diff --git a/e2e/settings-persistence.spec.ts b/e2e/settings-persistence.spec.ts
new file mode 100644
index 000000000..cdf7399de
--- /dev/null
+++ b/e2e/settings-persistence.spec.ts
@@ -0,0 +1,82 @@
+import type { Page } from "@playwright/test";
+
+import { expect, test } from "./setup/test";
+
+import {
+  criticalAccount,
+  criticalStorageState,
+  resetCriticalAccount,
+} from "./setup/critical-fixture";
+
+/**
+ * A settings change is the server's, not the tab's.
+ *
+ * The time-format choice is changed through the account settings, then read
+ * back three ways that a client-side cache could not fake: after a reload, in
+ * a second browser context that shares the session but none of the first
+ * one's storage, and on `/api/auth/me`, which every client renders from.
+ *
+ * The account is the settings journey's own, one per project
+ * (`critical-fixture.ts`), and is put back to the default first.
+ */
+
+async function openTimeFormat(page: Page) {
+  await page.goto("/settings/account");
+  const select = page.getByTestId("settings-time-format-select");
+  await expect(select).toBeVisible({ timeout: 30_000 });
+  // The control is disabled until the account payload has landed.
+  await expect(select).toBeEnabled();
+  return select;
+}
+
+async function publishedTimeFormat(page: Page): Promise<string> {
+  const res = await page.request.get("/api/auth/me");
+  expect(res.status()).toBe(200);
+  return ((await res.json()) as { data: { timeFormat: string } }).data
+    .timeFormat;
+}
+
+test.describe("settings persistence", () => {
+  test.use({ storageState: criticalStorageState("settings") });
+
+  test.beforeEach(async ({}, testInfo) => {
+    await resetCriticalAccount(criticalAccount("settings", testInfo), [
+      "UPDATE users SET time_format = 'AUTO' WHERE id = $1",
+    ]);
+  });
+
+  test("a time-format change survives a reload and reaches a second browser @critical", async ({
+    page,
+    browser,
+  }, testInfo) => {
+    const select = await openTimeFormat(page);
+    await expect(select).toHaveValue("AUTO");
+
+    const saved = page.waitForResponse(
+      (res) =>
+        new URL(res.url()).pathname === "/api/user/profile" &&
+        res.request().method() === "PATCH",
+    );
+    await select.selectOption("H12");
+    expect((await saved).status()).toBe(200);
+    expect(await publishedTimeFormat(page)).toBe("H12");
+
+    await page.reload();
+    await expect(page.getByTestId("settings-time-format-select")).toHaveValue(
+      "H12",
+    );
+
+    // Same session cookie, nothing else: no local storage, no IndexedDB
+    // query snapshot, no in-memory cache.
+    const other = await browser.newContext({
+      storageState: criticalAccount("settings", testInfo).storageStatePath,
+      baseURL: testInfo.project.use.baseURL,
+    });
+    try {
+      const second = await other.newPage();
+      await expect(await openTimeFormat(second)).toHaveValue("H12");
+    } finally {
+      await other.close();
+    }
+  });
+});
diff --git a/src/__tests__/e2e-critical-no-api-stub-guard.test.ts b/src/__tests__/e2e-critical-no-api-stub-guard.test.ts
index 6e85af0a0..6486deaf0 100644
--- a/src/__tests__/e2e-critical-no-api-stub-guard.test.ts
+++ b/src/__tests__/e2e-critical-no-api-stub-guard.test.ts
@@ -50,7 +50,7 @@ const E2E = join(process.cwd(), "e2e");
  * somebody should see here. It counts title literals in the source, so a
  * title written once inside a loop counts once however many tests it names.
  */
-const CRITICAL_TEST_FLOOR = 14;
+const CRITICAL_TEST_FLOOR = 15;
 
 /** A test whose title literal carries the tag, however the call is spelled. */
 const CRITICAL_TEST =