Handling of stderr on STDIO servers

[cliffhall]

Description

• Remove handling of out-of-spec notification/stderr messages. It's not a thing. See Remove out-of-specification sending of notifications/stderr servers#2469

• Inspect the stderr output of STDIO servers and attempt to assign an appropriate RFC 5424 Syslog Protocol level before sending as leveled logging message to the client, as per MCP Logging Spec.

Motivation and Context

A recent change to the server-everything reference server raised the point that notification/stderr is not a valid MCP message type. It was only added to the server because handling for it existed in the Inspector, added some time ago.

Clearly, handling for this message type needs to also be removed from the Inspector so as not to confuse developers who may come across it as I myself did, leading to my erroneously adding the periodic sending of notification/stderr messages from server-everything.

As far as I can tell, it only existed as a way to indicate that the message came from thestderr output of a STDIO server, so that it could be handled differently from other logging messages, placing the output prominently in the sidebar in red to indicate error. However, another recent issue highlights that this is not a valid approach, since all output, even innocuous server welcome messages must be sent via stderr in STDIO servers.

Therefore, I have removed the handling of this out-of-spec message type and am sending all stderr output to the client as MCP compliant logging messages. I attempt to set an appropriate logging level by inspecting the contents of the message and searching for levels. So if a STDIO server output the following line on stderr:

[info] The fetch tool was used.

...the following message would be sent to the client...

{
  "jsonrpc": "2.0",
  "method": "notifications/message",
  "params": {
    "level": "info",
    "logger": "stdio",
    "data": {
      "message": "[info] The fetch tool was used",
    },
  },
}

How Has This Been Tested?

Using an STDIO server that outputs a welcome message on stderr

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• [] I have added or updated documentation as needed

Additional context

[olaservo]

Tested with everything-server and saw notifications logged but no noise in the sidebar: