Add reporting of the Proxy <-> Server sessionId. It's only possible to get the session id for this leg of the proxy if it is a StreamableHttp connection.

* In server/src/index.ts
  - slight format tweak of the Client <-> Proxy message so that the two messages will line up when displayed
* In server/src/mcpProxy.ts
  - add reportedServerSession boolean initialized to false
  - in transportToServer.onmessage,
    - if reportedServerSession isn't set yet, report the sessionId if present on the transport, and then set reportedServerSession to true

Author: cliffhall

server/src/index.ts

@@ -197,7 +197,7 @@ app.post("/mcp", async (req, res) => {
         onsessioninitialized: (sessionId) => {
           webAppTransports.set(sessionId, webAppTransport);
           serverTransports.set(sessionId, serverTransport!);
-          console.log("New client <-> proxy server sessionId: " + sessionId);
+          console.log("Client <-> Proxy  sessionId: " + sessionId);
         },
       });
       console.log("Created StreamableHttp client transport");

server/src/mcpProxy.ts

@@ -27,6 +27,8 @@ export default function mcpProxy({
   let transportToClientClosed = false;
   let transportToServerClosed = false;
 
+  let reportedServerSession = false;
+
   transportToClient.onmessage = (message) => {
     transportToServer.send(message).catch((error) => {
       // Send error response back to client if it was a request (has id) and connection is still open
@@ -46,6 +48,15 @@ export default function mcpProxy({
   };
 
   transportToServer.onmessage = (message) => {
+    if (!reportedServerSession) {
+      if (transportToServer.sessionId) {
+        // Can only report for StreamableHttp
+        console.error(
+          "Proxy  <-> Server sessionId: " + transportToServer.sessionId,
+        );
+      }
+      reportedServerSession = true;
+    }
     transportToClient.send(message).catch(onClientError);
   };