diff --git a/README.md b/README.md
index 577c48c..83b5332 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,9 @@ is a Tabata whether or not anything is labelled "Tabata".
 - **Accurate by construction** — state is derived from a wall-clock timestamp with a pure
   `phaseAt(elapsed, routine)`; nothing accumulates ticks, so nothing drifts.
 - **Offline-first** — precached service worker, no backend, no accounts, no analytics. Installable as
-  **Timer**; updates are picked up on page reload, not via a modal prompt.
+  **Timer**; waiting updates apply immediately when no Routine is active.
+- **Automatic updates** — an active or paused Routine defers a waiting worker until Done or Reset. The
+  new build reloads automatically, then the bottom version shows green **NEW vx.x.x** for that reload only.
 - **Progressive enhancement** — audio session, Wake Lock and vibration are feature-detected and no-ops
   where absent. The app is fully correct without any of them.
 - **Semantic and accessible** — [Pico CSS](https://picocss.com/) classless, WCAG 2.2 AA in every slice.
diff --git a/astro.config.mjs b/astro.config.mjs
index 2035987..fd86400 100644
--- a/astro.config.mjs
+++ b/astro.config.mjs
@@ -6,7 +6,7 @@ import { defineConfig } from "astro/config";
 export default defineConfig({
   integrations: [
     AstroPWA({
-      registerType: "autoUpdate",
+      registerType: "prompt",
       manifest: {
         name: "Timer",
         short_name: "Timer",
diff --git a/docs/spec.md b/docs/spec.md
index 2c278c9..5705bf3 100644
--- a/docs/spec.md
+++ b/docs/spec.md
@@ -328,7 +328,7 @@ No theme override — `prefers-color-scheme` only, and nothing is stored for it.
 
 The whole prototype is one `.astro` file: plain DOM, CSS custom properties, `rAF`, native
 `<input inputmode="numeric">`. Nothing in the UI justifies an island, and nothing in the PWA layer
-does either — registration, update prompt, Wake Lock, audio session and haptics are all plain DOM APIs.
+does either — registration, update activation, Wake Lock, audio session and haptics are all plain DOM APIs.
 
 ---
 
@@ -416,11 +416,26 @@ layer below is feature-detected, adds capability where present, and is a no-op w
   `virtual:pwa-register` by hand; (2) **`workbox-window` must be an explicit devDependency** under
   pnpm, or the build fails with `Rolldown failed to resolve import "workbox-window"`.
 - Add `/// <reference types="vite-plugin-pwa/client" />` to `src/env.d.ts` for the virtual-module types.
-- **`registerType: 'prompt'`, never `autoUpdate`** — an auto-reload mid-Routine would destroy the
-  running clock. The update prompt is a plain `.astro` component with a `<script>`.
+- **`registerType: 'prompt'`, never `autoUpdate`** — activate a waiting worker immediately when no
+  Routine is active. The activation path is a thin Astro script around the service-worker registration wrapper.
 - Precache everything: the app is static with no API. Measured cost: 1.3 KB `sw.js` + 15 KB workbox
   (SW scope only), 1.27 KB gzip on the page.
 
+#### Update activation
+
+- Use `registerType: 'prompt'`, never `autoUpdate`. Capture `updateSW` from
+  `registerSW({ onNeedRefresh })`. When the callback fires, activate immediately if no Routine is active;
+  otherwise leave the worker waiting.
+- A running or paused Routine counts as active. When it reaches Done or the user presses Reset, emit an
+  inactive event and retry `updateSW(true)`. This applies update without discarding an in-memory Clock.
+- No update button, confirmation prompt, toast dependency, popover/polyfill, polling or custom
+  service-worker manager. Standalone Home Screen PWAs use the same service-worker lifecycle: on launch,
+  registration checks for an update; `updateSW(true)` activates it and reloads the page.
+- Before activation, store a session marker. The new build consumes the marker on first render, prefixes
+  the bottom version with green **NEW vX.X.X**, then clears it. Version highlighting is optional; storage
+  failures do not block update activation or timer behavior.
+- Registration errors are swallowed; offline use of the current version remains unaffected.
+
 ### Install and manifest
 
 - Chromium installability requires `name`/`short_name`, **192px and 512px** icons, `start_url`,
@@ -578,8 +593,9 @@ Genuinely undecided. An implementer should raise these rather than assume.
   app is live at `timer.marcosgilf.com`.
 - **Default values for `Prefs`** — only `prepareMs` has a stated default (10s). Starting `muted`,
   `volume`, `ticks` and `vibrate` values are unspecified.
-- ~~**Update-prompt UI**~~ — resolved: no prompt. The service worker auto-updates and the new build
-  is picked up on page reload, so timing is never interrupted.
+- ~~**Update-prompt UI**~~ — resolved: no update control. A waiting worker activates immediately when
+  idle, or after an active Routine reaches Done/Reset; the new build labels its bottom version **NEW vX.X.X**
+  in green for that reload only.
 - **Install guidance copy for iOS** — decided that it must be static copy (no
   `beforeinstallprompt`), but not what it says or which screen hosts it.
 - ~~**App name, iconography, favicon/manifest icon set, `theme_color`**~~ — resolved: app name
diff --git a/src/client/pwa-update.test.ts b/src/client/pwa-update.test.ts
new file mode 100644
index 0000000..99650b6
--- /dev/null
+++ b/src/client/pwa-update.test.ts
@@ -0,0 +1,124 @@
+import { describe, expect, it, vi } from "vitest";
+import { createPwaUpdateController, type RegisterSW } from "./pwa-update.ts";
+
+const createHarness = () => {
+  let routineActive = false;
+  let onNeedRefresh = () => {};
+  const updateSW = vi.fn<ReturnType<RegisterSW>>();
+  updateSW.mockResolvedValue(undefined);
+  const registerSW = vi.fn<RegisterSW>((options) => {
+    onNeedRefresh = options.onNeedRefresh;
+    return updateSW;
+  });
+  const onActivationStarted = vi.fn();
+  const onActivationFailed = vi.fn();
+  const controller = createPwaUpdateController({
+    registerSW,
+    isRoutineActive: () => routineActive,
+    onActivationStarted,
+    onActivationFailed,
+  });
+
+  return {
+    controller,
+    get onNeedRefresh() {
+      return onNeedRefresh;
+    },
+    updateSW,
+    onActivationStarted,
+    onActivationFailed,
+    setRoutineActive: (active: boolean) => {
+      routineActive = active;
+    },
+  };
+};
+
+describe("PWA update controller", () => {
+  it("activates waiting update immediately when no Routine is active", () => {
+    const harness = createHarness();
+    harness.controller.register();
+
+    harness.onNeedRefresh();
+
+    expect(harness.controller.ready).toBe(true);
+    expect(harness.onActivationStarted).toHaveBeenCalledOnce();
+    expect(harness.updateSW).toHaveBeenCalledOnce();
+    expect(harness.updateSW).toHaveBeenCalledWith(true);
+  });
+
+  it("keeps waiting update while Routine is active", async () => {
+    const harness = createHarness();
+    harness.setRoutineActive(true);
+    harness.controller.register();
+
+    harness.onNeedRefresh();
+
+    expect(harness.controller.ready).toBe(true);
+    expect(harness.updateSW).not.toHaveBeenCalled();
+    await expect(harness.controller.activate()).resolves.toBe(false);
+    expect(harness.updateSW).not.toHaveBeenCalled();
+  });
+
+  it("activates deferred update when Routine becomes inactive", async () => {
+    const harness = createHarness();
+    harness.setRoutineActive(true);
+    harness.controller.register();
+    harness.onNeedRefresh();
+
+    harness.setRoutineActive(false);
+
+    await expect(harness.controller.activate()).resolves.toBe(true);
+    expect(harness.onActivationStarted).toHaveBeenCalledOnce();
+    expect(harness.updateSW).toHaveBeenCalledOnce();
+  });
+
+  it("allows retry when waiting-version activation fails", async () => {
+    const harness = createHarness();
+    harness.updateSW.mockRejectedValueOnce(new Error("activation failed"));
+    harness.controller.register();
+    harness.onNeedRefresh();
+
+    await expect(harness.controller.activate()).resolves.toBe(false);
+    await expect(harness.controller.activate()).resolves.toBe(true);
+
+    expect(harness.onActivationFailed).toHaveBeenCalledOnce();
+    expect(harness.updateSW).toHaveBeenCalledTimes(2);
+  });
+
+  it("does not activate twice while first activation is in flight", async () => {
+    const harness = createHarness();
+    harness.setRoutineActive(true);
+    let resolveUpdate = () => {};
+    harness.updateSW.mockImplementation(
+      () =>
+        new Promise<void>((resolve) => {
+          resolveUpdate = resolve;
+        }),
+    );
+    harness.controller.register();
+    harness.onNeedRefresh();
+    harness.setRoutineActive(false);
+
+    const first = harness.controller.activate();
+    const second = harness.controller.activate();
+    resolveUpdate();
+
+    await expect(first).resolves.toBe(true);
+    await expect(second).resolves.toBe(false);
+    expect(harness.updateSW).toHaveBeenCalledOnce();
+  });
+
+  it("leaves timer behavior alone when registration fails", () => {
+    const registerSW = vi.fn(() => {
+      throw new Error("registration failed");
+    });
+    const controller = createPwaUpdateController({
+      registerSW,
+      isRoutineActive: () => false,
+      onActivationStarted: vi.fn(),
+    });
+
+    expect(() => controller.register()).not.toThrow();
+    expect(controller.ready).toBe(false);
+  });
+});
diff --git a/src/client/pwa-update.ts b/src/client/pwa-update.ts
new file mode 100644
index 0000000..1e39129
--- /dev/null
+++ b/src/client/pwa-update.ts
@@ -0,0 +1,62 @@
+export type UpdateSW = (reloadPage?: boolean) => Promise<void>;
+
+export type RegisterSW = (options: {
+  onNeedRefresh: () => void;
+  onRegisterError: (error: unknown) => void;
+}) => UpdateSW;
+
+type PwaUpdateControllerOptions = {
+  registerSW: RegisterSW;
+  isRoutineActive: () => boolean;
+  onActivationStarted: () => void;
+  onActivationFailed?: () => void;
+};
+
+export const createPwaUpdateController = ({
+  registerSW,
+  isRoutineActive,
+  onActivationStarted,
+  onActivationFailed,
+}: PwaUpdateControllerOptions) => {
+  let updateSW: UpdateSW | null = null;
+  let updateReady = false;
+  let activationStarted = false;
+
+  const activate = async (): Promise<boolean> => {
+    if (!updateReady || !updateSW || activationStarted || isRoutineActive()) return false;
+
+    activationStarted = true;
+
+    try {
+      onActivationStarted();
+      await updateSW(true);
+      return true;
+    } catch {
+      activationStarted = false;
+      onActivationFailed?.();
+      return false;
+    }
+  };
+
+  const register = () => {
+    try {
+      updateSW = registerSW({
+        onNeedRefresh: () => {
+          updateReady = true;
+          if (!isRoutineActive()) void activate();
+        },
+        onRegisterError: () => {},
+      });
+    } catch {
+      // Service-worker support is an enhancement. Timer behavior stays independent.
+    }
+  };
+
+  return {
+    register,
+    activate,
+    get ready() {
+      return updateReady;
+    },
+  };
+};
diff --git a/src/env.d.ts b/src/env.d.ts
index 7bfa427..ea29537 100644
--- a/src/env.d.ts
+++ b/src/env.d.ts
@@ -4,5 +4,6 @@ declare module "virtual:pwa-register" {
   export function registerSW(options?: {
     onNeedRefresh?: () => void;
     onOfflineReady?: () => void;
+    onRegisterError?: (error: unknown) => void;
   }): (reloadPage?: boolean) => Promise<void>;
 }
diff --git a/src/layouts/AppLayout.astro b/src/layouts/AppLayout.astro
index 38039e1..8a88c0f 100644
--- a/src/layouts/AppLayout.astro
+++ b/src/layouts/AppLayout.astro
@@ -35,17 +35,56 @@ const deployLabel =
     <link rel="manifest" href="/manifest.webmanifest" />
     <title>{title}</title>
   </head>
-  <body>
+  <body data-routine-active="false">
     <slot />
     <footer class="app-footer">
       {deployLabel && <small class="deploy-clue" aria-label={`Deployment context ${deployLabel}`}>{deployLabel}</small>}
-      <small class="app-version" aria-label={`Timer version ${version}`}>v{version}</small>
+      <small class="app-version" data-app-version aria-label={`Timer version ${version}`}>v{version}</small>
     </footer>
 
     <script>
       import { registerSW } from "virtual:pwa-register";
+      import { createPwaUpdateController, type RegisterSW } from "../client/pwa-update.ts";
 
-      registerSW();
+      const appVersion = document.querySelector<HTMLElement>("[data-app-version]")!;
+      const updateMarker = "timer:update-activated";
+
+      try {
+        if (sessionStorage.getItem(updateMarker) === "true") {
+          sessionStorage.removeItem(updateMarker);
+          const versionLabel = appVersion.getAttribute("aria-label") ?? "Timer version";
+          appVersion.textContent = `NEW ${appVersion.textContent ?? ""}`;
+          appVersion.dataset.updated = "true";
+          appVersion.setAttribute("aria-label", `New ${versionLabel}`);
+        }
+      } catch {
+        // Version highlighting is optional. Timer behavior stays independent.
+      }
+
+      const registerServiceWorker: RegisterSW = (options) => registerSW(options);
+      const update = createPwaUpdateController({
+        registerSW: registerServiceWorker,
+        isRoutineActive: () => document.body.dataset.routineActive === "true",
+        onActivationStarted: () => {
+          try {
+            sessionStorage.setItem(updateMarker, "true");
+          } catch {
+            // Session storage is optional. Update activation still proceeds.
+          }
+        },
+        onActivationFailed: () => {
+          try {
+            sessionStorage.removeItem(updateMarker);
+          } catch {
+            // Session storage is optional. Timer behavior stays independent.
+          }
+        },
+      });
+
+      window.addEventListener("timer:counter-inactive", () => {
+        void update.activate();
+      });
+      update.register();
     </script>
   </body>
 </html>
diff --git a/src/pages/down.astro b/src/pages/down.astro
index d93c520..f96e076 100644
--- a/src/pages/down.astro
+++ b/src/pages/down.astro
@@ -111,6 +111,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     configuredDuration.textContent = formatElapsed(durationMs);
     configuredDuration.dateTime = `PT${durationMs / 1000}S`;
     const status = isFinished ? "Done" : isRunning(clock) ? "Running" : clock.startedAt === null ? "" : "Paused";
+    document.body.dataset.routineActive = String(!isFinished && clock.startedAt !== null);
     const nextTitle = status ? `${status} · ${visible} - Down Timer` : `${visible} - Down Timer`;
     if (nextTitle !== pageTitle) {
       pageTitle = nextTitle;
@@ -134,6 +135,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     setUrlState("done");
     announce(`Done count down: ${announceElapsed(durationMs)}.`);
     paint();
+    window.dispatchEvent(new Event("timer:counter-inactive"));
   };
 
   const paint = () => {
@@ -201,6 +203,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     stopTicking();
     announce("Reset.");
     paint();
+    window.dispatchEvent(new Event("timer:counter-inactive"));
   });
 
   const configurationUrl = new URL("/configuration", location.origin);
@@ -226,8 +229,8 @@ import AppLayout from "../layouts/AppLayout.astro";
   let confirmedNavigation = false;
   document.querySelectorAll<HTMLAnchorElement>("a[href]").forEach((link) => {
     link.addEventListener("click", (event) => {
-      if (!isRunning(clock) || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
-      if (!window.confirm("Leave the running Routine?")) {
+      if (document.body.dataset.routineActive !== "true" || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
+      if (!window.confirm("Leave the active Routine?")) {
         event.preventDefault();
         return;
       }
@@ -237,7 +240,7 @@ import AppLayout from "../layouts/AppLayout.astro";
   });
 
   window.addEventListener("beforeunload", (event) => {
-    if (!isRunning(clock) || confirmedNavigation) return;
+    if (document.body.dataset.routineActive !== "true" || confirmedNavigation) return;
     event.preventDefault();
   });
 
diff --git a/src/pages/index.astro b/src/pages/index.astro
index 42d4f84..2a8e8ef 100644
--- a/src/pages/index.astro
+++ b/src/pages/index.astro
@@ -99,6 +99,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     display.dateTime = `PT${state.displayMs / 1000}S`;
     display.setAttribute("aria-label", announceElapsed(state.displayMs));
     const status = isFinished ? "Done" : isRunning(clock) ? "Running" : clock.startedAt === null ? "" : "Paused";
+    document.body.dataset.routineActive = String(!isFinished && clock.startedAt !== null);
     const nextTitle = status ? `${status} · ${visible} - Timer` : `${visible} - Timer`;
     if (nextTitle !== pageTitle) {
       pageTitle = nextTitle;
@@ -124,6 +125,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     setUrlState("done");
     announce(`Done: ${announceElapsed(MAX_ELAPSED_MS)}.`);
     paint();
+    window.dispatchEvent(new Event("timer:counter-inactive"));
   };
 
   const paint = () => {
@@ -191,6 +193,7 @@ import AppLayout from "../layouts/AppLayout.astro";
     stopTicking();
     announce("Reset.");
     paint();
+    window.dispatchEvent(new Event("timer:counter-inactive"));
   });
 
   document.addEventListener("visibilitychange", () => {
@@ -209,8 +212,8 @@ import AppLayout from "../layouts/AppLayout.astro";
   let confirmedNavigation = false;
   document.querySelectorAll<HTMLAnchorElement>("a[href]").forEach((link) => {
     link.addEventListener("click", (event) => {
-      if (!isRunning(clock) || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
-      if (!window.confirm("Leave the running Routine?")) {
+      if (document.body.dataset.routineActive !== "true" || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
+      if (!window.confirm("Leave the active Routine?")) {
         event.preventDefault();
         return;
       }
@@ -220,7 +223,7 @@ import AppLayout from "../layouts/AppLayout.astro";
   });
 
   window.addEventListener("beforeunload", (event) => {
-    if (!isRunning(clock) || confirmedNavigation) return;
+    if (document.body.dataset.routineActive !== "true" || confirmedNavigation) return;
     event.preventDefault();
   });
 
diff --git a/src/styles/app.css b/src/styles/app.css
index a8cd3b3..e05cc56 100644
--- a/src/styles/app.css
+++ b/src/styles/app.css
@@ -23,6 +23,7 @@ body > main {
   width: 100%;
   max-width: none;
   padding: 1rem max(0.5rem, env(safe-area-inset-left)) 1rem max(0.5rem, env(safe-area-inset-right));
+  padding-bottom: calc(3rem + env(safe-area-inset-bottom));
   overflow-x: hidden;
   text-align: center;
 }
@@ -249,6 +250,12 @@ body > main {
   user-select: none;
 }
 
+.app-version[data-updated="true"] {
+  color: #39d353;
+  font-weight: 700;
+  opacity: 1;
+}
+
 .deploy-clue {
   letter-spacing: 0.08em;
   text-transform: uppercase;