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;