add SSE connection ability

server/src/mcpProxy.ts

@@ -9,6 +9,9 @@ export default function mcpProxy({
   transportToServer: Transport;
   onerror: (error: Error) => void;
 }) {
+  let transportToClientClosed = false;
+  let transportToServerClosed = false;
+
   transportToClient.onmessage = (message) => {
     transportToServer.send(message).catch(onerror);
   };
@@ -18,10 +21,20 @@ export default function mcpProxy({
   };
 
   transportToClient.onclose = () => {
+    if (transportToServerClosed) {
+      return;
+    }
+
+    transportToClientClosed = true;
     transportToServer.close().catch(onerror);
   };
 
   transportToServer.onclose = () => {
+    if (transportToClientClosed) {
+      return;
+    }
+    transportToServerClosed = true;
+
     transportToClient.close().catch(onerror);
   };