Use MCP_PROXY_FULL_ADDRESS for SSE transport backed endpoints

The STDIO and SSE transports use SSE between the client and the proxy
server.  SSE requires the client to initiate the MCP conversation
(event-stream) at the endpoint provided by the server.  The endpoint
provided by the server must obey where the client sees the server,
rather than being assumed to be server-root `/message`.

The Streamable HTTP endpoint does not need the full proxy address
because the response is an event-stream directly, and does not use SSE.

client/src/lib/hooks/useConnection.ts

@@ -409,11 +409,20 @@ export function useConnection({
 
       let mcpProxyServerUrl;
       switch (transportType) {
-        case "stdio":
+        case "stdio": {
           mcpProxyServerUrl = new URL(`${getMCPProxyAddress(config)}/stdio`);
           mcpProxyServerUrl.searchParams.append("command", command);
           mcpProxyServerUrl.searchParams.append("args", args);
           mcpProxyServerUrl.searchParams.append("env", JSON.stringify(env));
+
+          const proxyFullAddress = config.MCP_PROXY_FULL_ADDRESS
+            .value as string;
+          if (proxyFullAddress) {
+            mcpProxyServerUrl.searchParams.append(
+              "proxyFullAddress",
+              proxyFullAddress,
+            );
+          }
           transportOptions = {
             authProvider: serverAuthProvider,
             eventSourceInit: {
@@ -431,10 +440,20 @@ export function useConnection({
             },
           };
           break;
+        }
 
-        case "sse":
+        case "sse": {
           mcpProxyServerUrl = new URL(`${getMCPProxyAddress(config)}/sse`);
           mcpProxyServerUrl.searchParams.append("url", sseUrl);
+
+          const proxyFullAddressSSE = config.MCP_PROXY_FULL_ADDRESS
+            .value as string;
+          if (proxyFullAddressSSE) {
+            mcpProxyServerUrl.searchParams.append(
+              "proxyFullAddress",
+              proxyFullAddressSSE,
+            );
+          }
           transportOptions = {
             eventSourceInit: {
               fetch: (
@@ -451,6 +470,7 @@ export function useConnection({
             },
           };
           break;
+        }
 
         case "streamable-http":
           mcpProxyServerUrl = new URL(`${getMCPProxyAddress(config)}/mcp`);

server/src/index.ts

@@ -391,7 +391,11 @@ app.get(
         throw error;
       }
 
-      const webAppTransport = new SSEServerTransport("/message", res);
+      const proxyFullAddress = (req.query.proxyFullAddress as string) || "";
+      const prefix = proxyFullAddress || "";
+      const endpoint = `${prefix}/message`;
+
+      const webAppTransport = new SSEServerTransport(endpoint, res);
       console.log("Created client transport");
 
       webAppTransports.set(webAppTransport.sessionId, webAppTransport);
@@ -511,7 +515,11 @@ app.get(
       }
 
       if (serverTransport) {
-        const webAppTransport = new SSEServerTransport("/message", res);
+        const proxyFullAddress = (req.query.proxyFullAddress as string) || "";
+        const prefix = proxyFullAddress || "";
+        const endpoint = `${prefix}/message`;
+
+        const webAppTransport = new SSEServerTransport(endpoint, res);
         webAppTransports.set(webAppTransport.sessionId, webAppTransport);
         console.log("Created client transport");
         serverTransports.set(webAppTransport.sessionId, serverTransport!);