diff --git a/.gitignore b/.gitignore
index 4bf6d5386..d7ddf951c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -593,3 +593,7 @@ src/ingestion/tools/seed/manifest.json
 # run, exactly like .env.compose.test-stand and the generated realm.
 .artifacts/
 coverage.md
+
+# playwright-cli session output: snapshots, console logs and screenshots from
+# driving the UI against a stand.
+.playwright-cli/
diff --git a/src/frontend/src/routes/index.test.tsx b/src/frontend/src/routes/index.test.tsx
new file mode 100644
index 000000000..088a5842c
--- /dev/null
+++ b/src/frontend/src/routes/index.test.tsx
@@ -0,0 +1,55 @@
+// @vitest-environment jsdom
+/**
+ * The preview flag can flip while "/" is mounted, and the route's `beforeLoad`
+ * guard does not run again for that — only the component can send the reader
+ * into the portal without a reload.
+ */
+vi.mock("@tanstack/react-router", async () => {
+  const { portalRouterMock } = await import("@/test/portal-router");
+  return {
+    ...portalRouterMock(),
+    redirect: vi.fn(),
+    createFileRoute: () => (options: Record<string, unknown>) => options,
+  };
+});
+vi.mock("@/auth", () => ({
+  useViewer: () => ({ personId: "person-1" }),
+}));
+vi.mock("@/screens/dashboard", () => ({
+  DashboardScreen: () => <div data-testid="dashboard" />,
+}));
+
+import { act, render, screen } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { setPortalEnabled } from "@/lib/portal/portal-store";
+import { portalRouter } from "@/test/portal-router";
+
+import { Route } from "./index";
+
+const Component = (Route as unknown as { component: () => React.ReactNode })
+  .component;
+
+beforeEach(() => {
+  act(() => {
+    setPortalEnabled(false);
+    portalRouter.reset("/");
+  });
+});
+
+describe("/", () => {
+  it("renders the dashboard while the preview is off", () => {
+    render(<Component />);
+    expect(screen.getByTestId("dashboard")).toBeInTheDocument();
+    expect(portalRouter.navigations).toEqual([]);
+  });
+
+  it("enters the portal when the preview is turned on", () => {
+    render(<Component />);
+    act(() => setPortalEnabled(true));
+    expect(screen.queryByTestId("dashboard")).not.toBeInTheDocument();
+    expect(portalRouter.navigations).toEqual([
+      { to: "/portal", replace: true },
+    ]);
+  });
+});
diff --git a/src/frontend/src/routes/index.tsx b/src/frontend/src/routes/index.tsx
index 9af267ec0..6b31e1a01 100644
--- a/src/frontend/src/routes/index.tsx
+++ b/src/frontend/src/routes/index.tsx
@@ -1,7 +1,7 @@
-import { createFileRoute, redirect } from "@tanstack/react-router";
+import { Navigate, createFileRoute, redirect } from "@tanstack/react-router";
 
 import { useViewer } from "@/auth";
-import { readPortalEnabled } from "@/lib/portal/portal-store";
+import { readPortalEnabled, usePortalEnabled } from "@/lib/portal/portal-store";
 import { FullScreenLoading } from "@/components/full-screen-loading";
 import { DashboardScreen } from "@/screens/dashboard";
 
@@ -17,6 +17,10 @@ export const Route = createFileRoute("/")({
 
 function IndexRoute() {
   const { personId } = useViewer();
+  const portal = usePortalEnabled();
+  // `beforeLoad` only runs on navigation, so a reader who turns the preview on
+  // while standing here would sit on the dashboard until a reload.
+  if (portal) return <Navigate to="/portal" replace />;
   // An authenticated session always carries the person id (the gateway JWT
   // `sub`); the loading fallback only shows in the brief window before the
   // store resolves.