diff --git a/src/server/mcp/transport-v2.test.ts b/src/server/mcp/transport-v2.test.ts
index 3262193..80d65f8 100644
--- a/src/server/mcp/transport-v2.test.ts
+++ b/src/server/mcp/transport-v2.test.ts
@@ -1,7 +1,35 @@
 import { createMcpHandler, getMcpAuthContext } from "agents/mcp/server";
 import { McpServer } from "@modelcontextprotocol/server";
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import { z } from "zod";
+import { createWorkersOAuthMcpProps } from "@/server/mcp/context";
+import { handleAuthenticatedOpenSeoMcpRequest } from "@/server/mcp/transport";
+
+vi.mock("@/lib/auth", () => ({
+  getHostedBaseUrl: () => "https://open-seo.test",
+}));
+
+vi.mock("@/middleware/ensure-user/cloudflareAccess", () => ({
+  resolveCloudflareAccessContext: vi.fn(),
+}));
+
+vi.mock("@/middleware/ensure-user/delegated", () => ({
+  resolveLocalNoAuthContext: vi.fn(),
+}));
+
+vi.mock("@/server/mcp/server", async () => {
+  const { McpServer: ActualMcpServer } =
+    await import("@modelcontextprotocol/server");
+  return {
+    createOpenSeoMcpServer: () => {
+      const server = new ActualMcpServer({ name: "test", version: "1.0.0" });
+      server.registerTool("ping", {}, () => ({
+        content: [{ type: "text" as const, text: "pong" }],
+      }));
+      return server;
+    },
+  };
+});
 
 const ctx: ExecutionContext = {
   waitUntil() {},
@@ -136,4 +164,50 @@ describe("Agents SDK v2 MCP transport", () => {
     expect(surfMindResponse.status).toBe(200);
     expect(unrelatedOriginResponse.status).toBe(403);
   });
+
+  it("enforces exact hosted origins around the real SDK handler", async () => {
+    const props = createWorkersOAuthMcpProps({
+      userId: "user-1",
+      userEmail: "user@example.com",
+      organizationId: "org-1",
+      baseUrl: "https://open-seo.test",
+      clientId: "client-1",
+      scopes: ["mcp"],
+    });
+    const modernToolsList = {
+      jsonrpc: "2.0",
+      id: 1,
+      method: "tools/list",
+      params: {
+        _meta: {
+          "io.modelcontextprotocol/protocolVersion": "2026-07-28",
+          "io.modelcontextprotocol/clientCapabilities": {},
+        },
+      },
+    };
+    const call = (origin?: string) =>
+      handleAuthenticatedOpenSeoMcpRequest(
+        request("POST", modernToolsList, {
+          "Mcp-Method": "tools/list",
+          ...(origin ? { Origin: origin } : {}),
+        }),
+        props,
+        {},
+        { ...ctx, props },
+      );
+
+    await expect(
+      call("chrome-extension://pghallcbnfabbgfijhbcldaapmgidnaa"),
+    ).resolves.toMatchObject({ status: 200 });
+    await expect(call("https://open-seo.test")).resolves.toMatchObject({
+      status: 200,
+    });
+    await expect(call()).resolves.toMatchObject({ status: 200 });
+    await expect(
+      call("https://pghallcbnfabbgfijhbcldaapmgidnaa"),
+    ).resolves.toMatchObject({ status: 403 });
+    await expect(
+      call("chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
+    ).resolves.toMatchObject({ status: 403 });
+  });
 });
diff --git a/src/server/mcp/transport.test.ts b/src/server/mcp/transport.test.ts
index 961ec87..dfd535e 100644
--- a/src/server/mcp/transport.test.ts
+++ b/src/server/mcp/transport.test.ts
@@ -84,12 +84,13 @@ function createMcpRequest(headers?: Record<string, string>) {
 
 // The modern (2026-07-28) era is selected by the per-request `_meta` envelope
 // claim; without it every POST classifies as legacy traffic.
-function createModernMcpRequest() {
+function createModernMcpRequest(headers?: Record<string, string>) {
   return new Request("https://open-seo.test/mcp", {
     method: "POST",
     headers: {
       Accept: "application/json, text/event-stream",
       "Content-Type": "application/json",
+      ...headers,
     },
     body: JSON.stringify({
       jsonrpc: "2.0",
@@ -244,6 +245,22 @@ describe("handleAuthenticatedOpenSeoMcpRequest", () => {
     expect(selfHostedAuthMocks.createOpenSeoMcpServer).not.toHaveBeenCalled();
   });
 
+  it("accepts a modern request from the exact SurfMind extension origin", async () => {
+    const props = hostedProps();
+
+    const response = await handleAuthenticatedOpenSeoMcpRequest(
+      createModernMcpRequest({
+        Origin: "chrome-extension://pghallcbnfabbgfijhbcldaapmgidnaa",
+      }),
+      props,
+      {},
+      { ...ctx, props },
+    );
+
+    expect(response.status).toBe(202);
+    expect(await response.json()).toEqual({ handledBy: "modern" });
+  });
+
   it("rejects a legacy request from a disallowed Origin", async () => {
     const props = hostedProps();
 
@@ -276,6 +293,28 @@ describe("handleAuthenticatedOpenSeoMcpRequest", () => {
     );
   });
 
+  it.each([
+    [
+      "the SurfMind hostname over HTTPS",
+      "https://pghallcbnfabbgfijhbcldaapmgidnaa",
+    ],
+    [
+      "another Chrome extension",
+      "chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+    ],
+  ])("rejects a request from %s", async (_label, origin) => {
+    const props = hostedProps();
+
+    const response = await handleAuthenticatedOpenSeoMcpRequest(
+      createMcpRequest({ Origin: origin }),
+      props,
+      {},
+      { ...ctx, props },
+    );
+
+    expect(response.status).toBe(403);
+  });
+
   it("rejects provider props missing the OAuth client identity", async () => {
     // Hosted tokens always carry clientId/scopes; a token without them must
     // fail closed rather than skip scope enforcement.
diff --git a/src/server/mcp/transport.ts b/src/server/mcp/transport.ts
index 196db45..de1cd02 100644
--- a/src/server/mcp/transport.ts
+++ b/src/server/mcp/transport.ts
@@ -33,6 +33,7 @@ const MCP_CORS_HEADERS = {
 } as const;
 
 const SURFMIND_CHROME_EXTENSION_HOSTNAME = "pghallcbnfabbgfijhbcldaapmgidnaa";
+const SURFMIND_CHROME_EXTENSION_ORIGIN = `chrome-extension://${SURFMIND_CHROME_EXTENSION_HOSTNAME}`;
 
 function withMcpCors(response: Response) {
   const headers = new Headers(response.headers);
@@ -113,11 +114,12 @@ async function handleLegacyJsonRequest(request: Request, props: McpProps) {
   }
 }
 
-// Hosted pins browser Origins to the configured base URL. Self-hosted leaves
-// the option unset so the handler's localhost-class default applies — an
-// allowlist derived from the request's own Host would accept a DNS-rebinding
-// page trivially. Non-browser MCP clients send no Origin and are unaffected
-// either way.
+// Hosted applies exact-origin validation before passing the corresponding
+// hostname allowlist to the SDK as defense in depth. Self-hosted leaves the
+// option unset so the handler's localhost-class default applies — an allowlist
+// derived from the request's own Host would accept a DNS-rebinding page
+// trivially. Non-browser MCP clients send no Origin and are unaffected either
+// way.
 function createRequestHandler(
   props: McpProps,
   allowedOriginHostnames?: string[],
@@ -158,8 +160,18 @@ export async function handleAuthenticatedOpenSeoMcpRequest(
     return new Response("MCP scope required", { status: 403 });
   }
 
+  const hostedUrl = new URL(getHostedBaseUrl());
+  const origin = request.headers.get("Origin");
+  if (
+    origin &&
+    origin !== hostedUrl.origin &&
+    origin !== SURFMIND_CHROME_EXTENSION_ORIGIN
+  ) {
+    return withMcpCors(new Response("Invalid Origin", { status: 403 }));
+  }
+
   return createRequestHandler(result.data, [
-    new URL(getHostedBaseUrl()).hostname,
+    hostedUrl.hostname,
     SURFMIND_CHROME_EXTENSION_HOSTNAME,
   ])(request, env, ctx);
 }