diff --git a/lib/mcp-gateway.ts b/lib/mcp-gateway.ts
new file mode 100644
index 0000000..b57735b
--- /dev/null
+++ b/lib/mcp-gateway.ts
@@ -0,0 +1,15 @@
+// Only the Moshcode product host exposes this gateway. Other tenant domains
+// remain independent, and neither a query parameter nor a forwarded host can
+// choose the destination receiving a client's bearer token.
+const MCP_UPSTREAM = "https://app.moshcode.sh";
+
+export function mcpGatewayTarget(host: string, pathname: string, search = ""): URL | null {
+  if (host.toLowerCase() !== "moshcode.sh") return null;
+  const metadata = pathname === "/.well-known/oauth-authorization-server"
+    || /^\/\.well-known\/oauth-protected-resource(?:\/mcp|\/api\/v1\/mcp\/[A-Za-z0-9_-]{1,128})?$/.test(pathname);
+  const share = /^\/api\/v1\/mcp\/[A-Za-z0-9_-]{1,128}(?:\/[A-Za-z0-9_-]{1,128})?$/.test(pathname);
+  if (!metadata && !share) return null;
+  const target = new URL(pathname, MCP_UPSTREAM);
+  target.search = search;
+  return target;
+}
diff --git a/middleware.ts b/middleware.ts
index 8d57bc2..64521bb 100644
--- a/middleware.ts
+++ b/middleware.ts
@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from "next/server";
+import { mcpGatewayTarget } from "./lib/mcp-gateway";
 
 // Parked domains allowed to frame the app (Porkbun masked/frameset forwarding,
 // e.g. moshcode.sh). Read at RUNTIME so FRAME_ANCESTORS env changes take effect
@@ -33,6 +34,13 @@ export function middleware(req: NextRequest) {
     url.port = "";
     return NextResponse.redirect(url, 308);
   }
+  const mcpTarget = mcpGatewayTarget(host, req.nextUrl.pathname, req.nextUrl.search);
+  if (mcpTarget) {
+    // A rewrite preserves methods, request bodies, Authorization and streaming
+    // responses. OAuth consent and token endpoints remain on the app origin;
+    // the resource URL the client added remains on moshcode.sh.
+    return NextResponse.rewrite(mcpTarget);
+  }
   const res = NextResponse.next();
   res.headers.set("Content-Security-Policy", `frame-ancestors ${frameAncestors(req)}`);
 
diff --git a/tests/mcp-gateway.test.mjs b/tests/mcp-gateway.test.mjs
new file mode 100644
index 0000000..ae8d30f
--- /dev/null
+++ b/tests/mcp-gateway.test.mjs
@@ -0,0 +1,26 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { mcpGatewayTarget } from "../lib/mcp-gateway.ts";
+
+test("Moshcode share and discovery URLs reach the fixed app authority", () => {
+  for (const path of [
+    "/api/v1/mcp/mcs_example", "/api/v1/mcp/shares", "/api/v1/mcp/shares/mcs_example",
+    "/.well-known/oauth-authorization-server", "/.well-known/oauth-protected-resource",
+    "/.well-known/oauth-protected-resource/mcp",
+    "/.well-known/oauth-protected-resource/api/v1/mcp/mcs_example",
+  ]) assert.equal(mcpGatewayTarget("moshcode.sh", path).href, `https://app.moshcode.sh${path}`);
+});
+
+test("a caller cannot change the bearer-token destination through path or query input", () => {
+  assert.equal(mcpGatewayTarget("moshcode.sh", "/api/v1/mcp/mcs_example", "?upstream=https://evil.example").origin,
+    "https://app.moshcode.sh");
+  for (const path of ["//evil.example/api/v1/mcp/x", "/api/v1/mcp/../oauth/token", "/api/v1/mcp/%2f%2fevil.example", "/api/v1/mcp/x/../../../oauth/token"])
+    assert.equal(mcpGatewayTarget("moshcode.sh", path), null);
+});
+
+test("tenant sites, account routes and ordinary pages stay outside the gateway", () => {
+  for (const host of ["moshcoding.com", "moshscript.com", "moshcode.sh.evil.example", "moshcode.sh:8080", ""])
+    assert.equal(mcpGatewayTarget(host, "/api/v1/mcp/mcs_example"), null);
+  for (const path of ["/", "/oauth/authorize", "/oauth/token", "/api/v1/me", "/api/sessions", "/api/v1/mcp"])
+    assert.equal(mcpGatewayTarget("moshcode.sh", path), null);
+});