diff --git a/src/app/main.ts b/src/app/main.ts
index 7ab2103..f578cb7 100644
--- a/src/app/main.ts
+++ b/src/app/main.ts
@@ -7,7 +7,12 @@
  */
 import "./styles.css";
 import { deleteShellCaches } from "../platform/cache/shell-cache.js";
-import { registerSW, unregisterAllServiceWorkers } from "../platform/sw/register.js";
+import {
+  registerSW,
+  skipWaitingForUpdate,
+  unregisterAllServiceWorkers,
+  watchForWaitingUpdate,
+} from "../platform/sw/register.js";
 import { evaluateBootReadiness } from "./readiness.js";
 import type { BootReadinessReport } from "../domain/readiness/types.js";
 import { chipForState } from "../domain/readiness/logic.js";
@@ -30,6 +35,7 @@ import { Router, routeForPath, normalizePath } from "../ui/router/router.js";
 import { createHomeView } from "../ui/views/home/home.js";
 import { createStatusView, createPrepGuidanceView } from "../ui/views/status/status.js";
 import { createEmergencyView } from "../ui/views/emergency/emergency.js";
+import { showUpdateBanner } from "../ui/components/update-banner.js";
 import { createMapView } from "../ui/views/map/map.js";
 import { loadMapData, type MapHandle } from "./map.js";
 import type { MapViewData } from "../domain/map/types.js";
@@ -378,9 +384,31 @@ function mount(): { router: Router; cleanup: () => void } {
     void deleteShellCaches();
   } else {
     // Stage 3: register SW after boot — never blocks rendering (offline-safe)
-    void registerSW().catch(() => {
-      // registration failure is non-blocking — shell remains usable
-    });
+    void registerSW()
+      .then((result) => {
+        // "Restart to update": when a newer shell has installed and waits
+        // behind the current one, offer an explicit restart. The SW never
+        // claims clients mid-session on its own (ARCH §8) — this is the
+        // sanctioned opt-in.
+        if (result.status === "registered") {
+          watchForWaitingUpdate(result.registration, (reg) => {
+            showUpdateBanner(root, () => {
+              // One reload per controllerchange; the swap lands right after
+              // SKIP_WAITING wakes the waiting worker.
+              let reloaded = false;
+              navigator.serviceWorker.addEventListener("controllerchange", () => {
+                if (reloaded) return;
+                reloaded = true;
+                location.reload();
+              });
+              skipWaitingForUpdate(reg);
+            });
+          });
+        }
+      })
+      .catch(() => {
+        // registration failure is non-blocking — shell remains usable
+      });
   }
 
   // Ensure the deterministic deep-link set exists: all four destinations must be reachable
diff --git a/src/app/styles.css b/src/app/styles.css
index bc6887e..0d35296 100644
--- a/src/app/styles.css
+++ b/src/app/styles.css
@@ -921,3 +921,44 @@ html.dark .app-main__icon {
   background: var(--chip-amber-bg);
   color: var(--chip-amber-fg);
 }
+
+/* Restart-to-update banner — appears only when a newer shell is installed
+   and waiting; tap applies (SKIP_WAITING + reload), Later dismisses. */
+.update-banner {
+  position: fixed;
+  inset: auto 0 0 0;
+  z-index: 20;
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  padding: 0.75rem max(1rem, env(safe-area-inset-right)) max(0.75rem, env(safe-area-inset-bottom))
+    max(1rem, env(safe-area-inset-left));
+  background: color-mix(in srgb, var(--bg) 92%, transparent);
+  -webkit-backdrop-filter: blur(12px);
+  backdrop-filter: blur(12px);
+  border-top: 1px solid var(--border-accent);
+}
+.update-banner__text {
+  flex: 1;
+  font-size: 0.9375rem;
+  color: var(--fg);
+}
+.update-banner__apply {
+  min-height: 44px;
+  padding: 0.5rem 1rem;
+  border: 1px solid var(--accent);
+  border-radius: var(--radius-sm);
+  background: var(--accent);
+  color: var(--accent-contrast);
+  font-weight: 600;
+  font-size: 0.875rem;
+}
+.update-banner__dismiss {
+  min-height: 44px;
+  padding: 0.5rem 0.75rem;
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  background: var(--surface);
+  color: var(--muted);
+  font-size: 0.875rem;
+}
diff --git a/src/platform/sw/register.ts b/src/platform/sw/register.ts
index a2ca05b..89878ca 100644
--- a/src/platform/sw/register.ts
+++ b/src/platform/sw/register.ts
@@ -32,6 +32,37 @@ export function skipWaitingForUpdate(registration?: ServiceWorkerRegistration):
   if (waiting) waiting.postMessage({ type: "SKIP_WAITING" });
 }
 
+/**
+ * Notify `onWaiting` when a newer shell has installed and is waiting behind
+ * the current one (never mid-session by itself — the user opts in via
+ * skipWaitingForUpdate). Covers both a registration that already has a
+ * `waiting` worker and one discovered later through `updatefound`.
+ * Only fires when a controller exists (a true update, not first install).
+ */
+export function watchForWaitingUpdate(
+  registration: ServiceWorkerRegistration,
+  onWaiting: (registration: ServiceWorkerRegistration) => void,
+): void {
+  const hasController =
+    typeof navigator !== "undefined" &&
+    "serviceWorker" in navigator &&
+    navigator.serviceWorker.controller !== null;
+  if (!hasController) return; // first install activates on its own — nothing to offer
+  if (registration.waiting) {
+    onWaiting(registration);
+    return;
+  }
+  registration.addEventListener("updatefound", () => {
+    const installing = registration.installing;
+    if (!installing) return;
+    installing.addEventListener("statechange", () => {
+      if (installing.state === "installed" && registration.waiting) {
+        onWaiting(registration);
+      }
+    });
+  });
+}
+
 /**
  * Unregister every service worker (dev-mode scrub only — production never
  * calls this). Resolves true when a controlling SW was removed and the
diff --git a/src/ui/components/update-banner.ts b/src/ui/components/update-banner.ts
new file mode 100644
index 0000000..b0a6817
--- /dev/null
+++ b/src/ui/components/update-banner.ts
@@ -0,0 +1,41 @@
+/**
+ * "Restart to update" banner — shown when a newer shell is installed and
+ * waiting behind the current one. The SW never claims clients mid-session
+ * (ARCH §8: no auto skipWaiting/clients.claim), so activating an update is
+ * an explicit user action wired by the app layer via onApply. Dismiss hides
+ * it; the next full start picks the update up anyway. Text-only DOM via
+ * textContent (B-7); no storage, no network, no platform imports (B-1).
+ */
+export function showUpdateBanner(root: HTMLElement, onApply: () => void): void {
+  if (document.querySelector(".update-banner")) return; // idempotent
+
+  const banner = document.createElement("div");
+  banner.className = "update-banner";
+  banner.setAttribute("role", "status");
+
+  const label = document.createElement("span");
+  label.className = "update-banner__text";
+  label.textContent = "A new version of Lumen is ready.";
+  banner.append(label);
+
+  const apply = document.createElement("button");
+  apply.type = "button";
+  apply.className = "update-banner__apply";
+  apply.textContent = "Restart to update";
+  apply.addEventListener("click", () => {
+    onApply();
+  });
+  banner.append(apply);
+
+  const dismiss = document.createElement("button");
+  dismiss.type = "button";
+  dismiss.className = "update-banner__dismiss";
+  dismiss.textContent = "Later";
+  dismiss.setAttribute("aria-label", "Dismiss update notice");
+  dismiss.addEventListener("click", () => {
+    banner.remove();
+  });
+  banner.append(dismiss);
+
+  root.append(banner);
+}
diff --git a/tests/unit/sw.test.ts b/tests/unit/sw.test.ts
index 0db2dc2..aac8514 100644
--- a/tests/unit/sw.test.ts
+++ b/tests/unit/sw.test.ts
@@ -6,7 +6,11 @@
  */
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
 import { readFileSync, existsSync } from "node:fs";
-import { registerSW, skipWaitingForUpdate } from "../../src/platform/sw/register.js";
+import {
+  registerSW,
+  skipWaitingForUpdate,
+  watchForWaitingUpdate,
+} from "../../src/platform/sw/register.js";
 
 const SW_PATH = "public/sw.js";
 
@@ -148,6 +152,68 @@ describe("SW registration — page bridge", () => {
   });
 });
 
+describe("watchForWaitingUpdate — restart-to-update discovery", () => {
+  function withController(controller: unknown): void {
+    Object.defineProperty(globalThis, "navigator", {
+      value: { serviceWorker: { controller } },
+      writable: true,
+      configurable: true,
+    });
+  }
+
+  it("no controller (first install) → never fires", () => {
+    withController(null);
+    const onWaiting = vi.fn();
+    const listeners: Record<string, () => void> = {};
+    const reg = {
+      waiting: { postMessage: vi.fn() },
+      addEventListener: (ev: string, cb: () => void) => {
+        listeners[ev] = cb;
+      },
+    } as unknown as ServiceWorkerRegistration;
+    watchForWaitingUpdate(reg, onWaiting);
+    expect(onWaiting).not.toHaveBeenCalled();
+    expect(listeners.updatefound).toBeUndefined();
+  });
+
+  it("already-waiting worker fires immediately", () => {
+    withController({ scriptURL: "/sw.js" });
+    const onWaiting = vi.fn();
+    const reg = {
+      waiting: { postMessage: vi.fn() },
+      addEventListener: vi.fn(),
+    } as unknown as ServiceWorkerRegistration;
+    watchForWaitingUpdate(reg, onWaiting);
+    expect(onWaiting).toHaveBeenCalledWith(reg);
+  });
+
+  it("updatefound → installed waiting worker fires", () => {
+    withController({ scriptURL: "/sw.js" });
+    const onWaiting = vi.fn();
+    let foundCb: (() => void) | undefined;
+    const stateCbs: (() => void)[] = [];
+    const installed = {
+      state: "installed",
+      addEventListener: (_ev: string, cb: () => void) => {
+        stateCbs.push(cb);
+      },
+    };
+    const reg = {
+      waiting: null as unknown,
+      installing: installed,
+      addEventListener: (ev: string, cb: () => void) => {
+        if (ev === "updatefound") foundCb = cb;
+      },
+    } as unknown as ServiceWorkerRegistration;
+    watchForWaitingUpdate(reg, onWaiting);
+    expect(onWaiting).not.toHaveBeenCalled();
+    foundCb?.();
+    (reg as { waiting: unknown }).waiting = { postMessage: vi.fn() };
+    for (const cb of stateCbs) cb();
+    expect(onWaiting).toHaveBeenCalledTimes(1);
+  });
+});
+
 describe("Cache safety — versioning and shell assets", () => {
   it("vite build injects hashed assets into precache manifest", () => {
     const viteConfig = readFileSync("vite.config.ts", "utf8");
diff --git a/tests/unit/update-banner.test.ts b/tests/unit/update-banner.test.ts
new file mode 100644
index 0000000..e7037f9
--- /dev/null
+++ b/tests/unit/update-banner.test.ts
@@ -0,0 +1,54 @@
+/**
+ * @vitest-environment jsdom
+ * "Restart to update" banner — appears once, apply fires the opt-in,
+ * Later removes it. Pure DOM: no SW, no storage, no network.
+ */
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { showUpdateBanner } from "../../src/ui/components/update-banner.js";
+
+beforeEach(() => {
+  document.body.replaceChildren();
+});
+
+function root(): HTMLElement {
+  const el = document.createElement("div");
+  document.body.append(el);
+  return el;
+}
+
+describe("update banner", () => {
+  it("renders banner with apply and dismiss once (idempotent)", () => {
+    const host = root();
+    showUpdateBanner(host, () => undefined);
+    showUpdateBanner(host, () => undefined);
+    expect(document.querySelectorAll(".update-banner").length).toBe(1);
+    expect(host.textContent).toContain("new version");
+    expect(host.querySelector(".update-banner__apply")?.textContent).toBe("Restart to update");
+  });
+
+  it("apply invokes the opt-in callback exactly once per click", () => {
+    const host = root();
+    const onApply = vi.fn();
+    showUpdateBanner(host, onApply);
+    host.querySelector(".update-banner__apply")?.dispatchEvent(new MouseEvent("click"));
+    expect(onApply).toHaveBeenCalledTimes(1);
+  });
+
+  it("Later removes the banner without calling apply", () => {
+    const host = root();
+    const onApply = vi.fn();
+    showUpdateBanner(host, onApply);
+    host.querySelector(".update-banner__dismiss")?.dispatchEvent(new MouseEvent("click"));
+    expect(document.querySelector(".update-banner")).toBeNull();
+    expect(onApply).not.toHaveBeenCalled();
+    // Re-show after dismiss is allowed (a later update can re-offer).
+    showUpdateBanner(host, onApply);
+    expect(document.querySelectorAll(".update-banner").length).toBe(1);
+  });
+
+  it("renders no raw HTML from anywhere (textContent-only)", () => {
+    const host = root();
+    showUpdateBanner(host, () => undefined);
+    expect(host.querySelector(".update-banner")?.innerHTML).not.toMatch(/<img|<script/);
+  });
+});