diff --git a/src/server/routes/viewers.ts b/src/server/routes/viewers.ts
index 436904d..7fcae24 100644
--- a/src/server/routes/viewers.ts
+++ b/src/server/routes/viewers.ts
@@ -23,9 +23,11 @@ import {
 
 const OPAQUE_ID = /^[\w][\w.:-]{0,199}$/u;
 
-function store(context: ModuleRouteHandlerContext): SignageStore {
-  return createSignageStore(context.services.database);
-}
+/** How a route reaches Signage's tables. Production uses the module database; the display contract
+ * test supplies the in-memory store so the real handlers can answer the real display controller. */
+export type ViewerRouteStore = (context: ModuleRouteHandlerContext) => SignageStore;
+
+const databaseStore: ViewerRouteStore = (context) => createSignageStore(context.services.database);
 
 /**
  * The safe Core device identifier, when the caller is a paired display.
@@ -57,6 +59,7 @@ export function registerViewerRoutes(
   http: ModuleHttpRouteRegistrar,
   playback: () => SignagePlaybackService,
   registry: () => SignageProviderRegistry,
+  store: ViewerRouteStore = databaseStore,
 ): void {
   http.route({
     method: "POST",
diff --git a/tests/browser/display-contract.test.ts b/tests/browser/display-contract.test.ts
new file mode 100644
index 0000000..09e3c2a
--- /dev/null
+++ b/tests/browser/display-contract.test.ts
@@ -0,0 +1,223 @@
+// @vitest-environment happy-dom
+import type { ModuleRouteDefinitionInput, ModuleRouteHandlerContext } from "@ekklesia/module-sdk";
+import { beforeEach, describe, expect, it } from "vitest";
+import { createMemoryMediaCache, createMemoryStore } from "../../src/browser/storage.js";
+import { createDisplayController } from "../../src/display/controller.js";
+import { ingestNativeImage } from "../../src/server/images/service.js";
+import {
+  createPlaybackService,
+  type SignagePlaybackService,
+} from "../../src/server/playback/service.js";
+import { createNativeImageProvider } from "../../src/server/providers/native-provider.js";
+import {
+  createProviderRegistry,
+  type SignageProviderRegistry,
+} from "../../src/server/providers/registry.js";
+import { registerViewerRoutes } from "../../src/server/routes/viewers.js";
+import { addSlot, createTimeline } from "../../src/server/timelines/service.js";
+import { SIGNAGE_NATIVE_PROVIDER_ID } from "../../src/shared/native-images.js";
+import { SIGNAGE_DEFAULT_SETTINGS } from "../../src/shared/settings.js";
+import { createFakeStorage, type FakeStorage } from "../support/fake-storage.js";
+import { makeImage } from "../support/images.js";
+import { createMemorySignageStore, type MemorySignageStore } from "../support/memory-store.js";
+import { fakeConfiguration, fakeHandlerContext } from "../support/module-harness.js";
+import { createFakeHost, type FakeHost } from "./support/fake-host.js";
+
+/**
+ * The display and the server, speaking to each other for real.
+ *
+ * Every other display test answers the controller with a hand-written stub, and every server test
+ * calls the service with a hand-written object. Each side was therefore only ever tested against its
+ * own idea of the other, and in production every heartbeat a display sent was refused with a 400:
+ * the display sent `null` for an absent value and the route accepted only a missing field.
+ *
+ * Here the **real** display controller's requests are answered by the **real** viewer routes, over
+ * Signage's in-memory store. A request one side builds and the other refuses fails this suite.
+ */
+
+const START = Date.parse("2026-10-05T15:00:00.000Z");
+const DEVICE_ID = "6f1c2d3e-4b5a-4c6d-8e7f-901a2b3c4d5e";
+
+interface RouteCall {
+  readonly method: string;
+  readonly path: string;
+  readonly status: number;
+  readonly code: string | undefined;
+}
+
+/** Matches a relative module path against the routes the module registered, and runs the handler
+ * exactly as the platform would: one Request, one handler context. */
+function routeTable(routes: readonly ModuleRouteDefinitionInput[]) {
+  return routes.map((route) => {
+    const names: string[] = [];
+    const pattern = new RegExp(
+      `^${route.path.replace(/:(\w+)/gu, (_match, name: string) => {
+        names.push(name);
+        return "([^/]+)";
+      })}$`,
+      "u",
+    );
+    return { names, pattern, route };
+  });
+}
+
+describe("display ↔ server contract", () => {
+  let clock: number;
+  let counter: number;
+  let store: MemorySignageStore;
+  let storage: FakeStorage;
+  let registry: SignageProviderRegistry;
+  let playback: SignagePlaybackService;
+  let calls: RouteCall[];
+  let host: FakeHost;
+  let root: HTMLElement;
+
+  const newId = () => {
+    counter += 1;
+    return `00000000-0000-4000-8000-${String(counter).padStart(12, "0")}`;
+  };
+
+  beforeEach(async () => {
+    clock = START;
+    counter = 0;
+    calls = [];
+    store = createMemorySignageStore({ now: () => new Date(clock) });
+    storage = createFakeStorage({ now: () => new Date(clock) });
+    registry = createProviderRegistry({ now: () => new Date(clock), timeoutMs: 1000 });
+    registry.capability.register(
+      createNativeImageProvider({ defaultDurationMs: 7000, now: () => new Date(clock), store })
+        .registration,
+    );
+    playback = createPlaybackService({
+      newId,
+      now: () => clock,
+      registry,
+      setTimer: () => undefined,
+      settings: SIGNAGE_DEFAULT_SETTINGS,
+      store,
+    });
+    await playback.ensureDefaultGroup();
+
+    // One native image on a pinned slot: the most ordinary thing a wall shows, and exactly the case
+    // whose heartbeat carries no contribution id.
+    const image = await ingestNativeImage({
+      bytes: await makeImage({ format: "png", height: 1080, width: 1920 }),
+      displayName: "Welcome",
+      expiresAt: null,
+      filename: "welcome.png",
+      storage,
+      store,
+    });
+    const timeline = await createTimeline({ name: "Lobby", newId, store });
+    await addSlot({
+      newId,
+      pinned: { itemId: image.image.id, providerId: SIGNAGE_NATIVE_PROVIDER_ID },
+      store,
+      timelineId: timeline.timeline.id,
+    });
+    const groups = await store.listChannelGroups();
+    await playback.activate(groups[0]?.group.id as string, timeline.timeline.id, "now");
+
+    const registered: ModuleRouteDefinitionInput[] = [];
+    registerViewerRoutes(
+      { route: (definition) => registered.push(definition) },
+      () => playback,
+      () => registry,
+      () => store,
+    );
+    const table = routeTable(registered);
+
+    host = createFakeHost({
+      permissions: ["signage.display.view"],
+      principal: { deviceId: DEVICE_ID, kind: "device" } as never,
+      routes: [],
+    });
+    Object.assign(host.context.api, {
+      async request(path: string, init?: RequestInit): Promise<Response> {
+        const method = (init?.method ?? "GET").toUpperCase();
+        const relative = `/${path.replace(/^\//u, "").split("?")[0]}`;
+        for (const { names, pattern, route } of table) {
+          const match = pattern.exec(relative);
+          if (route.method !== method || !match) {
+            continue;
+          }
+          const params = Object.fromEntries(names.map((name, index) => [name, match[index + 1]]));
+          const context: ModuleRouteHandlerContext = fakeHandlerContext({
+            config: fakeConfiguration({}),
+            params: Object.freeze(params) as never,
+            principal: { deviceId: DEVICE_ID, kind: "device" } as never,
+            storage,
+          });
+          const request = new Request(
+            `https://api.example.test/api/v1/modules/signage${relative}`,
+            {
+              ...(init?.body === undefined ? {} : { body: init.body }),
+              headers: init?.headers ?? {},
+              method,
+            },
+          );
+          const response = await route.handler(request, context);
+          const body = (await response
+            .clone()
+            .json()
+            .catch(() => null)) as { code?: string } | null;
+          calls.push({ code: body?.code, method, path: route.path, status: response.status });
+          return response;
+        }
+        throw new Error(
+          `The display asked for ${method} ${relative}, which Signage does not serve.`,
+        );
+      },
+    });
+
+    root = document.createElement("div");
+    document.body.append(root);
+  });
+
+  async function settle(): Promise<void> {
+    for (let index = 0; index < 20; index += 1) {
+      await new Promise((resolve) => setTimeout(resolve, 0));
+    }
+  }
+
+  function controller() {
+    return createDisplayController({
+      context: host.context,
+      decodeImage: () => Promise.resolve(),
+      media: createMemoryMediaCache(),
+      newViewerKey: () => "kiosk-contract-0001",
+      now: () => clock,
+      root,
+      store: createMemoryStore(),
+    });
+  }
+
+  it("the server accepts every request the display sends", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    await display.stop();
+
+    const sent = calls.map((call) => `${call.method} ${call.path}`);
+    expect(sent).toContain("POST /viewers/session");
+    expect(sent).toContain("GET /channels/:channelId/display-state");
+    expect(sent).toContain("POST /viewers/:viewerId/heartbeat");
+    // Not one refusal, of any kind, for anything the display built itself.
+    expect(calls.filter((call) => call.status >= 400)).toEqual([]);
+  });
+
+  it("a heartbeat from a screen showing a native image is recorded", async () => {
+    const display = controller();
+    await display.start();
+    await settle();
+    await display.stop();
+
+    const [viewer] = await store.listViewers();
+    expect(viewer).toBeDefined();
+    // What the display reported is what the roster now holds — which is only true if the server
+    // took the heartbeat, rather than refusing it and leaving the session's values in place.
+    expect(viewer?.reportedSelectionId).not.toBeNull();
+    expect(viewer?.reportedProviderId).toBe(SIGNAGE_NATIVE_PROVIDER_ID);
+    expect(viewer?.reportedContributionId).toBeNull();
+  });
+});