Expected usage for concurrent calls

[goelhardik]

I am trying to make multiple parallel calls to an MCP server over stdio transport. But that seems to throw an error sometimes. Should I instantiate a new client (and therefore server) for each request?

This is the error when reusing the same client object returned by McpClientFactory.CreateAsync() for multiple calls:

System.InvalidOperationException
The stream is currently in use by a previous operation on the stream.
   at System.IO.StreamWriter.ThrowAsyncIOInProgress()
   at System.IO.StreamWriter.WriteLineAsync(ReadOnlyMemory`1 buffer, CancellationToken cancellationToken)
   at ModelContextProtocol.Protocol.Transport.StdioClientTransport.SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken)

[stephentoub]

It's possibly broken right now; we'll take a look. We should make this safe to do.

[goelhardik]

Just fyi, I had a unit test to make 20 concurrent calls to the server to list tools and it kept failing randomly for a few hours. But it has been stable since then without more changes. I am not sure why. I will let you know if I repro it again.

Anyway if this issue doesn't happen, it sounds like reusing the same client is recommended - there shouldn't be any multi-threading concerns?

[PederHP]

Just fyi, I had a unit test to make 20 concurrent calls to the server to list tools and it kept failing randomly for a few hours. But it has been stable since then without more changes. I am not sure why. I will let you know if I repro it again.

Anyway if this issue doesn't happen, it sounds like reusing the same client is recommended - there shouldn't be any multi-threading concerns?

In terms of the spec, for Stdio servers, there's a functional difference between reusing the client or not. Each client scopes the lifetime of a server instance. This could be useful for having multiple instances with different env arguments (for some servers this makes them point at different integration points).

In practice parallel calls are usually parallel tool (execution) calls. This is a valid use case against a single server and should work.