Do you have a recommendation for what to do here? I assume the Process.Start actually successfully starts the process, which means the output you're seeing, coming on either stdout or stderr, would need to be interpreted as error information rather than something meaningful from a running server (some of which may redirect their logs to stderr).

Ah, the woes of using STDIO as an RPC channel. I hate it for this reason, and because it's so dang easy for code running in that process to inadvertently pollute the stream by console logging.

At a minimum, I think you should collect the STDERR stream content and include it as a property on the exception you throw so we can share it with the user.

At a minimum, I think you should collect the STDERR stream content and include it as a property on the exception you throw so we can share it with the user.

We could. The reason we haven't is, because stdout can't be used for logging, MCP actually suggests that stderr be used not just for errors but for general purpose logging. It's why examples like in our README:
https://github.com/modelcontextprotocol/csharp-sdk?tab=readme-ov-file#getting-started-server
do things like:

consoleLogOptions.LogToStandardErrorThreshold = LogLevel.Trace;

So we'd be tracking potentially unbounded amounts of state that might then be included in an exception error message. We could try to limit this, maybe putting a limit on how much we store, and only doing so until the initialization handshake has been successfully completed...

(It's also a bit challenging to factor well, as the knowledge of the handshake and the failure happen at the client/session layer while the tracking of stderr happens at the transport layer.)

because stdout can't be used for logging, MCP actually suggests that stderr be used not just for errors but for general purpose logging

gah. Another reason to add to my distaste of STDIO for IPC. (not your fault, I know.)

Can we attach our own logger to stderr of the server then, so we can point the user to the log?

Can we attach our own logger to stderr of the server then, so we can point the user to the log?

The stdio client automatically logs all of the server process' stderr, so if you provide your own logger to the client, you should be getting all of that information already. The challenge I expect would be filtering it in a robust/maintainable manner. We might be able to improve that, though, especially if we can't come up with anything better, e.g. adding a callback to the stdio client constructor that would receive all of the stderr redirected output.

I don't think it would be a bad idea to include the last 10 lines of stderror output in at the end of the "The server shut down unexpectedly" exception message. It might be generic logs, but if the server shutdown unexpectedly, that could still be very useful information.

Furthermore, this should also be an IOException rather than an InvalidOperationException, because the caller didn't necessarily do anything wrong to cause the stdio server to crash before attempting to send a message or response.

I don't think it would be a bad idea to include the last 10 lines of stderror output in at the end of the "The server shut down unexpectedly" exception message

Thoughts on how to structure this? The exception is coming from McpSession.ProcessMessagesAsync, which only has access to an arbitrary ITransport. I don't think we want to hardcode knowledge of that being a stdio transport, which suggests the answer is to expose something from ITransport, which is kind of gross, but I don't have a better answer. Another option would be to expose something from StdioClientTransport, and then type test / special-case in ProcessMessagesAsync, which is also kind of gross.

Another option would be to treat any process exit in StdioClientSessionTransport prior to disposal as ungraceful and complete the ChannelWriter with an exception including the last 10 lines of stderr (and stdout if we haven't gotten past initialization) if any.

I'm a little surprised this isn't how we handled things initially. Not necessarily the including the 10 lines in the exception message part but throwing an exception in the first place. It's not like HTTP where server authors are more likely to want to kill idle sessions to save on backend costs. We could consider only raising an exception for non-zero exit codes if we worried throwing on any early exit will be too noisy, but I don't think it will be. I think we should raise an exception for any early exit and include the status code in the exception message even if it's 0.

And I want to reiterate that it doesn't make sense for StreamClientSessionTransport.SendMessageAsync to throw an InvalidOperationException if it's not connected. You cannot guard against it as a caller. That only makes sense if it was possible to call that method before connecting, but that isn't possible. StreamClientSessionTransport sets IsConnected to true in its constructor. If the transport isn't connected because it's disposed, it should be an ODE. And if it isn't connected because the peer/server disconnected unilaterally, that should be an IOException.