Merge pull request #5834 from continuedev/nate/mcp-debug

show stderr in the error message for failed MCP servers

Author: sestinj

core/context/mcp/MCPConnection.test.ts

@@ -4,7 +4,7 @@ import MCPConnection from "./MCPConnection";
 
 describe("MCPConnection", () => {
   beforeEach(() => {
-    jest.clearAllMocks();
+    jest.restoreAllMocks();
   });
 
   describe("constructor", () => {
@@ -173,11 +173,11 @@ describe("MCPConnection", () => {
     });
 
     it("should handle custom connection timeout", async () => {
-      const conn = new MCPConnection({ ...options, timeout: 11 });
+      const conn = new MCPConnection({ ...options, timeout: 1500 });
       const mockConnect = jest
         .spyOn(Client.prototype, "connect")
         .mockImplementation(
-          () => new Promise((resolve) => setTimeout(resolve, 10)),
+          () => new Promise((resolve) => setTimeout(resolve, 1000)),
         );
 
       const abortController = new AbortController();
@@ -229,6 +229,38 @@ describe("MCPConnection", () => {
       expect(conn.errors[0]).toContain('command "test-cmd" not found');
       expect(mockConnect).toHaveBeenCalled();
     });
+
+    it.skip("should include stderr output in error message when stdio command fails", async () => {
+      // Clear any existing mocks to ensure we get real behavior
+      jest.restoreAllMocks();
+
+      // Use a command that will definitely fail and produce stderr output
+      const failingOptions = {
+        name: "failing-mcp",
+        id: "failing-id",
+        transport: {
+          type: "stdio" as const,
+          command: "node",
+          args: [
+            "-e",
+            "console.error('Custom error message from stderr'); process.exit(1);",
+          ],
+        },
+        timeout: 5000, // Give enough time for the command to run and fail
+      };
+
+      const conn = new MCPConnection(failingOptions);
+      const abortController = new AbortController();
+
+      await conn.connectClient(false, abortController.signal);
+
+      expect(conn.status).toBe("error");
+      expect(conn.errors).toHaveLength(1);
+      expect(conn.errors[0]).toContain("Failed to connect");
+      expect(conn.errors[0]).toContain("Process output:");
+      expect(conn.errors[0]).toContain("STDERR:");
+      expect(conn.errors[0]).toContain("Custom error message from stderr");
+    });
   });
 
   describe.skip("actually connect to Filesystem MCP", () => {

core/context/mcp/MCPConnection.ts

@@ -38,6 +38,10 @@ class MCPConnection {
   public resources: MCPResource[] = [];
   private transport: Transport;
   private connectionPromise: Promise<unknown> | null = null;
+  private stdioOutput: { stdout: string; stderr: string } = {
+    stdout: "",
+    stderr: "",
+  };
 
   constructor(public options: MCPOptions) {
     this.transport = this.constructTransport(options);
@@ -91,6 +95,7 @@ class MCPConnection {
     this.prompts = [];
     this.resources = [];
     this.errors = [];
+    this.stdioOutput = { stdout: "", stderr: "" };
 
     this.abortController.abort();
     this.abortController = new AbortController();
@@ -215,6 +220,20 @@ class MCPConnection {
             }
           }
 
+          // Include stdio output if available for stdio transport
+          if (
+            this.options.transport.type === "stdio" &&
+            (this.stdioOutput.stdout || this.stdioOutput.stderr)
+          ) {
+            errorMessage += "\n\nProcess output:";
+            if (this.stdioOutput.stdout) {
+              errorMessage += `\nSTDOUT:\n${this.stdioOutput.stdout}`;
+            }
+            if (this.stdioOutput.stderr) {
+              errorMessage += `\nSTDERR:\n${this.stdioOutput.stderr}`;
+            }
+          }
+
           this.status = "error";
           this.errors.push(errorMessage);
         } finally {
@@ -284,11 +303,20 @@ class MCPConnection {
           options.transport.args || [],
         );
 
-        return new StdioClientTransport({
+        const transport = new StdioClientTransport({
           command,
           args,
           env,
+          stderr: "pipe",
         });
+
+        // Capture stdio output for better error reporting
+
+        transport.stderr?.on("data", (data: Buffer) => {
+          this.stdioOutput.stderr += data.toString();
+        });
+
+        return transport;
       case "websocket":
         return new WebSocketClientTransport(new URL(options.transport.url));
       case "sse":