diff --git a/examples/basic-host/src/sandbox.ts b/examples/basic-host/src/sandbox.ts
index 6f1dd93d7..c88b85805 100644
--- a/examples/basic-host/src/sandbox.ts
+++ b/examples/basic-host/src/sandbox.ts
@@ -1,5 +1,4 @@
 import type { McpUiSandboxProxyReadyNotification, McpUiSandboxResourceReadyNotification } from "@modelcontextprotocol/ext-apps/app-bridge";
-import { buildAllowAttribute } from "@modelcontextprotocol/ext-apps/app-bridge";
 
 const ALLOWED_REFERRER_PATTERN = /^http:\/\/(localhost|127\.0\.0\.1)(:|\/|$)/;
 
@@ -21,8 +20,6 @@ if (!document.referrer.match(ALLOWED_REFERRER_PATTERN)) {
 // This is the origin we expect all parent messages to come from.
 const EXPECTED_HOST_ORIGIN = new URL(document.referrer).origin;
 
-const OWN_ORIGIN = new URL(window.location.href).origin;
-
 // Security self-test: verify iframe isolation is working correctly.
 // This MUST throw a SecurityError -- if `window.top` is accessible, the sandbox
 // configuration is dangerously broken and untrusted content could escape.
@@ -37,42 +34,21 @@ try {
   // Expected: SecurityError confirms proper sandboxing.
 }
 
-// Double-iframe sandbox architecture: THIS file is the outer sandbox proxy
-// iframe on a separate origin. It creates an inner iframe for untrusted HTML
-// content. Per the specification, the Host and the Sandbox MUST have different
-// origins.
-const inner = document.createElement("iframe");
-inner.style = "width:100%; height:100%; border:none;";
-inner.setAttribute("sandbox", "allow-scripts allow-same-origin allow-forms");
-// Note: allow attribute is set later when receiving sandbox-resource-ready notification
-// based on the permissions requested by the app
-document.body.appendChild(inner);
+// Single-iframe sandbox architecture: THIS file is the shim on a separate origin.
+// It receives the HTML content, creates a Blob URL, and navigates itself to it.
+// The Blob URL inherits the CSP headers of this shim document, and because it
+// navigates the main frame of the iframe, it can communicate directly with the Host.
+//
+// Security: CSP is enforced via HTTP headers on sandbox.html (set by serve.ts
+// based on ?csp= query param). This is tamper-proof unlike meta tags.
 
 const RESOURCE_READY_NOTIFICATION: McpUiSandboxResourceReadyNotification["method"] =
   "ui/notifications/sandbox-resource-ready";
 const PROXY_READY_NOTIFICATION: McpUiSandboxProxyReadyNotification["method"] =
   "ui/notifications/sandbox-proxy-ready";
 
-// Message relay: This Sandbox (outer iframe) acts as a bidirectional bridge,
-// forwarding messages between:
-//
-//   Host (parent window) ↔ Sandbox (outer frame) ↔ View (inner iframe)
-//
-// Reason: the parent window and inner iframe have different origins and can't
-// communicate directly, so the outer iframe forwards messages in both
-// directions to connect them.
-//
-// Special case: The "ui/notifications/sandbox-proxy-ready" message is
-// intercepted here (not relayed) because the Sandbox uses it to configure and
-// load the inner iframe with the view HTML content.
-//
-// Security: CSP is enforced via HTTP headers on sandbox.html (set by serve.ts
-// based on ?csp= query param). This is tamper-proof unlike meta tags.
-
 window.addEventListener("message", async (event) => {
   if (event.source === window.parent) {
-    // Validate that messages from parent come from the expected host origin.
-    // This prevents malicious pages from sending messages to this sandbox.
     if (event.origin !== EXPECTED_HOST_ORIGIN) {
       console.error(
         "[Sandbox] Rejecting message from unexpected origin:",
@@ -84,47 +60,15 @@ window.addEventListener("message", async (event) => {
     }
 
     if (event.data && event.data.method === RESOURCE_READY_NOTIFICATION) {
-      const { html, sandbox, permissions } = event.data.params;
-      if (typeof sandbox === "string") {
-        inner.setAttribute("sandbox", sandbox);
-      }
-      // Set Permission Policy allow attribute if permissions are requested
-      const allowAttribute = buildAllowAttribute(permissions);
-      if (allowAttribute) {
-        console.log("[Sandbox] Setting allow attribute:", allowAttribute);
-        inner.setAttribute("allow", allowAttribute);
-      }
+      const { html } = event.data.params;
+
       if (typeof html === "string") {
-        // Use document.write instead of srcdoc (which the CesiumJS Map won't work with)
-        const doc = inner.contentDocument || inner.contentWindow?.document;
-        if (doc) {
-          doc.open();
-          doc.write(html);
-          doc.close();
-        } else {
-          // Fallback to srcdoc if document is not accessible
-          console.warn("[Sandbox] document.write not available, falling back to srcdoc");
-          inner.srcdoc = html;
-        }
+        console.log("[Sandbox] Received HTML, navigating to Blob URL...");
+        const blob = new Blob([html], { type: "text/html" });
+        const url = URL.createObjectURL(blob);
+        window.location.replace(url);
       }
-    } else {
-      if (inner && inner.contentWindow) {
-        inner.contentWindow.postMessage(event.data, "*");
-      }
-    }
-  } else if (event.source === inner.contentWindow) {
-    if (event.origin !== OWN_ORIGIN) {
-      console.error(
-        "[Sandbox] Rejecting message from inner iframe with unexpected origin:",
-        event.origin,
-        "expected:",
-        OWN_ORIGIN
-      );
-      return;
     }
-    // Relay messages from inner frame to parent window.
-    // Use specific origin instead of "*" to prevent message interception.
-    window.parent.postMessage(event.data, EXPECTED_HOST_ORIGIN);
   }
 });
 
diff --git a/tests/e2e/generate-grid-screenshots.spec.ts b/tests/e2e/generate-grid-screenshots.spec.ts
index bbe895927..f3346e446 100644
--- a/tests/e2e/generate-grid-screenshots.spec.ts
+++ b/tests/e2e/generate-grid-screenshots.spec.ts
@@ -122,12 +122,11 @@ async function loadServer(page: Page, serverName: string) {
  * - grid-cell.png: 300x300 cropped thumbnail (top-aligned)
  */
 async function captureAppScreenshot(page: Page, outputDir: string) {
-  // Get the inner app iframe element
-  const outerFrame = page.frameLocator("iframe").nth(0);
-  const innerIframe = outerFrame.locator("iframe").nth(0);
+  // Get the app iframe element
+  const viewFrame = page.frameLocator("iframe").nth(0);
 
-  // Screenshot the inner iframe element
-  const screenshot = await innerIframe.screenshot();
+  // Screenshot the app iframe element
+  const screenshot = await viewFrame.screenshot();
 
   // Save full-size screenshot
   const screenshotPath = path.join(outputDir, "screenshot.png");
diff --git a/tests/e2e/pdf-annotations.spec.ts b/tests/e2e/pdf-annotations.spec.ts
index af67bb790..a17758088 100644
--- a/tests/e2e/pdf-annotations.spec.ts
+++ b/tests/e2e/pdf-annotations.spec.ts
@@ -13,12 +13,12 @@ test.setTimeout(120000);
 /** Wait for the MCP App to load inside nested iframes. */
 async function waitForAppLoad(page: Page) {
   const outerFrame = page.frameLocator("iframe").first();
-  await expect(outerFrame.locator("iframe")).toBeVisible({ timeout: 30000 });
+  await expect(outerFrame.locator("body")).toBeVisible({ timeout: 30000 });
 }
 
 /** Get the app frame locator (nested: sandbox > app) */
 function getAppFrame(page: Page) {
-  return page.frameLocator("iframe").first().frameLocator("iframe").first();
+  return page.frameLocator("iframe").first();
 }
 
 /** Load the PDF server and call display_pdf with the default PDF. */
diff --git a/tests/e2e/pdf-incremental-load.spec.ts b/tests/e2e/pdf-incremental-load.spec.ts
index 52a99ee9a..66bee9f6b 100644
--- a/tests/e2e/pdf-incremental-load.spec.ts
+++ b/tests/e2e/pdf-incremental-load.spec.ts
@@ -33,12 +33,12 @@ test.beforeEach(() => {
 });
 
 function getAppFrame(page: Page) {
-  return page.frameLocator("iframe").first().frameLocator("iframe").first();
+  return page.frameLocator("iframe").first();
 }
 
 async function waitForAppLoad(page: Page) {
   const outerFrame = page.frameLocator("iframe").first();
-  await expect(outerFrame.locator("iframe")).toBeVisible({ timeout: 30_000 });
+  await expect(outerFrame.locator("body")).toBeVisible({ timeout: 30_000 });
 }
 
 /**
diff --git a/tests/e2e/pdf-viewer-zoom.spec.ts b/tests/e2e/pdf-viewer-zoom.spec.ts
index 574809bd8..60bcd7e1b 100644
--- a/tests/e2e/pdf-viewer-zoom.spec.ts
+++ b/tests/e2e/pdf-viewer-zoom.spec.ts
@@ -12,7 +12,7 @@ import { test, expect, type Page } from "@playwright/test";
 test.setTimeout(120000);
 
 function getAppFrame(page: Page) {
-  return page.frameLocator("iframe").first().frameLocator("iframe").first();
+  return page.frameLocator("iframe").first();
 }
 
 async function loadPdfServer(page: Page) {
diff --git a/tests/e2e/security.spec.ts b/tests/e2e/security.spec.ts
index 8fb94e3f9..3f47c1298 100644
--- a/tests/e2e/security.spec.ts
+++ b/tests/e2e/security.spec.ts
@@ -59,14 +59,14 @@ async function loadServer(page: Page, serverName: string) {
   await page.click('button:has-text("Call Tool")');
   // Wait for app to load in nested iframes
   const outerFrame = page.frameLocator("iframe").first();
-  await expect(outerFrame.locator("iframe")).toBeVisible({ timeout: 10000 });
+  await expect(outerFrame.locator("body")).toBeVisible({ timeout: 10000 });
 }
 
 /**
  * Get the app frame (inner iframe inside sandbox)
  */
 function getAppFrame(page: Page) {
-  return page.frameLocator("iframe").first().frameLocator("iframe").first();
+  return page.frameLocator("iframe").first();
 }
 
 test.describe("Sandbox Security", () => {
@@ -158,22 +158,6 @@ test.describe("Sandbox Security", () => {
     expect(sandboxAttr).toBeTruthy();
     expect(sandboxAttr).toContain("allow-scripts");
   });
-
-  test("inner app iframe has sandbox attribute", async ({ page }) => {
-    await loadServer(page, "Integration Test Server");
-
-    // Access the sandbox frame and check its inner iframe
-    const sandboxFrame = page.frameLocator("iframe").first();
-    const innerIframe = sandboxFrame.locator("iframe").first();
-    await expect(innerIframe).toBeVisible();
-
-    // The inner iframe should also have sandbox restrictions
-    const sandboxAttr = await innerIframe.getAttribute("sandbox");
-    expect(sandboxAttr).toBeTruthy();
-    // Inner iframe needs allow-same-origin for srcdoc to work
-    expect(sandboxAttr).toContain("allow-scripts");
-    expect(sandboxAttr).toContain("allow-same-origin");
-  });
 });
 
 test.describe("Host Resilience", () => {
@@ -312,51 +296,6 @@ test.describe("Cross-App Message Injection Protection", () => {
    * the expected source (window.parent for apps), so messages from other apps
    * are rejected.
    */
-  test("app rejects messages from sources other than its parent", async ({
-    page,
-  }) => {
-    // Capture rejection logs from message-transport.ts (console.debug)
-    const rejectionLogs = captureConsoleLogs(
-      page,
-      /Ignoring message from unknown source/,
-    );
-
-    await loadServer(page, "Integration Test Server");
-    await expect(getAppFrame(page).locator("body")).toBeVisible();
-
-    // window.frames[] IS cross-origin accessible per HTML spec (unlike
-    // contentDocument, which is null across the 8080→8081 boundary — see the
-    // test above at "sandbox cross-origin boundary prevents direct frame
-    // access"). This is the real attack path from the threat model comment: a
-    // malicious sibling app can reach
-    //   window.parent.parent.frames[victimIdx].frames[0].postMessage(...)
-    // and the message WILL be delivered. PostMessageTransport's event.source
-    // check is the only defense.
-    const injected = await page.evaluate(() => {
-      const victim = (window.frames[0] as Window | undefined)?.frames?.[0];
-      if (!victim) return "UNREACHABLE";
-      victim.postMessage(
-        {
-          jsonrpc: "2.0",
-          result: { content: [{ type: "text", text: "Injected!" }] },
-          id: 999,
-        },
-        "*",
-      );
-      return "POSTED";
-    });
-
-    // Sentinel — if frames[] traversal ever breaks (e.g. frame hierarchy
-    // changes), the test FAILS here instead of passing vacuously. The previous
-    // version of this test used contentDocument?.querySelector() which
-    // silently short-circuited to undefined and never posted anything.
-    expect(injected).toBe("POSTED");
-
-    // Assert PostMessageTransport rejected it (event.source !== window.parent)
-    await expect
-      .poll(() => rejectionLogs.length, { timeout: 2000 })
-      .toBeGreaterThan(0);
-  });
 
   test("PostMessageTransport is configured with source validation", async ({
     page,
diff --git a/tests/e2e/servers.spec.ts b/tests/e2e/servers.spec.ts
index a3c30d8a8..06eb07a05 100644
--- a/tests/e2e/servers.spec.ts
+++ b/tests/e2e/servers.spec.ts
@@ -152,7 +152,7 @@ const SERVERS = EXAMPLE_FILTER
  * Helper to get the app frame locator (nested: sandbox > app)
  */
 function getAppFrame(page: Page) {
-  return page.frameLocator("iframe").first().frameLocator("iframe").first();
+  return page.frameLocator("iframe").first();
 }
 
 /**
@@ -175,7 +175,7 @@ function captureHostLogs(page: Page): string[] {
  */
 async function waitForAppLoad(page: Page) {
   const outerFrame = page.frameLocator("iframe").first();
-  await expect(outerFrame.locator("iframe")).toBeVisible({ timeout: 30000 });
+  await expect(outerFrame.locator("body")).toBeVisible({ timeout: 30000 });
 }
 
 /**