Merge pull request modelcontextprotocol#224 from cliffhall/fix-214

Gracefully handle proxy server and inspector client startup errors

Author: cliffhall

bin/cli.js

@@ -7,7 +7,7 @@ import { fileURLToPath } from "url";
 const __dirname = dirname(fileURLToPath(import.meta.url));
 
 function delay(ms) {
-  return new Promise((resolve) => setTimeout(resolve, ms));
+  return new Promise((resolve) => setTimeout(resolve, ms, true));
 }
 
 async function main() {
@@ -73,42 +73,40 @@ async function main() {
     cancelled = true;
     abort.abort();
   });
-
-  const server = spawnPromise(
-    "node",
-    [
-      inspectorServerPath,
-      ...(command ? [`--env`, command] : []),
-      ...(mcpServerArgs ? [`--args=${mcpServerArgs.join(" ")}`] : []),
-    ],
-    {
-      env: {
-        ...process.env,
-        PORT: SERVER_PORT,
-        MCP_ENV_VARS: JSON.stringify(envVars),
-      },
-      signal: abort.signal,
-      echoOutput: true,
-    },
-  );
-
-  const client = spawnPromise("node", [inspectorClientPath], {
-    env: { ...process.env, PORT: CLIENT_PORT },
-    signal: abort.signal,
-    echoOutput: true,
-  });
-
-  // Make sure our server/client didn't immediately fail
-  await Promise.any([server, client, delay(2 * 1000)]);
-  const portParam = SERVER_PORT === "3000" ? "" : `?proxyPort=${SERVER_PORT}`;
-  console.log(
-    `\n🔍 MCP Inspector is up and running at http://127.0.0.1:${CLIENT_PORT}${portParam} 🚀`,
-  );
-
+  let server, serverOk;
   try {
-    await Promise.any([server, client]);
-  } catch (e) {
-    if (!cancelled || process.env.DEBUG) throw e;
+    server = spawnPromise(
+      "node",
+      [
+        inspectorServerPath,
+        ...(command ? [`--env`, command] : []),
+        ...(mcpServerArgs ? [`--args=${mcpServerArgs.join(" ")}`] : []),
+      ],
+      {
+        env: {
+          ...process.env,
+          PORT: SERVER_PORT,
+          MCP_ENV_VARS: JSON.stringify(envVars),
+        },
+        signal: abort.signal,
+        echoOutput: true,
+      },
+    );
+
+    // Make sure server started before starting client
+    serverOk = await Promise.race([server, delay(2 * 1000)]);
+  } catch (error) {}
+
+  if (serverOk) {
+    try {
+      await spawnPromise("node", [inspectorClientPath], {
+        env: { ...process.env, PORT: CLIENT_PORT },
+        signal: abort.signal,
+        echoOutput: true,
+      });
+    } catch (e) {
+      if (!cancelled || process.env.DEBUG) throw e;
+    }
   }
 
   return 0;

client/bin/cli.js

@@ -16,4 +16,18 @@ const server = http.createServer((request, response) => {
 });
 
 const port = process.env.PORT || 5173;
-server.listen(port, () => {});
+server.on("listening", () => {
+  console.log(
+    `🔍 MCP Inspector is up and running at http://127.0.0.1:${port} 🚀`,
+  );
+});
+server.on("error", (err) => {
+  if (err.message.includes(`EADDRINUSE`)) {
+    console.error(
+      `❌  MCP Inspector PORT IS IN USE at http://127.0.0.1:${port} ❌ `,
+    );
+  } else {
+    throw err;
+  }
+});
+server.listen(port);

client/package.json

@@ -18,7 +18,7 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview",
+    "preview": "vite preview --port 5173",
     "test": "jest --config jest.config.cjs",
     "test:watch": "jest --config jest.config.cjs --watch"
   },

server/src/index.ts

@@ -184,15 +184,15 @@ app.get("/config", (req, res) => {
 
 const PORT = process.env.PORT || 3000;
 
-try {
-  const server = app.listen(PORT);
-
-  server.on("listening", () => {
-    const addr = server.address();
-    const port = typeof addr === "string" ? addr : addr?.port;
-    console.log(`Proxy server listening on port ${port}`);
-  });
-} catch (error) {
-  console.error("Failed to start server:", error);
+const server = app.listen(PORT);
+server.on("listening", () => {
+  console.log(`⚙️ Proxy server listening on port ${PORT}`);
+});
+server.on("error", (err) => {
+  if (err.message.includes(`EADDRINUSE`)) {
+    console.error(`❌  Proxy Server PORT IS IN USE at port ${PORT} ❌ `);
+  } else {
+    console.error(err.message);
+  }
   process.exit(1);
-}
+});