Merge pull request #418 from modelcontextprotocol/ihrpr/auth-sse-shttp-connect

Fix connection issue for servers with auth

Author: ihrpr

client/src/App.tsx

@@ -234,7 +234,6 @@ const App = () => {
   const onOAuthConnect = useCallback(
     (serverUrl: string) => {
       setSseUrl(serverUrl);
-      setTransportType("sse");
       setIsAuthDebuggerVisible(false);
       void connectMcpServer();
     },

client/src/lib/hooks/useConnection.ts

@@ -249,9 +249,16 @@ export function useConnection({
     }
   };
 
+  const is401Error = (error: unknown): boolean => {
+    return (
+      (error instanceof SseError && error.code === 401) ||
+      (error instanceof Error && error.message.includes("401")) ||
+      (error instanceof Error && error.message.includes("Unauthorized"))
+    );
+  };
+
   const handleAuthError = async (error: unknown) => {
-    if (error instanceof SseError && error.code === 401) {
-      // Create a new auth provider with the current server URL
+    if (is401Error(error)) {
       const serverAuthProvider = new InspectorOAuthClientProvider(sseUrl);
 
       const result = await auth(serverAuthProvider, { serverUrl: sseUrl });
@@ -330,7 +337,6 @@ export function useConnection({
           mcpProxyServerUrl = new URL(`${getMCPProxyAddress(config)}/sse`);
           mcpProxyServerUrl.searchParams.append("url", sseUrl);
           transportOptions = {
-            authProvider: serverAuthProvider,
             eventSourceInit: {
               fetch: (
                 url: string | URL | globalThis.Request,
@@ -347,7 +353,6 @@ export function useConnection({
           mcpProxyServerUrl = new URL(`${getMCPProxyAddress(config)}/mcp`);
           mcpProxyServerUrl.searchParams.append("url", sseUrl);
           transportOptions = {
-            authProvider: serverAuthProvider,
             eventSourceInit: {
               fetch: (
                 url: string | URL | globalThis.Request,
@@ -425,13 +430,14 @@ export function useConnection({
           `Failed to connect to MCP Server via the MCP Inspector Proxy: ${mcpProxyServerUrl}:`,
           error,
         );
+
         const shouldRetry = await handleAuthError(error);
         if (shouldRetry) {
           return connect(undefined, retryCount + 1);
         }
-
-        if (error instanceof SseError && error.code === 401) {
+        if (is401Error(error)) {
           // Don't set error state if we're about to redirect for auth
+
           return;
         }
         throw error;

package-lock.json

@@ -1,4 +1,5 @@
 import { Transport } from "@modelcontextprotocol/sdk/shared/transport.js";
+import { isJSONRPCRequest } from "@modelcontextprotocol/sdk/types.js";
 
 function onClientError(error: Error) {
   console.error("Error from inspector client:", error);
@@ -19,7 +20,21 @@ export default function mcpProxy({
   let transportToServerClosed = false;
 
   transportToClient.onmessage = (message) => {
-    transportToServer.send(message).catch(onServerError);
+    transportToServer.send(message).catch((error) => {
+      // Send error response back to client if it was a request (has id) and connection is still open
+      if (isJSONRPCRequest(message) && !transportToClientClosed) {
+        const errorResponse = {
+          jsonrpc: "2.0" as const,
+          id: message.id,
+          error: {
+            code: -32001,
+            message: error.message,
+            data: error,
+          },
+        };
+        transportToClient.send(errorResponse).catch(onClientError);
+      }
+    });
   };
 
   transportToServer.onmessage = (message) => {