diff --git a/apps/dev-playground/server/smoke.test.ts b/apps/dev-playground/server/smoke.test.ts
new file mode 100644
index 000000000..b1915f95c
--- /dev/null
+++ b/apps/dev-playground/server/smoke.test.ts
@@ -0,0 +1,121 @@
+import { createTestApp } from "@databricks/appkit/testing";
+import { describe, expect, test, vi } from "vitest";
+
+import { lakebaseExamples } from "./lakebase-examples-plugin";
+import { reconnect } from "./reconnect-plugin";
+import { telemetryExamples } from "./telemetry-example-plugin";
+
+/**
+ * Smoke tests for the playground's own server plugins.
+ *
+ * `tests/` holds Playwright specs that fake `/api` responses at the browser
+ * boundary (`page.route` + `fulfill`), so the Express server never runs there.
+ * These cover the other side: the plugins boot and answer over real HTTP with
+ * the Databricks data plane faked by the harness — no workspace, no
+ * credentials, no network.
+ */
+
+/**
+ * Read one SSE payload, then hang up.
+ *
+ * `expectStream` buffers a source to completion, and the reconnect stream is
+ * five messages three seconds apart — so asserting through it would cost ~12s
+ * for a smoke test.
+ */
+async function firstSSEPayload(res: Response): Promise<unknown> {
+  if (!res.body) throw new Error("expected a streaming body, got none");
+  const reader = res.body.getReader();
+  const decoder = new TextDecoder();
+  let buffered = "";
+  try {
+    while (!buffered.includes("\n\n")) {
+      const { value, done } = await reader.read();
+      if (done) break;
+      buffered += decoder.decode(value, { stream: true });
+    }
+    const data = buffered.split("\n").find((line) => line.startsWith("data:"));
+    return data ? JSON.parse(data.slice("data:".length).trim()) : undefined;
+  } finally {
+    await reader.cancel();
+  }
+}
+
+describe("dev-playground server plugins", () => {
+  test("all three boot together and register under their manifest names", async () => {
+    await using app = await createTestApp({
+      plugins: [reconnect(), telemetryExamples(), lakebaseExamples()],
+    });
+
+    expect(app.plugins.reconnect).toBeDefined();
+    expect(app.plugins["telemetry-examples"]).toBeDefined();
+    expect(app.plugins["lakebase-examples"]).toBeDefined();
+  });
+
+  test("GET /api/reconnect answers", async () => {
+    await using app = await createTestApp({ plugins: [reconnect()] });
+
+    const res = await app.get("/api/reconnect");
+
+    expect(res.status).toBe(200);
+    await expect(res.json()).resolves.toEqual({ message: "Reconnected" });
+  });
+
+  test("the reconnect stream opens as SSE and emits its first message", async () => {
+    await using app = await createTestApp({ plugins: [reconnect()] });
+
+    const res = await app.get("/api/reconnect/stream?sessionId=smoke");
+
+    expect(res.status).toBe(200);
+    expect(res.headers.get("content-type")).toContain("text/event-stream");
+    await expect(firstSSEPayload(res)).resolves.toMatchObject({
+      type: "message",
+      count: 1,
+      total: 5,
+      content: "Message 1 of 5",
+    });
+  });
+
+  test("POST /api/telemetry-examples/combined threads the userId through every span", async () => {
+    // This route really calls `fetch("https://example.com")` (its
+    // external-api span). Left alone the suite would need the internet, so
+    // non-loopback requests are stubbed — loopback must pass through, because
+    // that is how the harness reaches its own server.
+    const realFetch = globalThis.fetch;
+    vi.stubGlobal("fetch", (input: RequestInfo | URL, init?: RequestInit) => {
+      const url = String(input instanceof Request ? input.url : input);
+      return /127\.0\.0\.1|localhost/.test(url)
+        ? realFetch(input, init)
+        : Promise.resolve(new Response("stubbed", { status: 200 }));
+    });
+
+    try {
+      await using app = await createTestApp({ plugins: [telemetryExamples()] });
+
+      const res = await app.post("/api/telemetry-examples/combined", {
+        body: { userId: "smoke-user" },
+      });
+
+      expect(res.status).toBe(200);
+      // A 200 means the whole nested-span body ran against the real
+      // TelemetryProvider: tracer, meter, and logger.
+      await expect(res.json()).resolves.toMatchObject({
+        success: true,
+        result: { userId: "smoke-user" },
+      });
+    } finally {
+      vi.unstubAllGlobals();
+    }
+  });
+
+  test("lakebase-examples degrades to no routes when Lakebase is unconfigured", async () => {
+    // Its setup() and injectRoutes() both bail on missing PGHOST/LAKEBASE_ENDPOINT.
+    // The app must still boot; the routes must simply be absent.
+    await using app = await createTestApp({
+      plugins: [lakebaseExamples()],
+      env: { PGHOST: "", LAKEBASE_ENDPOINT: "" },
+    });
+
+    expect(app.plugins["lakebase-examples"]).toBeDefined();
+    expect((await app.get("/api/lakebase-examples/raw")).status).toBe(404);
+  });
+});
diff --git a/apps/dev-playground/server/testing-kit.integration.test.ts b/apps/dev-playground/server/testing-kit.integration.test.ts
new file mode 100644
index 000000000..ccf39a5f2
--- /dev/null
+++ b/apps/dev-playground/server/testing-kit.integration.test.ts
@@ -0,0 +1,239 @@
+import { ApiError, files, genie } from "@databricks/appkit";
+import {
+  createApiError,
+  createMockRequest,
+  createMockResponse,
+  createMockWorkspaceClient,
+  createTestApp,
+  createTestPlugin,
+  createTestPluginContext,
+  expectStream,
+  getMock,
+  resetTestCache,
+  useServiceContextMock,
+  useTestApp,
+  useTestCache,
+  withEnv,
+} from "@databricks/appkit/testing";
+import { describe, expect, test, vi } from "vitest";
+
+import { reconnect } from "./reconnect-plugin";
+
+/**
+ * One worked example per `@databricks/appkit/testing` helper. `smoke.test.ts`
+ * covers that the plugins boot and answer; this covers the kit itself.
+ */
+
+// Dummy values: the strict validator only checks the vars are present.
+const FILES_ENV = {
+  DATABRICKS_VOLUME_FILES: "/Volumes/main/default/vol",
+  DATABRICKS_VOLUME_REPORTS: "/Volumes/main/default/vol",
+};
+const filesPlugin = () =>
+  files({ volumes: { reports: { policy: files.policy.allowAll() } } });
+
+describe("getMock — assert the workspace-client call a route made", () => {
+  test("GET /api/files/reports/metadata calls files.getMetadata", async () => {
+    await using app = await createTestApp({
+      plugins: [filesPlugin()],
+      env: FILES_ENV,
+      responses: {
+        "files.getMetadata": {
+          "content-length": "42",
+          "content-type": "text/plain",
+        },
+      },
+    });
+
+    const res = await app.get("/api/files/reports/metadata?path=report.csv");
+
+    expect(res.status).toBe(200);
+    expect(getMock(app.client, "files.getMetadata")).toHaveBeenCalled();
+  });
+});
+
+describe("createMockWorkspaceClient — build the fake client yourself", () => {
+  test("declared paths answer; undeclared ones resolve undefined", async () => {
+    const client = createMockWorkspaceClient({
+      responses: { "jobs.getRun": { state: "TERMINATED" } },
+    });
+
+    await expect(client.jobs.getRun({ run_id: 1 })).resolves.toEqual({
+      state: "TERMINATED",
+    });
+    expect(getMock(client, "jobs.getRun")).toHaveBeenCalledWith({ run_id: 1 });
+    await expect(
+      client.genie.getMessage({ id: "m-1" }),
+    ).resolves.toBeUndefined();
+  });
+});
+
+describe("createApiError — simulate a typed workspace error", () => {
+  test("produces a genuine ApiError instance", () => {
+    const err = createApiError({
+      statusCode: 404,
+      message: "No such directory",
+      errorCode: "NOT_FOUND",
+    });
+
+    expect(err).toBeInstanceOf(ApiError);
+    expect(err.statusCode).toBe(404);
+  });
+
+  test("seeded as a rejection, a route surfaces the failure instead of crashing", async () => {
+    await using app = await createTestApp({
+      plugins: [filesPlugin()],
+      env: FILES_ENV,
+      responses: {
+        "files.getMetadata": () => {
+          throw createApiError({
+            statusCode: 404,
+            message: "No such file",
+            errorCode: "NOT_FOUND",
+          });
+        },
+      },
+    });
+
+    const res = await app.get("/api/files/reports/metadata?path=report.csv");
+
+    expect(res.ok).toBe(false);
+  });
+});
+
+describe("useTestApp — a fresh app per test, closed for you", () => {
+  const app = useTestApp({ plugins: [reconnect()] });
+
+  test("the first test gets its own app", async () => {
+    expect((await app.current.get("/api/reconnect")).status).toBe(200);
+  });
+
+  test("the second gets a fresh one — no close() to forget", async () => {
+    await expect(
+      app.current.get("/api/reconnect").then((r) => r.json()),
+    ).resolves.toEqual({ message: "Reconnected" });
+  });
+});
+
+describe("withEnv — set env for a block, restored after", () => {
+  test("the variable is present inside the block and gone after", async () => {
+    expect(process.env.DEMO_FLAG).toBeUndefined();
+
+    await withEnv({ DEMO_FLAG: "on" }, async () => {
+      expect(process.env.DEMO_FLAG).toBe("on");
+    });
+
+    expect(process.env.DEMO_FLAG).toBeUndefined();
+  });
+});
+
+describe("useTestCache — assert caching against the real CacheManager", () => {
+  const cache = useTestCache();
+
+  test("generateKey is stable for identical inputs and scoped per user", () => {
+    const parts = ["analytics:query", "top_users"];
+    expect(cache.current.generateKey(parts, "")).toBe(
+      cache.current.generateKey(parts, ""),
+    );
+    expect(cache.current.generateKey(parts, "alice")).not.toBe(
+      cache.current.generateKey(parts, "bob"),
+    );
+  });
+
+  test("a second identical call is a hit; resetTestCache forces a miss", async () => {
+    const work = vi.fn(async () => "value");
+
+    await cache.current.getOrExecute(["report"], work, "");
+    await cache.current.getOrExecute(["report"], work, "");
+    expect(work).toHaveBeenCalledTimes(1); // second served from cache
+
+    await resetTestCache();
+    await cache.current.getOrExecute(["report"], work, "");
+    expect(work).toHaveBeenCalledTimes(2); // cleared, so recomputed
+  });
+});
+
+describe("createTestPlugin — instantiate a factory the way production does", () => {
+  test("applies the config merge and manifest name", () => {
+    const plugin = createTestPlugin(genie, { spaces: { demo: "space-test" } });
+
+    expect(plugin.name).toBe("genie");
+  });
+});
+
+describe("createTestPluginContext — unit-test wiring with no boot, no socket", () => {
+  test("dispatches a cross-plugin tool call on-behalf-of the user", async () => {
+    // No playground plugin dispatches tools hermetically (agents needs a live
+    // model), so drive the context directly with a faked analytics tool.
+    const mock = createTestPluginContext({
+      analytics: { top_users: (args) => [{ user: "alice", args }] },
+    });
+
+    const req = createMockRequest({ obo: { userId: "analyst@example.com" } });
+    const result = await mock.ctx.executeTool(req, "analytics", "top_users", {
+      limit: 5,
+    });
+
+    expect(result).toEqual([{ user: "alice", args: { limit: 5 } }]);
+    expect(mock.toolCalls[0]).toMatchObject({
+      plugin: "analytics",
+      tool: "top_users",
+      asUser: true,
+      userId: "analyst@example.com",
+    });
+  });
+
+  test("a token-less request rejects on the on-behalf-of path", async () => {
+    const mock = createTestPluginContext({
+      analytics: { top_users: () => [] },
+    });
+
+    await expect(
+      mock.ctx.executeTool(createMockRequest(), "analytics", "top_users", {}),
+    ).rejects.toThrow();
+  });
+});
+
+describe("expectStream — assert the ordered events a stream emits", () => {
+  // reconnect's real stream is long-lived, so smoke.test.ts reads it by hand;
+  // expectStream buffers to completion, so demo it on a bounded source.
+  async function* runReport() {
+    yield { type: "warehouse_status", state: "RUNNING" };
+    yield { type: "row", n: 1 };
+    yield { type: "result", rows: 1 };
+  }
+
+  test("toEmit matches an in-order subsequence", async () => {
+    await expectStream(runReport()).toEmit("warehouse_status", "result");
+  });
+
+  test("toEmitExactly matches the full shape, in order", async () => {
+    await expectStream(runReport()).toEmitExactly(
+      "warehouse_status",
+      "row",
+      "result",
+    );
+  });
+
+  test("reads an SSE handler's writes back through createMockResponse", async () => {
+    const res = createMockResponse();
+    res.setHeader("content-type", "text/event-stream");
+    res.write(`event: status\ndata: ${JSON.stringify({ s: "go" })}\n\n`);
+    res.write(`event: result\ndata: ${JSON.stringify({ n: 1 })}\n\n`);
+    res.end();
+
+    await expectStream(res).toEmit("status", "result");
+  });
+});
+
+describe("useServiceContextMock — spy the data-plane singleton in one line", () => {
+  // Spies the ServiceContext singleton directly — the seam beneath the client
+  // createTestApp injects.
+  const ctx = useServiceContextMock();
+
+  test("installs live spies over the service context", () => {
+    expect(ctx.current.getSpy).toBeDefined();
+    expect(ctx.current.createUserContextSpy).toBeDefined();
+    expect(ctx.current.createUserContextSpy).not.toHaveBeenCalled();
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index d466c3a51..38f0c3e06 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -59,6 +59,19 @@ export default defineConfig({
           environment: "node",
         },
       },
+      {
+        plugins: [tsconfigPaths()],
+        test: {
+          name: "dev-playground",
+          root: "./apps/dev-playground",
+          environment: "node",
+          // tests/ holds Playwright specs. Vitest's default `**/*.spec.ts`
+          // glob would collect them and they fail on import with "Playwright
+          // Test did not expect test.describe() to be called here". They run
+          // via `pnpm test:integration`.
+          exclude: ["**/node_modules/**", "**/dist/**", "tests/**"],
+        },
+      },
     ],
   },
 });