diff --git a/e2e/mobile-density.spec.ts b/e2e/mobile-density.spec.ts
new file mode 100644
index 000000000..5478a0919
--- /dev/null
+++ b/e2e/mobile-density.spec.ts
@@ -0,0 +1,155 @@
+import { expect, test } from "@playwright/test";
+
+import { STORAGE_STATE_PATH } from "./setup/global-setup";
+import {
+  LONG_HEADLINE_BRIEFING,
+  MOCK_SCORE_RINGS,
+  mockDashboardSnapshot,
+  POPULATED_SUMMARIES,
+} from "./utils/mock-dashboard-snapshot";
+
+/**
+ * Phone-width density guard (390×844 iPhone class, 360×780 small
+ * Android class).
+ *
+ * Two invariants, per the report against the dashboard hero (a long
+ * briefing-signal headline squeezed into a one-word-per-line column
+ * beside a wide delta, the delta spilling past the tile edge at 360 px):
+ *
+ *   1. No horizontal page overflow — `scrollWidth` never exceeds the
+ *      viewport width (1 px sub-pixel tolerance).
+ *   2. Nothing escapes its tile — for every card / hero band / list
+ *      row, no descendant's visible box crosses the tile's left or
+ *      right edge (clip-aware: content genuinely hidden by an inner
+ *      `overflow-hidden` wrapper below the tile is fine; content cut
+ *      off at the tile's own edge is the bug).
+ *
+ * The snapshot mock pins the worst case: hero band on, three score
+ * rings, and a fresh briefing whose signal rows pair wrapping German
+ * headlines with wide delta strings.
+ *
+ * Desktop project only — the assertions are viewport-driven via
+ * `setViewportSize` (the `ipad-viewport.spec.ts` pattern).
+ */
+const VIEWPORTS = [
+  { width: 390, height: 844 },
+  { width: 360, height: 780 },
+] as const;
+
+const ROUTES = ["/", "/insights"] as const;
+
+interface TileEscape {
+  tile: string;
+  child: string;
+  overRight: number;
+  overLeft: number;
+  text: string;
+}
+
+/** Serialised into the page: measure children escaping their tile. */
+function collectTileEscapes(): TileEscape[] {
+  const escapes: TileEscape[] = [];
+  const tiles = document.querySelectorAll(
+    '[data-slot="card"], [data-slot="dashboard-hero"], [data-slot="list-row"]',
+  );
+  const TOLERANCE = 1;
+  for (const tile of tiles) {
+    const tileRect = tile.getBoundingClientRect();
+    if (tileRect.width === 0) continue;
+    for (const child of tile.querySelectorAll("*")) {
+      const rect = child.getBoundingClientRect();
+      if (rect.width === 0 && rect.height === 0) continue;
+      if (getComputedStyle(child).position === "fixed") continue;
+      // Clamp by clipping ancestors BELOW the tile only — the tile's
+      // own overflow-hidden cutting content off is exactly the bug.
+      let left = rect.left;
+      let right = rect.right;
+      let ancestor = child.parentElement;
+      while (ancestor && ancestor !== tile) {
+        if (getComputedStyle(ancestor).overflowX !== "visible") {
+          const ancestorRect = ancestor.getBoundingClientRect();
+          left = Math.max(left, ancestorRect.left);
+          right = Math.min(right, ancestorRect.right);
+        }
+        ancestor = ancestor.parentElement;
+      }
+      if (right <= left) continue;
+      const overRight = right - tileRect.right;
+      const overLeft = tileRect.left - left;
+      if (overRight > TOLERANCE || overLeft > TOLERANCE) {
+        escapes.push({
+          tile: tile.getAttribute("data-slot") ?? tile.tagName,
+          child:
+            child.tagName.toLowerCase() +
+            (child.getAttribute("data-slot")
+              ? `[${child.getAttribute("data-slot")}]`
+              : ""),
+          overRight: Math.round(overRight * 10) / 10,
+          overLeft: Math.round(overLeft * 10) / 10,
+          text: (child.textContent ?? "").trim().slice(0, 60),
+        });
+      }
+    }
+  }
+  return escapes;
+}
+
+test.describe("phone-width density guard", () => {
+  test.use({ storageState: STORAGE_STATE_PATH });
+
+  test.beforeEach(({}, testInfo) => {
+    test.skip(
+      testInfo.project.name !== "chromium-desktop",
+      "viewport-driven spec; desktop project only",
+    );
+  });
+
+  for (const viewport of VIEWPORTS) {
+    for (const route of ROUTES) {
+      test(`no overflow and no tile escape on ${route} at ${viewport.width}x${viewport.height}`, async ({
+        page,
+      }) => {
+        await page.setViewportSize(viewport);
+        await mockDashboardSnapshot(page, {
+          summaries: POPULATED_SUMMARIES,
+          heroVisible: true,
+          briefing: LONG_HEADLINE_BRIEFING,
+          scoreRings: MOCK_SCORE_RINGS,
+        });
+        await page.goto(route, { waitUntil: "domcontentloaded" });
+        await page.waitForLoadState("networkidle");
+
+        if (route === "/") {
+          // The guard must not pass vacuously — the hero band and its
+          // long-headline spotlight rows have to actually render.
+          await expect(
+            page.locator('[data-slot="dashboard-hero"]'),
+          ).toBeVisible();
+          await expect(
+            page.locator('[data-slot="dashboard-briefing-spotlight-row"]'),
+          ).toHaveCount(3);
+        }
+
+        const dims = await page.evaluate(() => ({
+          scrollWidth: document.documentElement.scrollWidth,
+          bodyScrollWidth: document.body.scrollWidth,
+          innerWidth: window.innerWidth,
+        }));
+        expect(
+          dims.scrollWidth,
+          `page scrollWidth=${dims.scrollWidth}, innerWidth=${dims.innerWidth}`,
+        ).toBeLessThanOrEqual(dims.innerWidth + 1);
+        expect(
+          dims.bodyScrollWidth,
+          `body scrollWidth=${dims.bodyScrollWidth}`,
+        ).toBeLessThanOrEqual(dims.innerWidth + 1);
+
+        const escapes = await page.evaluate(collectTileEscapes);
+        expect(
+          escapes,
+          `tile escapes: ${JSON.stringify(escapes, null, 2)}`,
+        ).toEqual([]);
+      });
+    }
+  }
+});
diff --git a/e2e/utils/mock-dashboard-snapshot.ts b/e2e/utils/mock-dashboard-snapshot.ts
index e1f626569..08a58c947 100644
--- a/e2e/utils/mock-dashboard-snapshot.ts
+++ b/e2e/utils/mock-dashboard-snapshot.ts
@@ -4,6 +4,8 @@ import {
   DASHBOARD_WIDGET_CATALOGUE_IDS,
   DEFAULT_DASHBOARD_LAYOUT,
 } from "@/lib/dashboard-layout";
+import type { DailyBriefing } from "@/lib/ai/schema";
+import type { DashboardScoreRing } from "@/lib/dashboard/score-rings";
 import type { DataSummary } from "@/lib/analytics/trends";
 import type {
   DashboardSnapshot,
@@ -172,6 +174,54 @@ function buildLayoutCatalogue(): DashboardLayoutCatalogueEntry[] {
   }));
 }
 
+/**
+ * Long-headline briefing fixture for the hero density guard: every
+ * signal pairs a wrapping German headline with a wide delta string —
+ * the exact shape that squeezed the spotlight rows on phone widths.
+ */
+export const LONG_HEADLINE_BRIEFING: DailyBriefing = {
+  paragraph:
+    "Dein systolischer Blutdruck liegt heute deutlich unter deinem Monatsmittel, während die Schlafdauer stabil bleibt.",
+  signalsOfDay: [
+    {
+      sourceMetric: "bp",
+      tone: "watch",
+      headline:
+        "Systolischer Blutdruck deutlich unter deinem Monatsmittel gemessen",
+      nudge: "Miss heute Abend erneut, um den Wert einzuordnen.",
+      delta: "−12 mmHg vs. 30-Tage-Mittel",
+    },
+    {
+      sourceMetric: "sleep",
+      tone: "info",
+      headline:
+        "Schlafdauer in den letzten sieben Nächten etwa eine Stunde kürzer als üblich",
+      nudge: "Plane heute eine frühere Nachtruhe ein.",
+      delta: "−58 min",
+    },
+    {
+      sourceMetric: "compliance",
+      tone: "good",
+      headline: "Alle geplanten Dosen der Woche bisher vollständig eingenommen",
+      nudge: "Weiter so — die Abenddosis steht um 20:00 an.",
+      delta: "7/7 Tage",
+    },
+  ],
+  keyFindings: [],
+};
+
+/** Three-ring hero row fixture (max the hero renders beside the score). */
+export const MOCK_SCORE_RINGS: DashboardScoreRing[] = [
+  { id: "READINESS", score: 82, band: "green" },
+  { id: "RECOVERY_SCORE", score: 74, band: "yellow" },
+  {
+    id: "MED_COMPLIANCE",
+    score: 33,
+    band: "yellow",
+    doses: { taken: 1, scheduled: 3 },
+  },
+];
+
 export interface MockSnapshotOptions {
   /**
    * Tile summaries keyed by `MeasurementType`. Defaults to the
@@ -181,6 +231,12 @@ export interface MockSnapshotOptions {
   summaries?: Record<string, DataSummary>;
   /** Artificial latency in ms before the route fulfils (race specs). */
   delayMs?: number;
+  /** Render the dashboard hero band (`layout.heroVisible` opt-in). */
+  heroVisible?: boolean;
+  /** Fresh (`ready`, non-stale) briefing for the hero spotlight rows. */
+  briefing?: DailyBriefing;
+  /** Selected hero score rings (renders beside the health-score ring). */
+  scoreRings?: DashboardScoreRing[];
 }
 
 /** Assemble a fully-typed snapshot body from the requested tile data. */
@@ -207,7 +263,9 @@ export function buildMockSnapshot(
       onboardingTourCompleted: true,
       greetingHour: 9,
     },
-    layout: DEFAULT_DASHBOARD_LAYOUT,
+    layout: options.heroVisible
+      ? { ...DEFAULT_DASHBOARD_LAYOUT, heroVisible: true }
+      : DEFAULT_DASHBOARD_LAYOUT,
     layoutCatalogue: buildLayoutCatalogue(),
     metricStates: {},
     targetBands: mockTargetBands(),
@@ -226,10 +284,13 @@ export function buildMockSnapshot(
       nextDueOverdue: false,
       nextDueMedicationName: null,
     },
-    healthScore: null,
-    briefing: null,
-    briefingState: "preparing",
-    briefingUpdatedAt: null,
+    healthScore: options.briefing
+      ? { score: 84, band: "green", delta: -2, restMode: null }
+      : null,
+    scoreRings: options.scoreRings,
+    briefing: options.briefing ?? null,
+    briefingState: options.briefing ? "ready" : "preparing",
+    briefingUpdatedAt: options.briefing ? now : null,
     briefingStale: false,
     generatedAt: now,
   };