fix(gateway): proxy browser server functions to workspace apps

packages/dev-server/src/gateway.ts

@@ -470,19 +470,37 @@ export function stripUntrustedInternalHeaders(headers: Headers): Headers {
 }
 
 /**
- * The reserved inter-app RPC namespace is refused at the gateway edge, before
- * any proxying — it is only ever mounted by a child app's own dev-server and
- * must never be reachable from outside the workspace.
+ * Private inter-app RPC routes are refused at the gateway edge. The exact
+ * prefix is the CSRF-protected browser-to-app server-function endpoint and is
+ * intentionally proxied to the selected child app.
  */
 export function isRpcGatewayPath(pathname: string): boolean {
   return (
-    pathname === RPC_PATH_PREFIX ||
     pathname.startsWith(`${RPC_PATH_PREFIX}/`) ||
     pathname === RPC_STREAM_PATH_PREFIX ||
     pathname.startsWith(`${RPC_STREAM_PATH_PREFIX}/`)
   );
 }
 
+/** Build the trusted internal hop for a browser server-function request. */
+export function gatewayBrowserRpcHeaders(
+  req: Request,
+  url: URL,
+  ip: string,
+  forwardedHeaders: boolean,
+  backendOrigin: string,
+): Headers {
+  const headers = stripUntrustedInternalHeaders(
+    gatewayProxyHeaders(req, url, ip, forwardedHeaders),
+  );
+  // The public request already passed the gateway's host and fetch-metadata
+  // checks. Present the internal proxy hop as same-origin to the child while
+  // retaining the double-submit CSRF cookie and header.
+  headers.set("origin", backendOrigin);
+  headers.delete("host");
+  return headers;
+}
+
 /** Boot every app as a child process, then route by Host on one gateway port. */
 export async function startGateway(opts: GatewayOptions): Promise<RunningGateway> {
   const port = opts.port ?? 3000;
@@ -497,7 +515,7 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
   );
   // Loopback-only origins, computed up front (ports are assigned by index
   // before any child spawns) so every child can reach every other child
-  // directly — bypassing the gateway, which 404s the RPC prefix by design.
+  // directly — bypassing the gateway, which 404s private nested RPC routes.
   const internalOriginsEnv: Readonly<Record<string, string>> = Object.freeze(
     Object.fromEntries(
       opts.apps.map((app, i) => [app.name, `http://127.0.0.1:${app.port ?? port + 1 + i}`]),
@@ -737,9 +755,10 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
         }
 
         // HTTP → reverse-proxy to the app, preserving method/headers/body.
-        const headers = stripUntrustedInternalHeaders(
-          gatewayProxyHeaders(req, url, ip, forwardedHeaders),
-        );
+        const headers =
+          url.pathname === RPC_PATH_PREFIX
+            ? gatewayBrowserRpcHeaders(req, url, ip, forwardedHeaders, target.origin)
+            : stripUntrustedInternalHeaders(gatewayProxyHeaders(req, url, ip, forwardedHeaders));
         const body =
           req.method === "GET" || req.method === "HEAD" ? undefined : await req.arrayBuffer();
         let res: Response;

packages/dev-server/test/gateway.test.ts

@@ -4,6 +4,7 @@ import {
   defaultGatewayHostname,
   forwardAuthFailure,
   forwardAuthHeaders,
+  gatewayBrowserRpcHeaders,
   gatewayProxyHeaders,
   gatewayWebSocketBackendHeaders,
   stripUntrustedInternalHeaders,
@@ -176,13 +177,40 @@ test("nested SSO proxy keeps the protected app's original request headers", () =
   expect(proxied.get("x-original-uri")).toBe("/settings");
 });
 
-test("the reserved RPC prefix is refused at the gateway before any proxying", () => {
-  expect(isRpcGatewayPath(RPC_PATH_PREFIX)).toBe(true);
+test("the gateway proxies browser server functions but refuses private RPC routes", () => {
+  expect(isRpcGatewayPath(RPC_PATH_PREFIX)).toBe(false);
   expect(isRpcGatewayPath(`${RPC_PATH_PREFIX}/billing/createInvoice`)).toBe(true);
   expect(isRpcGatewayPath("/api/billing")).toBe(false);
   expect(isRpcGatewayPath("/__wrnexus/rpcfoo")).toBe(false);
 });
 
+test("browser RPC proxy preserves CSRF credentials and trusts only the internal hop", () => {
+  const request = new Request(`http://web.localhost:3000${RPC_PATH_PREFIX}`, {
+    method: "POST",
+    headers: {
+      host: "web.localhost:3000",
+      origin: "http://web.localhost:3000",
+      cookie: "wrn-csrf=token",
+      "x-csrf-token": "token",
+      [RPC_INTERNAL_HEADER]: "forged",
+    },
+  });
+  const headers = gatewayBrowserRpcHeaders(
+    request,
+    new URL(request.url),
+    "127.0.0.1",
+    true,
+    "http://127.0.0.1:3001",
+  );
+
+  expect(headers.get("origin")).toBe("http://127.0.0.1:3001");
+  expect(headers.get("cookie")).toBe("wrn-csrf=token");
+  expect(headers.get("x-csrf-token")).toBe("token");
+  expect(headers.get("x-forwarded-host")).toBe("web.localhost:3000");
+  expect(headers.has(RPC_INTERNAL_HEADER)).toBe(false);
+  expect(headers.has("host")).toBe(false);
+});
+
 test("an inbound internal-marker header from outside is stripped regardless of casing", () => {
   for (const name of [
     RPC_INTERNAL_HEADER,