feat(gateway): forward application identity on WebSocket upgrades

Adds gatewayWebSocketBackendHeaders so proxied upgrades carry application
identity while Bun keeps ownership of WebSocket framing.

Pre-existing working-tree change, committed as-is rather than authored
here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/dev-server/src/gateway.ts

@@ -113,6 +113,7 @@ interface Target extends GatewayApp {
 interface WsBridge {
   origin: string;
   path: string;
+  headers: Record<string, string>;
   backend?: WebSocket;
   queue: Array<string | ArrayBuffer>;
   maxMessageBytes: number;
@@ -433,6 +434,30 @@ export function gatewayProxyHeaders(
   return headers;
 }
 
+/** Forward application identity to the child while leaving WebSocket framing to Bun. */
+export function gatewayWebSocketBackendHeaders(
+  req: Request,
+  url: URL,
+  ip: string,
+  forwardedHeaders: boolean,
+  backendOrigin: string,
+): Record<string, string> {
+  const headers = stripUntrustedInternalHeaders(
+    gatewayProxyHeaders(req, url, ip, forwardedHeaders),
+  );
+  headers.delete("host");
+  headers.delete("connection");
+  headers.delete("upgrade");
+  headers.delete("accept-encoding");
+  for (const name of [...headers.keys()]) {
+    if (name.startsWith("sec-websocket-")) headers.delete(name);
+  }
+  // The public origin was validated at the gateway edge. The child receives a
+  // new, trusted same-origin connection from its private gateway listener.
+  headers.set("origin", backendOrigin);
+  return Object.fromEntries(headers);
+}
+
 /** Remove headers that only a direct workspace-to-app request may supply. */
 export function stripUntrustedInternalHeaders(headers: Headers): Headers {
   const sanitized = new Headers(headers);
@@ -692,6 +717,13 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
             data: {
               origin: target.origin,
               path: url.pathname + url.search,
+              headers: gatewayWebSocketBackendHeaders(
+                req,
+                url,
+                ip,
+                forwardedHeaders,
+                target.origin,
+              ),
               queue: [],
               maxMessageBytes: websocketSecurity.maxMessageBytes ?? 64 * 1024,
               maxQueuedMessages: websocketSecurity.maxQueuedMessages ?? 100,
@@ -753,7 +785,11 @@ export async function startGateway(opts: GatewayOptions): Promise<RunningGateway
       websocket: {
         open(ws) {
           const backendUrl = ws.data.origin.replace(/^http/, "ws") + ws.data.path;
-          const backend = new WebSocket(backendUrl);
+          const BackendWebSocket = WebSocket as unknown as new (
+            url: string,
+            options: Bun.WebSocketOptions,
+          ) => WebSocket;
+          const backend = new BackendWebSocket(backendUrl, { headers: ws.data.headers });
           ws.data.backend = backend;
           backend.addEventListener("open", () => {
             for (const m of ws.data.queue) backend.send(m);