diff --git a/frontend/e2e/zeitstrahl-grouping.spec.ts b/frontend/e2e/zeitstrahl-grouping.spec.ts
new file mode 100644
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+++ b/frontend/e2e/zeitstrahl-grouping.spec.ts
@@ -0,0 +1,123 @@
+import AxeBuilder from '@axe-core/playwright';
+import { test, expect, type APIRequestContext } from '@playwright/test';
+
+/**
+ * Global /zeitstrahl grouping toggle (#827). Runs against the real stack with the seeded admin
+ * session (auth.setup). Covers REQ-002 (switching modes issues zero extra GET /api/timeline
+ * requests — the regroup is client-side), REQ-011 (the control stays usable and overflow-free at
+ * 320px with full-word aria-labels and ≥44px tap targets), and REQ-010g (a 320px axe pass over
+ * the control in both light and dark mode).
+ *
+ * Per e2e/CLAUDE.md, E2E is not yet wired into CI — this gate runs locally for now, like the
+ * #780 layer-filter spec it mirrors.
+ */
+
+const stamp = () => new Date().toISOString().replace(/[^0-9]/g, '');
+
+async function createPerson(request: APIRequestContext, firstName: string, lastName: string) {
+	const res = await request.post('/api/persons', {
+		data: { personType: 'PERSON', firstName, lastName }
+	});
+	if (!res.ok()) throw new Error(`create person failed: ${res.status()}`);
+	return (await res.json()).id as string;
+}
+
+/** Seeds one dated letter so the timeline has a loose letter and the grouping control is enabled. */
+async function seedDatedLetter(request: APIRequestContext, isoDate: string, title: string) {
+	const senderId = await createPerson(request, 'Group-Test', `Absender ${stamp()}`);
+	const receiverId = await createPerson(request, 'Group-Test', `Empfaenger ${stamp()}`);
+
+	const createRes = await request.post('/api/documents', { multipart: { title } });
+	if (!createRes.ok()) throw new Error(`create document failed: ${createRes.status()}`);
+	const docId = (await createRes.json()).id as string;
+
+	const put = await request.put(`/api/documents/${docId}`, {
+		multipart: {
+			title,
+			documentDate: isoDate,
+			metaDatePrecision: 'DAY',
+			senderId,
+			receiverIds: receiverId
+		}
+	});
+	if (!put.ok()) throw new Error(`update document failed: ${put.status()}`);
+}
+
+test.describe('Zeitstrahl — grouping toggle (#827)', () => {
+	test('switching grouping modes issues no extra timeline fetch (REQ-002)', async ({
+		page,
+		request
+	}) => {
+		await seedDatedLetter(request, '1909-05-05', `E2E Group Brief ${stamp()}`);
+
+		let timelineRequests = 0;
+		page.on('request', (req) => {
+			if (req.url().includes('/api/timeline')) timelineRequests++;
+		});
+
+		await page.goto('/zeitstrahl');
+		await page.waitForSelector('[data-hydrated]');
+		await expect(page.getByTestId('grouping-control')).toBeVisible();
+
+		const afterLoad = timelineRequests;
+		await page.locator('[data-value="event"]').click();
+		await page.locator('[data-value="thema"]').click();
+		await page.locator('[data-value="date"]').click();
+
+		// the regroup is a pure client-side transform — not one more GET /api/timeline
+		expect(timelineRequests).toBe(afterLoad);
+	});
+
+	test('the control stays overflow-free and operable at 320px (REQ-011)', async ({
+		page,
+		request
+	}) => {
+		await seedDatedLetter(request, '1911-02-02', `E2E Group 320 ${stamp()}`);
+
+		await page.setViewportSize({ width: 320, height: 800 });
+		await page.goto('/zeitstrahl');
+		await page.waitForSelector('[data-hydrated]');
+
+		const control = page.getByTestId('grouping-control');
+		await expect(control).toBeVisible();
+
+		// the control fits inside the 320px viewport — no horizontal overflow
+		const box = await control.boundingBox();
+		expect(box).not.toBeNull();
+		expect(box!.x + box!.width).toBeLessThanOrEqual(321);
+
+		for (const [value, fullWord] of [
+			['date', 'Datum'],
+			['event', 'Ereignis'],
+			['thema', 'Thema']
+		]) {
+			const radio = page.locator(`[data-value="${value}"]`);
+			const radioBox = await radio.boundingBox();
+			expect(radioBox!.height).toBeGreaterThanOrEqual(44);
+			expect(radioBox!.width).toBeGreaterThanOrEqual(44);
+			// the abbreviated segment still announces its full word
+			expect(await radio.getAttribute('aria-label')).toBe(fullWord);
+		}
+	});
+
+	test('no wcag2a/wcag2aa violations on the grouping control at 320px (light + dark) (REQ-010g)', async ({
+		page,
+		request
+	}) => {
+		await seedDatedLetter(request, '1915-06-15', `E2E Group A11y ${stamp()}`);
+
+		await page.setViewportSize({ width: 320, height: 800 });
+		await page.goto('/zeitstrahl');
+		await page.waitForSelector('[data-hydrated]');
+		await expect(page.getByTestId('grouping-control')).toBeVisible();
+
+		const scan = () => new AxeBuilder({ page }).withTags(['wcag2a', 'wcag2aa']).analyze();
+
+		const light = await scan();
+		expect(light.violations, JSON.stringify(light.violations, null, 2)).toEqual([]);
+
+		await page.evaluate(() => document.documentElement.setAttribute('data-theme', 'dark'));
+		const dark = await scan();
+		expect(dark.violations, JSON.stringify(dark.violations, null, 2)).toEqual([]);
+	});
+});