stop handler", () => {
+    const onSend = vi.fn();
+    const onStop = vi.fn();
+    render(
+      <ThreadComposer
+        onSend={onSend}
+        onStop={onStop}
+        isStreaming
+        placeholder="Type your message..."
+      />,
+    );
+
+    const input = screen.getByLabelText("Message input");
+    fireEvent.change(input, { target: { value: "/stop" } });
+    fireEvent.keyDown(input, { key: "Escape" });
+    fireEvent.keyDown(input, { key: "Enter" });
+
+    expect(onStop).toHaveBeenCalledTimes(1);
+    expect(onSend).not.toHaveBeenCalled();
+  });
+
   it("keeps goal task commands on the normal agent turn path", () => {
     const onSend = vi.fn();
     render(

webui/src/tests/useNanobotStream.test.tsx

@@ -1653,6 +1653,63 @@ describe("useNanobotStream", () => {
     ]);
   });
 
+  it("finalizes active streaming before turn-ending side-channel commands", async () => {
+    const fake = fakeClient();
+    const { result } = renderHook(() => useNanobotStream("chat-new", EMPTY_MESSAGES), {
+      wrapper: wrap(fake.client),
+    });
+
+    act(() => {
+      result.current.send("long task");
+    });
+    const activeTurnId = fake.client.sendMessage.mock.calls.at(-1)![3]?.turnId;
+
+    act(() => {
+      fake.emit("chat-new", {
+        event: "delta",
+        chat_id: "chat-new",
+        text: "partial answer",
+        turn_id: activeTurnId,
+      });
+    });
+    await flushStreamFrame();
+
+    expect(result.current.isStreaming).toBe(true);
+    expect(result.current.messages.find((message) => message.content === "partial answer"))
+      .toMatchObject({ isStreaming: true });
+
+    act(() => {
+      result.current.send("/new", undefined, {
+        sideChannel: true,
+        finalizeActiveTurn: true,
+      });
+    });
+
+    const newCall = fake.client.sendMessage.mock.calls.at(-1)!;
+    expect(newCall[3]).not.toHaveProperty("sideChannel");
+    expect(newCall[3]).not.toHaveProperty("finalizeActiveTurn");
+    expect(result.current.isStreaming).toBe(false);
+    expect(result.current.messages.find((message) => message.content === "partial answer"))
+      .toMatchObject({ isStreaming: false });
+
+    act(() => {
+      fake.emit("chat-new", {
+        event: "message",
+        chat_id: "chat-new",
+        text: "New session started.",
+        turn_id: newCall[3]?.turnId,
+      });
+    });
+
+    expect(result.current.isStreaming).toBe(false);
+    expect(result.current.messages.map((message) => message.content)).toEqual([
+      "long task",
+      "partial answer",
+      "/new",
+      "New session started.",
+    ]);
+  });
+
   it("lets stream_end finish streaming while side-channel status replies arrive", () => {
     vi.useFakeTimers();
     try {