diff --git a/runner/AGENTS.md b/runner/AGENTS.md
index 49f29a64..5f17b89b 100644
--- a/runner/AGENTS.md
+++ b/runner/AGENTS.md
@@ -111,6 +111,12 @@ What a green E2E run does and does not prove:
 - **Interaction states need a real pointer and `getComputedStyle`** — see
   [ADR-0026](docs/adr/0026-shell-styling-inline-vs-stylesheet.md). A synthetic `mouseover` does not
   fire CSS `:hover`, and a screenshot cannot tell a subtle live hover from a dead one.
+- **Backend-bound specs self-gate on `E2E_BASE_URL`** (`share-view.spec.ts` and friends):
+  `vite preview` has no `/api`, `/d` or `/embed` routes, so they skip unless pointed at a
+  deployment. `e2e/share-view.spec.ts` additionally depends on a **permanent fixture demo**
+  (`FIXTURE_ID` in the spec — currently `r-react-18-0-0`). Never revoke it; if it is lost,
+  mint a replacement titled "E2E fixture — do not revoke" from any signed-in session and
+  update the constant.
 
 ## Build & deploy
 
diff --git a/runner/e2e/share-view.spec.ts b/runner/e2e/share-view.spec.ts
new file mode 100644
index 00000000..fd53054f
--- /dev/null
+++ b/runner/e2e/share-view.spec.ts
@@ -0,0 +1,83 @@
+import { test, expect } from "@playwright/test";
+
+// The share viewer, read from the outside (DEV-2203).
+//
+// `/d/:id` is what a client actually receives when someone shares a demo: a
+// prebuilt static page served from R2 by the API worker, framed by `/share`
+// and `/embed`. Everything the app-side specs prove happens *before* this
+// point — `authed-actions.spec.ts` stubs `/api/demos` and never leaves the
+// SPA. Nothing exercised the deployed contract: the redirect, the frame
+// policies that make `/embed` docs-only, and the promise that a share never
+// leaks its source snapshot.
+//
+// Read-only on purpose. These tests hit a permanent fixture demo, so they run
+// against production with zero containers, zero writes and zero flake budget —
+// cheap enough for the post-deploy smoke. The authed write path (create →
+// build → view → revoke) is `share-create-live.spec.ts`, which needs a real
+// broker token and is gated separately.
+//
+//   E2E_BASE_URL=https://demos.handsontable.com pnpm e2e e2e/share-view.spec.ts
+//
+// FIXTURE_ID names a demo that must keep existing. `r-react-18-0-0` is the
+// IT-540 launch-verification share; if it is ever revoked, mint a replacement
+// titled "E2E fixture — do not revoke" from any signed-in session and update
+// the constant (see AGENTS.md § E2E).
+
+const FIXTURE_ID = "r-react-18-0-0";
+
+test.describe("share viewer — /d and /embed", () => {
+  test.skip(!process.env.E2E_BASE_URL, "needs a deployed API origin — vite preview has no /api or /d routes");
+
+  test("the built demo renders for an anonymous viewer", { tag: "@smoke" }, async ({ page }) => {
+    // The static page is the demo itself — no editor shell, no preview iframe.
+    await page.goto(`/d/${FIXTURE_ID}/`);
+    await expect
+      .poll(async () => page.locator(".handsontable .htCore td").count(), { timeout: 30_000 })
+      .toBeGreaterThan(0);
+  });
+
+  test("the slashless URL redirects permanently instead of serving a copy", async ({ request, baseURL }) => {
+    const res = await request.get(`${baseURL}/d/${FIXTURE_ID}`, { maxRedirects: 0 });
+    expect(res.status()).toBe(308);
+    expect(res.headers()["location"]).toContain(`/d/${FIXTURE_ID}/`);
+  });
+
+  test("the viewer is frame-locked to itself", async ({ request, baseURL }) => {
+    const res = await request.get(`${baseURL}/d/${FIXTURE_ID}/`);
+    expect(res.status()).toBe(200);
+    expect(res.headers()["content-security-policy"]).toContain("frame-ancestors 'self'");
+    expect(res.headers()["x-frame-options"]).toBe("SAMEORIGIN");
+    // HTML must revalidate so a re-shared id can never serve a stale build.
+    expect(res.headers()["cache-control"]).toContain("must-revalidate");
+  });
+
+  test("the embed is docs-only: handsontable.com may frame it, nobody may fetch it cross-origin", async ({
+    request,
+    baseURL,
+  }) => {
+    const res = await request.get(`${baseURL}/embed/${FIXTURE_ID}/`);
+    expect(res.status()).toBe(200);
+    const csp = res.headers()["content-security-policy"] ?? "";
+    expect(csp).toContain("frame-ancestors");
+    expect(csp).toContain("https://handsontable.com");
+    expect(csp).toContain("https://*.handsontable.com");
+    // `frame-ancestors` with an allowlist is incompatible with X-Frame-Options —
+    // the worker must not send both, or the stricter header wins in old engines.
+    expect(res.headers()["x-frame-options"]).toBeUndefined();
+    // /embed is deliberately outside the cors() wrap: framing is allowed,
+    // fetching from another origin is not.
+    expect(res.headers()["access-control-allow-origin"]).toBeUndefined();
+  });
+
+  test("the source snapshot behind a share is never served", async ({ request, baseURL }) => {
+    // __source.json sits in R2 next to the built assets; the worker refuses
+    // any `__`-prefixed segment so the un-built files cannot leak.
+    const res = await request.get(`${baseURL}/d/${FIXTURE_ID}/__source.json`);
+    expect(res.status()).toBe(404);
+  });
+
+  test("an unknown id is a 404, not an error page with a 200 on it", async ({ request, baseURL }) => {
+    const res = await request.get(`${baseURL}/d/zzzznope00/`);
+    expect(res.status()).toBe(404);
+  });
+});