diff --git a/ui/src/components/AppUpdateBanner.tsx b/ui/src/components/AppUpdateBanner.tsx
new file mode 100644
index 00000000000..64e196dcbb5
--- /dev/null
+++ b/ui/src/components/AppUpdateBanner.tsx
@@ -0,0 +1,57 @@
+import { useEffect, useState } from "react";
+import { RefreshCw, X } from "lucide-react";
+import { getLoadedBundleId, startAppUpdateWatcher } from "../lib/app-update-check";
+
+/**
+ * Shows a persistent, dismissible banner when a newer frontend build has been
+ * deployed while this tab was open, so long-lived tabs don't keep running a
+ * stale (possibly buggy) bundle until they happen to reload. See
+ * `lib/app-update-check.ts` for how a new build is detected.
+ */
+export function AppUpdateBanner() {
+  const [updateAvailable, setUpdateAvailable] = useState(false);
+  const [dismissed, setDismissed] = useState(false);
+
+  useEffect(() => {
+    const stop = startAppUpdateWatcher({
+      currentBundleId: getLoadedBundleId(),
+      onUpdateAvailable: () => setUpdateAvailable(true),
+    });
+    return stop;
+  }, []);
+
+  if (!updateAvailable || dismissed) return null;
+
+  return (
+    <aside
+      role="status"
+      aria-live="polite"
+      className="fixed bottom-3 right-3 z-(--z-120) w-full max-w-sm px-1"
+    >
+      <div className="pointer-events-auto rounded-sm border border-sky-300 bg-sky-50 text-sky-900 shadow-lg backdrop-blur-xl dark:border-sky-500/25 dark:bg-sky-950/60 dark:text-sky-100">
+        <div className="flex items-start gap-3 px-3 py-2.5">
+          <RefreshCw className="mt-0.5 h-4 w-4 shrink-0" aria-hidden />
+          <div className="min-w-0 flex-1">
+            <p className="text-sm font-semibold leading-5">A new version is available</p>
+            <p className="mt-1 text-xs leading-4 opacity-70">Reload to get the latest update.</p>
+            <button
+              type="button"
+              onClick={() => window.location.reload()}
+              className="mt-2 inline-flex text-xs font-medium underline underline-offset-4 hover:opacity-90"
+            >
+              Reload
+            </button>
+          </div>
+          <button
+            type="button"
+            aria-label="Dismiss update notice"
+            onClick={() => setDismissed(true)}
+            className="mt-0.5 shrink-0 rounded p-1 opacity-50 hover:bg-black/10 hover:opacity-100 dark:hover:bg-white/10"
+          >
+            <X className="h-3.5 w-3.5" />
+          </button>
+        </div>
+      </div>
+    </aside>
+  );
+}
diff --git a/ui/src/components/Layout.tsx b/ui/src/components/Layout.tsx
index ccb9fb3f4f9..f12cdbae9bb 100644
--- a/ui/src/components/Layout.tsx
+++ b/ui/src/components/Layout.tsx
@@ -15,6 +15,7 @@ import { NewGoalDialog } from "./NewGoalDialog";
 import { NewAgentDialog } from "./NewAgentDialog";
 import { KeyboardShortcutsCheatsheet } from "./KeyboardShortcutsCheatsheet";
 import { ToastViewport } from "./ToastViewport";
+import { AppUpdateBanner } from "./AppUpdateBanner";
 import { MobileBottomNav } from "./MobileBottomNav";
 import { WorktreeBanner } from "./WorktreeBanner";
 import { DevRestartBanner } from "./DevRestartBanner";
@@ -658,6 +659,7 @@ export function Layout() {
       <NewAgentDialog />
       <KeyboardShortcutsCheatsheet open={shortcutsOpen} onOpenChange={setShortcutsOpen} />
       <ToastViewport />
+      <AppUpdateBanner />
       </div>
     </GeneralSettingsProvider>
   );
diff --git a/ui/src/lib/app-update-check.test.ts b/ui/src/lib/app-update-check.test.ts
new file mode 100644
index 00000000000..0cb5d98fc46
--- /dev/null
+++ b/ui/src/lib/app-update-check.test.ts
@@ -0,0 +1,87 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  getLoadedBundleId,
+  parseBundleIdFromHtml,
+  startAppUpdateWatcher,
+} from "./app-update-check";
+
+describe("parseBundleIdFromHtml", () => {
+  it("extracts the hashed entry bundle from index.html", () => {
+    const html = `<!doctype html><html><head>
+      <script type="module" crossorigin src="/assets/index-B85r4qUa.js"></script>
+      <link rel="stylesheet" href="/assets/index-abc123.css"></head><body></body></html>`;
+    expect(parseBundleIdFromHtml(html)).toBe("index-B85r4qUa.js");
+  });
+
+  it("returns null when no entry bundle is present", () => {
+    expect(parseBundleIdFromHtml("<html><head></head><body></body></html>")).toBeNull();
+  });
+});
+
+describe("getLoadedBundleId", () => {
+  it("reads the bundle filename from a script tag", () => {
+    const doc = {
+      querySelectorAll: () => [
+        { getAttribute: () => "/assets/vendor-x.js" },
+        { getAttribute: () => "https://host/assets/index-CB24WRl4.js" },
+      ],
+    } as unknown as Document;
+    expect(getLoadedBundleId(doc)).toBe("index-CB24WRl4.js");
+  });
+});
+
+describe("startAppUpdateWatcher", () => {
+  beforeEach(() => vi.useFakeTimers());
+  afterEach(() => vi.useRealTimers());
+
+  it("fires once when the deployed bundle differs, then latches", async () => {
+    const onUpdate = vi.fn();
+    let deployed = "index-AAA.js";
+    const stop = startAppUpdateWatcher({
+      currentBundleId: "index-AAA.js",
+      onUpdateAvailable: onUpdate,
+      intervalMs: 1000,
+      getDeployedBundleId: () => Promise.resolve(deployed),
+    });
+
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(onUpdate).not.toHaveBeenCalled(); // same bundle
+
+    deployed = "index-BBB.js";
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(onUpdate).toHaveBeenCalledTimes(1);
+    expect(onUpdate).toHaveBeenCalledWith("index-BBB.js");
+
+    // latched — does not fire again on subsequent polls
+    await vi.advanceTimersByTimeAsync(5000);
+    expect(onUpdate).toHaveBeenCalledTimes(1);
+    stop();
+  });
+
+  it("does nothing when the current bundle id is unknown", () => {
+    const onUpdate = vi.fn();
+    const stop = startAppUpdateWatcher({
+      currentBundleId: null,
+      onUpdateAvailable: onUpdate,
+      intervalMs: 1000,
+      getDeployedBundleId: () => Promise.resolve("index-BBB.js"),
+    });
+    vi.advanceTimersByTime(5000);
+    expect(onUpdate).not.toHaveBeenCalled();
+    stop();
+  });
+
+  it("stop() halts polling", async () => {
+    const onUpdate = vi.fn();
+    const getter = vi.fn(() => Promise.resolve("index-AAA.js"));
+    const stop = startAppUpdateWatcher({
+      currentBundleId: "index-AAA.js",
+      onUpdateAvailable: onUpdate,
+      intervalMs: 1000,
+      getDeployedBundleId: getter,
+    });
+    stop();
+    await vi.advanceTimersByTimeAsync(5000);
+    expect(getter).not.toHaveBeenCalled();
+  });
+});
diff --git a/ui/src/lib/app-update-check.ts b/ui/src/lib/app-update-check.ts
new file mode 100644
index 00000000000..1467d644e73
--- /dev/null
+++ b/ui/src/lib/app-update-check.ts
@@ -0,0 +1,91 @@
+/**
+ * Detects when a new frontend build has been deployed while a tab is still open.
+ *
+ * Long-lived tabs keep running the JS bundle they loaded at page-load, so a
+ * deployed fix (e.g. the React perf-measure memory fix) doesn't reach them until
+ * they happen to reload. The service worker (`public/sw.js`) is a static file
+ * that doesn't change between builds, so the normal SW `updatefound` flow never
+ * fires for a JS-only deploy. Instead we compare the *hashed bundle filename*
+ * (which Vite changes every build) against the freshly-served `index.html`.
+ */
+
+// Vite emits the entry as `assets/index-<hash>.js`. Match just the filename.
+const BUNDLE_FILENAME_RE = /(index-[A-Za-z0-9_-]+\.js)/;
+
+/** The bundle filename this page actually loaded, from its <script> tags. */
+export function getLoadedBundleId(doc: Document = document): string | null {
+  for (const script of Array.from(doc.querySelectorAll("script[src]"))) {
+    const src = script.getAttribute("src") ?? "";
+    const match = src.match(BUNDLE_FILENAME_RE);
+    if (match) return match[1]!;
+  }
+  return null;
+}
+
+/** The entry bundle filename referenced by a served index.html string. */
+export function parseBundleIdFromHtml(html: string): string | null {
+  const match = html.match(/<script[^>]+src=["'][^"']*?(index-[A-Za-z0-9_-]+\.js)["']/i);
+  return match ? match[1]! : null;
+}
+
+export async function fetchDeployedBundleId(
+  fetchImpl: typeof fetch = fetch,
+): Promise<string | null> {
+  try {
+    const res = await fetchImpl("/index.html", { cache: "no-store", credentials: "same-origin" });
+    if (!res.ok) return null;
+    return parseBundleIdFromHtml(await res.text());
+  } catch {
+    return null;
+  }
+}
+
+export interface AppUpdateWatcherOptions {
+  currentBundleId: string | null;
+  onUpdateAvailable: (deployedBundleId: string) => void;
+  /** Poll interval; also re-checks when the tab regains focus/visibility. */
+  intervalMs?: number;
+  /** Injectable for tests. */
+  getDeployedBundleId?: () => Promise<string | null>;
+}
+
+const DEFAULT_INTERVAL_MS = 5 * 60_000;
+
+/**
+ * Poll for a newer deployed bundle. Fires `onUpdateAvailable` at most once
+ * (updates are latched), then keeps quiet until the caller reloads. Returns a
+ * stop function.
+ */
+export function startAppUpdateWatcher(opts: AppUpdateWatcherOptions): () => void {
+  const { currentBundleId, onUpdateAvailable, intervalMs = DEFAULT_INTERVAL_MS } = opts;
+  // If we can't identify our own bundle, we can't compare — do nothing.
+  if (!currentBundleId) return () => {};
+  const getDeployed = opts.getDeployedBundleId ?? (() => fetchDeployedBundleId());
+
+  let stopped = false;
+  let notified = false;
+
+  const check = async () => {
+    if (stopped || notified) return;
+    const deployed = await getDeployed();
+    if (stopped || notified) return;
+    if (deployed && deployed !== currentBundleId) {
+      notified = true;
+      onUpdateAvailable(deployed);
+    }
+  };
+
+  const interval = setInterval(() => void check(), intervalMs);
+  const onVisible = () => {
+    if (typeof document === "undefined" || document.visibilityState === "visible") void check();
+  };
+  if (typeof document !== "undefined") document.addEventListener("visibilitychange", onVisible);
+  if (typeof window !== "undefined") window.addEventListener("focus", onVisible);
+
+  return () => {
+    stopped = true;
+    clearInterval(interval);
+    if (typeof document !== "undefined") document.removeEventListener("visibilitychange", onVisible);
+    if (typeof window !== "undefined") window.removeEventListener("focus", onVisible);
+  };
+}