Invalid MCP Server instantiation fails poorly

[domesticmouse]

I was attempting to run an MCP server incorrectly. It exited with an error message. This error message was not surfaced by the MCP Client sample code.

$ dart run bin/mcp_client.dart
connecting to server
server started
initializing server
Unhandled exception:
Bad state: The client closed with pending request "initialize".

The simplified code is as follows. For context, the mcp_server requires a command line argument that is not supplied here.

import 'package:dart_mcp/client.dart';

void main() async {
  final client = MCPClient(
    ClientImplementation(name: 'example dart client', version: '0.1.0'),
  );
  print('connecting to server');
  final server = await client.connectStdioServer('dart', [
    'run',
    '../mcp_server/bin/mcp_server.dart',
  ]);
  print('server started');

  print('initializing server');
  final initializeResult = await server.initialize(
    InitializeRequest(
      protocolVersion: protocolVersion,
      capabilities: client.capabilities,
      clientInfo: client.implementation,
    ),
  );
  print('initialized: $initializeResult');
  if (initializeResult.protocolVersion != protocolVersion) {
    throw StateError(
      'Protocol version mismatch, expected $protocolVersion, '
      'got ${initializeResult.protocolVersion}',
    );
  }

  await client.shutdown();
}

For context, attempting to run the server incorrectly failed as follows:

$ dart run ../mcp_server/bin/mcp_server.dart
Usage: dart mcp_server.dart <path to project>

Can we please expose this error text as part of the bad state exception?

[jakemac53]

#4 is likely related to this as well

[jakemac53]

In this case, the first message from the server is an invalid one (not JSON), I am a bit surprised we aren't seeing some other sort of error bubble up here.

[jakemac53]

Ok, so json_rpc_2 automatically handles invalid messages by sending a notification message back to the server that it had sent an invalid request.

So, the client never sees this message and it sends an initialize request. The server then exits, and then we get the error about the connection closing with the pending initialize request.

We don't really have a good opportunity to see the actual bad output at all because it's intercepted by the json_rpc_2 package, we may want to add something to that package to allow you to see these.

I did recently resolve #4 which allows you to see all protocol traffic which should help generally resolve this, but you have to explicitly pass a sink on which to emit these messages.

[domesticmouse]

Silly question, what happens with stderr for commands? I'm wondering if I put out the usage instructions to stderr instead of stdout will that work better?

[jakemac53]

Silly question, what happens with stderr for commands? I'm wondering if I put out the usage instructions to stderr instead of stdout will that work better?

Sorry I missed this reply - yes MCP servers talking over stdio in general should output anything that isn't JSON to stderr instead of stdout. Clients may or may not do anything with it, but at least they are less likely to get broken. And a good client would do something with it - especially if the connection failed.