diff --git a/apps/server/src/http.test.ts b/apps/server/src/http.test.ts
index 0ee1ca9c186f8bfc23e0160547203f25f1aca9f9..9d3686a46692b59fbb80841b685b828ed08fb9bf 100644
--- a/apps/server/src/http.test.ts
+++ b/apps/server/src/http.test.ts
@@ -18,6 +18,7 @@ import { openMediaFile } from "./assets/MediaFile.ts";
 
 import {
   injectMcpAppCsp,
+  injectMcpAppCookieBootstrap,
   injectMcpAppThemeBootstrap,
   MCP_APP_MAX_HTML_BYTES,
 } from "@t3tools/shared/mcpApp";
@@ -58,7 +59,7 @@ describe("MCP app asset responses", () => {
               method,
             ),
           );
-          const expected = injectMcpAppThemeBootstrap(html);
+          const expected = injectMcpAppCookieBootstrap(injectMcpAppThemeBootstrap(html));
           expect(response.status).toBe(200);
           expect(response.headers.get("content-length")).toBe(
             String(new TextEncoder().encode(expected).byteLength),
diff --git a/apps/server/src/http.ts b/apps/server/src/http.ts
index 1e9c3d714d1cea54e9ca8e59526ed7ae0390b58c..13f54ef352998efc9c9dda091c1304cb120f3b0b 100644
--- a/apps/server/src/http.ts
+++ b/apps/server/src/http.ts
@@ -5,7 +5,7 @@ import {
   EnvironmentHttpApi,
 } from "@t3tools/contracts";
 import { isDevProxiedPath } from "@t3tools/shared/devProxy";
-import { injectMcpAppThemeBootstrap } from "@t3tools/shared/mcpApp";
+import { injectMcpAppCookieBootstrap, injectMcpAppThemeBootstrap } from "@t3tools/shared/mcpApp";
 import { decodeOtlpTraceRecords } from "@t3tools/shared/observability";
 import * as Data from "effect/Data";
 import * as Effect from "effect/Effect";
@@ -185,7 +185,7 @@ export const assetFileResponse = Effect.fn("assetFileResponse")(function* (
       return HttpServerResponse.text("MCP app is too large to preview.", { status: 413 });
     }
     const body = new TextEncoder().encode(
-      injectMcpAppThemeBootstrap(new TextDecoder().decode(bytes)),
+      injectMcpAppCookieBootstrap(injectMcpAppThemeBootstrap(new TextDecoder().decode(bytes))),
     );
     headers["Content-Length"] = String(body.byteLength);
     return method === "HEAD"
diff --git a/packages/shared/src/mcpApp.test.ts b/packages/shared/src/mcpApp.test.ts
index bca145a4fba88dc4ee9daac8fa7377ff3fa68cc5..7dd50690adf389986f44330de3dc1dc797e47162 100644
--- a/packages/shared/src/mcpApp.test.ts
+++ b/packages/shared/src/mcpApp.test.ts
@@ -4,6 +4,7 @@ import { describe, expect, it } from "vite-plus/test";
 import {
   injectMcpAppCsp,
   injectMcpAppThemeBootstrap,
+  injectMcpAppCookieBootstrap,
   mcpAppContentSecurityPolicy,
   mcpAppToolCallableByApp,
   readMcpAppCsp,
@@ -209,3 +210,61 @@ describe("mcpAppFromToolItem", () => {
     expect(output?.t3McpApp?.server).toBe("weather");
   });
 });
+
+describe("MCP App cookie bootstrap", () => {
+  const vendor = `<script>document.cookie.split(";");document.cookie="analytics=1";document.root="rendered";</script>`;
+  const execute = (html: string, document: object) => {
+    for (const script of html.matchAll(/<script>([\s\S]*?)<\/script>/g)) {
+      NodeVM.runInNewContext(script[1]!, { document });
+    }
+  };
+
+  it("lets cookie-reading startup code render in an opaque document without granting storage", () => {
+    const blocked = new DOMException("Opaque origin", "SecurityError");
+    const prototype = Object.defineProperty({}, "cookie", {
+      get() {
+        throw blocked;
+      },
+      set() {
+        throw blocked;
+      },
+    });
+    const document = Object.assign(
+      Object.create(prototype) as { cookie: string; root: string; indexedDB: unknown },
+      { root: "" },
+    );
+    Object.defineProperty(document, "indexedDB", {
+      get() {
+        throw blocked;
+      },
+    });
+    const saved = injectMcpAppCsp(vendor, { connectDomains: ["https://analytics.example"] });
+    const html = injectMcpAppCookieBootstrap(saved);
+    expect(html.indexOf("Content-Security-Policy")).toBeLessThan(html.indexOf("<script>"));
+    expect(html).toContain("connect-src https://analytics.example");
+    execute(html, document);
+    expect(document.root).toBe("rendered");
+    expect(document.cookie).toBe("");
+    document.cookie = "session=secret";
+    expect(document.cookie).toBe("");
+    expect(() => document.indexedDB).toThrow(blocked);
+    expect(() => Reflect.get(prototype, "cookie")).toThrow(blocked);
+  });
+
+  it("leaves functioning cookie reads and writes unchanged", () => {
+    const document = { cookie: "existing=1", root: "" };
+    execute(injectMcpAppCookieBootstrap(vendor), document);
+    expect(document).toEqual({ cookie: "analytics=1", root: "rendered" });
+  });
+
+  it("leaves other cookie failures unchanged", () => {
+    const failure = new TypeError("Unexpected cookie failure");
+    const document = Object.defineProperty({}, "cookie", {
+      get() {
+        throw failure;
+      },
+    });
+    expect(() => execute(injectMcpAppCookieBootstrap(vendor), document)).toThrow(failure);
+    expect(() => Reflect.get(document, "cookie")).toThrow(failure);
+  });
+});
diff --git a/packages/shared/src/mcpApp.ts b/packages/shared/src/mcpApp.ts
index b99defbb504779a1ad5ead59d36015cb25419d5f..e525b1b68d07cc5ded73a20a50d94a595dd04f57 100644
--- a/packages/shared/src/mcpApp.ts
+++ b/packages/shared/src/mcpApp.ts
@@ -249,6 +249,22 @@ export function injectMcpAppThemeBootstrap(html: string): string {
     : `<!doctype html>${script}${html}`;
 }
 
+/**
+ * Analytics libraries often read cookies during startup. An opaque document
+ * has none, but browsers throw instead of returning an empty cookie string.
+ * Keep the sandbox and make that document-local case behave like no cookies.
+ */
+export function injectMcpAppCookieBootstrap(html: string): string {
+  const bootstrap = `<script>(function(){try{void document.cookie;}catch(error){if(error&&error.name==="SecurityError"){Object.defineProperty(document,"cookie",{configurable:true,get:function(){return "";},set:function(){}});}}})();</script>`;
+  // Saved captures begin with the CSP inserted by injectMcpAppCsp. Keep it
+  // ahead of executable code, including this bootstrap, without rewriting it.
+  const prefix =
+    /^(?:\uFEFF?\s*<!doctype[^>]*>\s*)?(?:<meta http-equiv="Content-Security-Policy" content="[^"]*">)?/i.exec(
+      html,
+    )![0];
+  return `${prefix}${bootstrap}${html.slice(prefix.length)}`;
+}
+
 /** The iframe `allow` attribute for the permissions an app declared. */
 export function mcpAppAllowAttribute(permissions: McpAppPermissions | undefined): string {
   const features: Array<string> = [];