diff --git a/src/__tests__/demo.test.ts b/src/__tests__/demo.test.ts
new file mode 100644
index 0000000..e613bfe
--- /dev/null
+++ b/src/__tests__/demo.test.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import express from "express";
+import request from "supertest";
+
+vi.mock("../services/browser.js", () => ({
+  renderPdf: vi.fn(),
+  renderUrlPdf: vi.fn(),
+}));
+
+let app: express.Express;
+
+beforeEach(async () => {
+  vi.clearAllMocks();
+  vi.resetModules();
+
+  const { renderPdf } = await import("../services/browser.js");
+  vi.mocked(renderPdf).mockResolvedValue(Buffer.from("%PDF-1.4 mock"));
+
+  const { demoRouter } = await import("../routes/demo.js");
+  app = express();
+  app.use(express.json({ limit: "500kb" }));
+  app.use("/v1/demo", demoRouter);
+});
+
+describe("POST /v1/demo/html", () => {
+  it("returns 400 for missing html", async () => {
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({});
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 415 for wrong content-type", async () => {
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "text/plain")
+      .send("html=<h1>hi</h1>");
+    expect(res.status).toBe(415);
+  });
+
+  it("returns 503 on QUEUE_FULL", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("QUEUE_FULL"));
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(503);
+  });
+
+  it("returns 504 on PDF_TIMEOUT", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("PDF_TIMEOUT"));
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(504);
+  });
+
+  it("returns 500 on unexpected error", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("something broke"));
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(500);
+    expect(res.body.error).toMatch(/PDF generation failed/);
+  });
+
+  it("returns PDF with watermark on success", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-type"]).toMatch(/application\/pdf/);
+    // Verify watermark was injected into the HTML passed to renderPdf
+    const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+    expect(calledHtml).toContain("DEMO");
+    expect(calledHtml).toContain("docfast.dev");
+  });
+});
+
+describe("POST /v1/demo/markdown", () => {
+  it("returns 400 for missing markdown", async () => {
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({});
+    expect(res.status).toBe(400);
+  });
+
+  it("returns 415 for wrong content-type", async () => {
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "text/plain")
+      .send("markdown=# hi");
+    expect(res.status).toBe(415);
+  });
+
+  it("returns 503 on QUEUE_FULL", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("QUEUE_FULL"));
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello" });
+    expect(res.status).toBe(503);
+  });
+
+  it("returns 504 on PDF_TIMEOUT", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("PDF_TIMEOUT"));
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello" });
+    expect(res.status).toBe(504);
+  });
+
+  it("returns 500 on unexpected error", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    vi.mocked(renderPdf).mockRejectedValue(new Error("something broke"));
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello" });
+    expect(res.status).toBe(500);
+    expect(res.body.error).toMatch(/PDF generation failed/);
+  });
+
+  it("returns PDF with watermark on success", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello World" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-type"]).toMatch(/application\/pdf/);
+    const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+    expect(calledHtml).toContain("DEMO");
+    expect(calledHtml).toContain("docfast.dev");
+  });
+});