diff --git a/ui/e2e/auth.spec.ts b/ui/e2e/auth.spec.ts
index 84c4842..4846a05 100644
--- a/ui/e2e/auth.spec.ts
+++ b/ui/e2e/auth.spec.ts
@@ -34,34 +34,34 @@ const test = base.extend<{ unauthedPage: Page }>({
   },
 });
 
-// TODO(#66): re-enable these once the UI renders a visible "sign in" /
-// "authentication required" affordance on 401. Today apiFetch throws an
-// ApiErrorResponse that bubbles into React Query error states without a
-// recognisable auth-copy surface. See flake-register issue #66.
 test.describe("unauthed API", () => {
-  test.fixme(
-    "home surfaces an auth-required affordance when /api/* returns 401",
-    async ({ unauthedPage: page }) => {
-      await page.goto("/");
-      await expect(page.locator("main#main")).toBeVisible();
-      await expect(
-        page
-          .getByText(/sign in|authenticat|authori|session expired|please log in/i)
-          .first(),
-      ).toBeVisible({ timeout: 5_000 });
-    },
-  );
+  test("home surfaces an auth-required affordance when /api/* returns 401", async ({
+    unauthedPage: page,
+  }) => {
+    await page.goto("/");
+    await expect(page.locator("main#main")).toBeVisible();
+    await expect(page.getByTestId("auth-required-banner")).toBeVisible({
+      timeout: 5_000,
+    });
+    await expect(
+      page
+        .getByText(/sign in|authenticat|authori|session expired|please log in/i)
+        .first(),
+    ).toBeVisible();
+  });
 
-  test.fixme(
-    "navigating to /notes with 401 shows the same affordance",
-    async ({ unauthedPage: page }) => {
-      await page.goto("/notes");
-      await expect(page.locator("main#main")).toBeVisible();
-      await expect(
-        page
-          .getByText(/sign in|authenticat|authori|session expired|please log in/i)
-          .first(),
-      ).toBeVisible({ timeout: 5_000 });
-    },
-  );
+  test("navigating to /notes with 401 shows the same affordance", async ({
+    unauthedPage: page,
+  }) => {
+    await page.goto("/notes");
+    await expect(page.locator("main#main")).toBeVisible();
+    await expect(page.getByTestId("auth-required-banner")).toBeVisible({
+      timeout: 5_000,
+    });
+    await expect(
+      page
+        .getByText(/sign in|authenticat|authori|session expired|please log in/i)
+        .first(),
+    ).toBeVisible();
+  });
 });
diff --git a/ui/src/components/layout/AuthRequiredBanner.tsx b/ui/src/components/layout/AuthRequiredBanner.tsx
new file mode 100644
index 0000000..91cbd12
--- /dev/null
+++ b/ui/src/components/layout/AuthRequiredBanner.tsx
@@ -0,0 +1,42 @@
+import { ShieldAlert } from "lucide-react";
+import { useAuthStore } from "@/stores/auth";
+
+// AuthRequiredBanner renders a visible "Authentication required"
+// affordance when the API has returned 401 anywhere in the app.
+// Mounted inside <main id="main"> so smoke tests that scope to that
+// landmark always find the copy.
+//
+// Copy keywords ("Sign in", "authentication required", "session") are
+// intentionally aligned with ui/e2e/auth.spec.ts so the Playwright
+// smoke can match without embedding brittle selectors.
+export function AuthRequiredBanner() {
+  const authRequired = useAuthStore((s) => s.authRequired);
+  if (!authRequired) return null;
+  return (
+    <div
+      role="alert"
+      aria-live="assertive"
+      data-testid="auth-required-banner"
+      className="state-card state-card--error"
+    >
+      <div className="state-card__icon state-card__icon--danger" aria-hidden="true">
+        <ShieldAlert className="size-6" />
+      </div>
+      <h3 className="state-card__title">Sign in required</h3>
+      <p className="state-card__description">
+        Your session has expired or is missing. Please sign in again —
+        run <code>docsiq login</code> on the server, or reload the page
+        after authentication is re-established.
+      </p>
+      <div className="state-card__action">
+        <button
+          type="button"
+          className="inline-flex items-center justify-center rounded-md border px-3 py-1.5 text-sm hover:bg-accent"
+          onClick={() => window.location.reload()}
+        >
+          Reload
+        </button>
+      </div>
+    </div>
+  );
+}
diff --git a/ui/src/components/layout/Providers.tsx b/ui/src/components/layout/Providers.tsx
index e029920..981bb47 100644
--- a/ui/src/components/layout/Providers.tsx
+++ b/ui/src/components/layout/Providers.tsx
@@ -1,12 +1,33 @@
-import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import {
+  MutationCache,
+  QueryCache,
+  QueryClient,
+  QueryClientProvider,
+} from "@tanstack/react-query";
 import { useEffect, useState, type ReactNode } from "react";
 import { BrowserRouter } from "react-router-dom";
 import { useUIStore } from "@/stores/ui";
+import { useAuthStore } from "@/stores/auth";
+
+// Global 401 gate: any /api/* fetch that throws an ApiErrorResponse
+// with status === 401 flips the auth store so AuthRequiredBanner can
+// render a visible "Sign in required" affordance. Wired to BOTH
+// QueryCache and MutationCache — a 401 on a write action (note
+// create/update/delete) must surface the banner just the same as a
+// read-path failure.
+function gateUnauthorized(error: unknown) {
+  const status = (error as { status?: number })?.status ?? 0;
+  if (status === 401) {
+    useAuthStore.getState().signalUnauthorized();
+  }
+}
 
 export function Providers({ children }: { children: ReactNode }) {
   const [client] = useState(
     () =>
       new QueryClient({
+        queryCache: new QueryCache({ onError: gateUnauthorized }),
+        mutationCache: new MutationCache({ onError: gateUnauthorized }),
         defaultOptions: {
           queries: {
             staleTime: 30_000,
diff --git a/ui/src/components/layout/Shell.tsx b/ui/src/components/layout/Shell.tsx
index 2c1a75c..d714dad 100644
--- a/ui/src/components/layout/Shell.tsx
+++ b/ui/src/components/layout/Shell.tsx
@@ -2,6 +2,7 @@ import { type ReactNode, useCallback, useRef, useState } from "react";
 import { useNavigate } from "react-router-dom";
 import { AppSidebar } from "@/components/app-sidebar";
 import { SiteHeader } from "@/components/site-header";
+import { AuthRequiredBanner } from "./AuthRequiredBanner";
 import { SkipLink } from "./SkipLink";
 import { SidebarInset, SidebarProvider } from "@/components/ui/sidebar";
 import { useHotkey } from "@/hooks/useHotkey";
@@ -60,6 +61,7 @@ export function Shell({ children }: { children: ReactNode }) {
           tabIndex={-1}
           className="flex flex-1 flex-col"
         >
+          <AuthRequiredBanner />
           {children}
         </main>
       </SidebarInset>
diff --git a/ui/src/components/layout/__tests__/AuthRequiredBanner.test.tsx b/ui/src/components/layout/__tests__/AuthRequiredBanner.test.tsx
new file mode 100644
index 0000000..7b7177e
--- /dev/null
+++ b/ui/src/components/layout/__tests__/AuthRequiredBanner.test.tsx
@@ -0,0 +1,32 @@
+import { render, screen } from "@testing-library/react";
+import { act } from "react";
+import { afterEach, describe, expect, it } from "vitest";
+import { AuthRequiredBanner } from "../AuthRequiredBanner";
+import { useAuthStore } from "@/stores/auth";
+
+afterEach(() => {
+  // Store is module-level; reset between tests or flipped state leaks.
+  act(() => useAuthStore.getState().clear());
+});
+
+describe("AuthRequiredBanner", () => {
+  it("renders nothing while authRequired is false", () => {
+    const { container } = render(<AuthRequiredBanner />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  it("renders a visible sign-in affordance once authRequired flips", () => {
+    render(<AuthRequiredBanner />);
+    act(() => useAuthStore.getState().signalUnauthorized());
+
+    const banner = screen.getByTestId("auth-required-banner");
+    expect(banner).toBeInTheDocument();
+    expect(banner).toHaveAttribute("role", "alert");
+    expect(banner).toHaveAttribute("aria-live", "assertive");
+    expect(screen.getByText(/sign in required/i)).toBeInTheDocument();
+    expect(
+      screen.getByText(/session has expired|please sign in/i),
+    ).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: /reload/i })).toBeInTheDocument();
+  });
+});
diff --git a/ui/src/components/layout/__tests__/Providers.gate.test.tsx b/ui/src/components/layout/__tests__/Providers.gate.test.tsx
new file mode 100644
index 0000000..bbd0cc9
--- /dev/null
+++ b/ui/src/components/layout/__tests__/Providers.gate.test.tsx
@@ -0,0 +1,86 @@
+import { render } from "@testing-library/react";
+import { QueryClient, QueryClientProvider, useMutation, useQuery } from "@tanstack/react-query";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { Providers } from "../Providers";
+import { useAuthStore } from "@/stores/auth";
+import { ApiErrorResponse } from "@/lib/api-client";
+
+// The real Providers wires QueryCache + MutationCache `onError` to the
+// auth store. Rather than render Providers (which pulls in the full
+// router + shell), we reach into the same factory semantics by
+// inlining the cache hooks — and assert the banner store flips for
+// BOTH query and mutation 401s.
+
+function unauthorized(): ApiErrorResponse {
+  return new ApiErrorResponse(401, { error: "unauthenticated" });
+}
+
+function TriggerQuery() {
+  useQuery({
+    queryKey: ["probe"],
+    queryFn: () => {
+      throw unauthorized();
+    },
+    retry: false,
+  });
+  return null;
+}
+
+function TriggerMutation() {
+  const m = useMutation({
+    mutationFn: async () => {
+      throw unauthorized();
+    },
+  });
+  if (!m.isPending && !m.isError && !m.isSuccess) m.mutate();
+  return null;
+}
+
+function mountWithRealProviders(children: React.ReactNode) {
+  return render(<Providers>{children}</Providers>);
+}
+
+afterEach(() => useAuthStore.getState().clear());
+
+describe("Providers auth gate", () => {
+  it("flips authRequired when a query throws 401", async () => {
+    mountWithRealProviders(<TriggerQuery />);
+    await vi.waitFor(() => {
+      expect(useAuthStore.getState().authRequired).toBe(true);
+    });
+  });
+
+  it("flips authRequired when a mutation throws 401", async () => {
+    mountWithRealProviders(<TriggerMutation />);
+    await vi.waitFor(() => {
+      expect(useAuthStore.getState().authRequired).toBe(true);
+    });
+  });
+
+  it("does not flip for non-401 errors", async () => {
+    // Build a sibling QueryClient wired the same way so we can
+    // simulate a 500 without waiting for the sentinel flag at the
+    // store. If the 500 ever flipped the flag we'd fail the prior
+    // assertions too, so this is belt-and-braces.
+    const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    function Child() {
+      useQuery({
+        queryKey: ["nope"],
+        queryFn: () => {
+          throw new ApiErrorResponse(500, { error: "boom" });
+        },
+        retry: false,
+      });
+      return null;
+    }
+    render(
+      <QueryClientProvider client={client}>
+        <Child />
+      </QueryClientProvider>,
+    );
+    // A 500 never reaches Providers' gate (we used a fresh client),
+    // so authRequired must stay false across one microtask flush.
+    await new Promise((r) => setTimeout(r, 10));
+    expect(useAuthStore.getState().authRequired).toBe(false);
+  });
+});
diff --git a/ui/src/stores/__tests__/auth.test.ts b/ui/src/stores/__tests__/auth.test.ts
new file mode 100644
index 0000000..4ec6391
--- /dev/null
+++ b/ui/src/stores/__tests__/auth.test.ts
@@ -0,0 +1,17 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { useAuthStore } from "../auth";
+
+afterEach(() => useAuthStore.getState().clear());
+
+describe("useAuthStore", () => {
+  it("starts clean", () => {
+    expect(useAuthStore.getState().authRequired).toBe(false);
+  });
+
+  it("signalUnauthorized flips the flag; clear resets it", () => {
+    useAuthStore.getState().signalUnauthorized();
+    expect(useAuthStore.getState().authRequired).toBe(true);
+    useAuthStore.getState().clear();
+    expect(useAuthStore.getState().authRequired).toBe(false);
+  });
+});
diff --git a/ui/src/stores/auth.ts b/ui/src/stores/auth.ts
new file mode 100644
index 0000000..6adcbdc
--- /dev/null
+++ b/ui/src/stores/auth.ts
@@ -0,0 +1,17 @@
+import { create } from "zustand";
+
+// authRequired flips to true the first time any React Query sees a 401
+// from /api/* or /mcp/*. Stays true until the user takes a sign-in
+// action (e.g. reload after OOB provisioning). Not persisted — if the
+// tab closes, the next session's cookies dictate the state afresh.
+interface AuthState {
+  authRequired: boolean;
+  signalUnauthorized: () => void;
+  clear: () => void;
+}
+
+export const useAuthStore = create<AuthState>()((set) => ({
+  authRequired: false,
+  signalUnauthorized: () => set({ authRequired: true }),
+  clear: () => set({ authRequired: false }),
+}));