fix(mcp): Let browsers send Origin on the consent POST

packages/junior/src/api/mcp/oauth/page.ts

@@ -1,6 +1,9 @@
 import { escapeHtml, renderHtmlPage } from "@/api/html";
 
-/** Render one MCP sign-in page. It sends no referrer and loads no resources. */
+/**
+ * Render one MCP sign-in page. It loads no resources and sends no referrer to
+ * other origins.
+ */
 export function renderMcpPage(page: {
   /** HTML that follows the message. Callers escape every value in it. */
   bodyHtml?: string;
@@ -19,7 +22,10 @@ export function renderMcpPage(page: {
     bodyHtml: page.bodyHtml ? `${message}\n${page.bodyHtml}` : message,
     headers: {
       "content-security-policy": `default-src 'none'; style-src 'unsafe-inline'; frame-ancestors 'none'; form-action ${formAction.join(" ")}`,
-      "referrer-policy": "no-referrer",
+      // Not `no-referrer`: browsers then send `Origin: null` on the consent
+      // POST, which fails the same-origin check. The client redirect is
+      // cross-origin, so it still gets no referrer.
+      "referrer-policy": "same-origin",
     },
     status: page.status,
     title: page.title,

packages/junior/tests/fixtures/mcp-http.ts

@@ -111,29 +111,44 @@ export async function openMcpConsentPage(
   app: Hono,
   consentUrl: string,
   email: string,
-): Promise<{ csrf?: string; html: string; status: number }> {
+): Promise<{
+  csrf?: string;
+  html: string;
+  referrerPolicy: string | null;
+  status: number;
+}> {
   const response = await app.request(consentUrl, {
     headers: { cookie: viewerCookie(email) },
   });
   const html = await response.text();
   return {
     csrf: html.match(/name="csrf" value="([^"]+)"/)?.[1],
     html,
+    referrerPolicy: response.headers.get("referrer-policy"),
     status: response.status,
   };
 }
 
-/** Approve consent as the viewer who opened the page. */
+/**
+ * Approve consent as the viewer who opened the page. The `Origin` header
+ * follows the page's referrer policy the way a browser form POST does: under
+ * `no-referrer`, browsers send `Origin: null`.
+ */
 export async function approveMcpConsent(
   app: Hono,
-  args: { consentUrl: string; csrf: string; email: string },
+  args: {
+    consentUrl: string;
+    csrf: string;
+    email: string;
+    referrerPolicy: string | null;
+  },
 ): Promise<Response> {
   return await app.request(args.consentUrl, {
     method: "POST",
     headers: {
       "content-type": "application/x-www-form-urlencoded",
       cookie: viewerCookie(args.email),
-      origin: MCP_TEST_ORIGIN,
+      origin: args.referrerPolicy === "no-referrer" ? "null" : MCP_TEST_ORIGIN,
     },
     body: new URLSearchParams({ action: "approve", csrf: args.csrf }),
   });
@@ -172,6 +187,7 @@ export async function approveMcpClient(
     consentUrl,
     csrf: page.csrf ?? "",
     email,
+    referrerPolicy: page.referrerPolicy,
   });
   const code = new URL(approved.headers.get("location") ?? "").searchParams.get(
     "code",

packages/junior/tests/integration/api/mcp/route.test.ts

@@ -110,6 +110,7 @@ describe("remote MCP HTTP", () => {
       consentUrl,
       csrf: page.csrf ?? "",
       email: MCP_TEST_VIEWER,
+      referrerPolicy: page.referrerPolicy,
     });
     const callback = new URL(approved.headers.get("location") ?? "");
     const issued = await exchangeMcpCode(app, {