* Remove handling of out-of-spec 'notification/stderr' messages. It's not a thing. See https://github.com/modelcontextprotocol/servers/pull/2469

* Inspect the stderr output of STDIO servers and attempt to assign an appropriate RFC 5424 Syslog Protocol level before sending a leveled logging message to the client

client/src/App.tsx

@@ -34,7 +34,6 @@ import {
   useDraggablePane,
   useDraggableSidebar,
 } from "./lib/hooks/useDraggablePane";
-import { StdErrNotification } from "./lib/notificationTypes";
 
 import { Tabs, TabsContent, TabsList, TabsTrigger } from "@/components/ui/tabs";
 import { Button } from "@/components/ui/button";
@@ -106,9 +105,6 @@ const App = () => {
   >(getInitialTransportType);
   const [logLevel, setLogLevel] = useState<LoggingLevel>("debug");
   const [notifications, setNotifications] = useState<ServerNotification[]>([]);
-  const [stdErrNotifications, setStdErrNotifications] = useState<
-    StdErrNotification[]
-  >([]);
   const [roots, setRoots] = useState<Root[]>([]);
   const [env, setEnv] = useState<Record<string, string>>({});
 
@@ -224,12 +220,6 @@ const App = () => {
     onNotification: (notification) => {
       setNotifications((prev) => [...prev, notification as ServerNotification]);
     },
-    onStdErrNotification: (notification) => {
-      setStdErrNotifications((prev) => [
-        ...prev,
-        notification as StdErrNotification,
-      ]);
-    },
     onPendingRequest: (request, resolve, reject) => {
       setPendingSampleRequests((prev) => [
         ...prev,
@@ -757,10 +747,6 @@ const App = () => {
     setLogLevel(level);
   };
 
-  const clearStdErrNotifications = () => {
-    setStdErrNotifications([]);
-  };
-
   const AuthDebuggerWrapper = () => (
     <TabsContent value="auth">
       <AuthDebugger
@@ -829,11 +815,9 @@ const App = () => {
           setOauthScope={setOauthScope}
           onConnect={connectMcpServer}
           onDisconnect={disconnectMcpServer}
-          stdErrNotifications={stdErrNotifications}
           logLevel={logLevel}
           sendLogLevelRequest={sendLogLevelRequest}
           loggingSupported={!!serverCapabilities?.logging || false}
-          clearStdErrNotifications={clearStdErrNotifications}
         />
         <div
           onMouseDown={handleSidebarDragStart}

client/src/components/Sidebar.tsx

@@ -24,7 +24,6 @@ import {
   SelectTrigger,
   SelectValue,
 } from "@/components/ui/select";
-import { StdErrNotification } from "@/lib/notificationTypes";
 import {
   LoggingLevel,
   LoggingLevelSchema,
@@ -62,8 +61,6 @@ interface SidebarProps {
   setOauthScope: (scope: string) => void;
   onConnect: () => void;
   onDisconnect: () => void;
-  stdErrNotifications: StdErrNotification[];
-  clearStdErrNotifications: () => void;
   logLevel: LoggingLevel;
   sendLogLevelRequest: (level: LoggingLevel) => void;
   loggingSupported: boolean;
@@ -93,8 +90,6 @@ const Sidebar = ({
   setOauthScope,
   onConnect,
   onDisconnect,
-  stdErrNotifications,
-  clearStdErrNotifications,
   logLevel,
   sendLogLevelRequest,
   loggingSupported,
@@ -760,36 +755,6 @@ const Sidebar = ({
                 </Select>
               </div>
             )}
-
-            {stdErrNotifications.length > 0 && (
-              <>
-                <div className="mt-4 border-t border-gray-200 pt-4">
-                  <div className="flex justify-between items-center">
-                    <h3 className="text-sm font-medium">
-                      Error output from MCP server
-                    </h3>
-                    <Button
-                      variant="outline"
-                      size="sm"
-                      onClick={clearStdErrNotifications}
-                      className="h-8 px-2"
-                    >
-                      Clear
-                    </Button>
-                  </div>
-                  <div className="mt-2 max-h-80 overflow-y-auto">
-                    {stdErrNotifications.map((notification, index) => (
-                      <div
-                        key={index}
-                        className="text-sm text-red-500 font-mono py-2 border-b border-gray-200 last:border-b-0"
-                      >
-                        {notification.params.content}
-                      </div>
-                    ))}
-                  </div>
-                </div>
-              </>
-            )}
           </div>
         </div>
       </div>

client/src/lib/hooks/useConnection.ts

@@ -36,7 +36,7 @@ import { useEffect, useState } from "react";
 import { useToast } from "@/lib/hooks/useToast";
 import { z } from "zod";
 import { ConnectionStatus } from "../constants";
-import { Notification, StdErrNotificationSchema } from "../notificationTypes";
+import { Notification } from "../notificationTypes";
 import {
   auth,
   discoverOAuthProtectedResourceMetadata,
@@ -92,7 +92,6 @@ export function useConnection({
   oauthScope,
   config,
   onNotification,
-  onStdErrNotification,
   onPendingRequest,
   onElicitationRequest,
   getRoots,
@@ -505,13 +504,6 @@ export function useConnection({
         };
       }
 
-      if (onStdErrNotification) {
-        client.setNotificationHandler(
-          StdErrNotificationSchema,
-          onStdErrNotification,
-        );
-      }
-
       let capabilities;
       try {
         const transport =

client/src/lib/notificationTypes.ts

@@ -5,18 +5,8 @@ import {
 } from "@modelcontextprotocol/sdk/types.js";
 import { z } from "zod";
 
-export const StdErrNotificationSchema = BaseNotificationSchema.extend({
-  method: z.literal("notifications/stderr"),
-  params: z.object({
-    content: z.string(),
-  }),
-});
-
 export const NotificationSchema = ClientNotificationSchema.or(
-  StdErrNotificationSchema,
-)
-  .or(ServerNotificationSchema)
-  .or(BaseNotificationSchema);
+  ServerNotificationSchema,
+).or(BaseNotificationSchema);
 
-export type StdErrNotification = z.infer<typeof StdErrNotificationSchema>;
 export type Notification = z.infer<typeof NotificationSchema>;

server/src/index.ts

@@ -401,24 +401,65 @@ app.get(
 
       (serverTransport as StdioClientTransport).stderr!.on("data", (chunk) => {
         if (chunk.toString().includes("MODULE_NOT_FOUND")) {
+          // Server command not found, remove transports
+          const message = "Command not found, transports removed";
           webAppTransport.send({
             jsonrpc: "2.0",
-            method: "notifications/stderr",
+            method: "notifications/message",
             params: {
-              content: "Command not found, transports removed",
+              level: "alert",
+              data: {
+                error: message,
+              },
             },
           });
           webAppTransport.close();
           serverTransport.close();
           webAppTransports.delete(webAppTransport.sessionId);
           serverTransports.delete(webAppTransport.sessionId);
-          console.error("Command not found, transports removed");
+          console.error(message);
         } else {
+          // Inspect message and attempt to assign a RFC 5424 Syslog Protocol level
+          let level;
+          let message = chunk.toString();
+          let ucMsg = chunk.toString().toUpperCase();
+          if (ucMsg.includes("DEBUG")) {
+            level = "debug";
+          } else if (ucMsg.includes("INFO")) {
+            level = "info";
+          } else if (ucMsg.includes("NOTICE")) {
+            level = "notice";
+          } else if (ucMsg.includes("WARN")) {
+            level = "warning";
+          } else if (ucMsg.includes("ERROR")) {
+            level = "error";
+          } else if (ucMsg.includes("CRITICAL")) {
+            level = "critical";
+          } else if (ucMsg.includes("ALERT")) {
+            level = "alert";
+          } else if (ucMsg.includes("EMERGENCY")) {
+            level = "emergency";
+          } else if (ucMsg.includes("SIGINT")) {
+            level = "alert";
+            message = "SIGINT received. Server shutdown.";
+          } else if (ucMsg.includes("SIGHUP")) {
+            level = "alert";
+            message = "SIGHUP received. Server shutdown.";
+          } else if (ucMsg.includes("SIGTERM")) {
+            level = "alert";
+            message = "SIGTERM received. Server shutdown.";
+          } else {
+            level = "info";
+          }
           webAppTransport.send({
             jsonrpc: "2.0",
-            method: "notifications/stderr",
+            method: "notifications/message",
             params: {
-              content: chunk.toString(),
+              level,
+              logger: "stdio",
+              data: {
+                error: message,
+              },
             },
           });
         }