Reusable server "specs" - don't re-create the server from scratch for each transport

[rnett]

Is your feature request related to a problem? Please describe.

With how each MCP server instance is limited to a single transport, you need to re-create the server instance (and any components, i.e. tools) for each different transport (and maybe session?). This can get decently expensive, especially if you're creating a bunch of JSON schemas.

Describe the solution you'd like

It would be great to be able to create a "spec" like object, quite like what Server is today, and then create an "instance" for each transport. That way you don't need to re-create all the tools and everything else.

Describe alternatives you've considered

Re-creating the server and any tools for each transport is currently the only alternative.

[devcrocod]

Hi @rnett ,
If I understood your request correctly, this pr should address it #198

[rnett]

Partially, but it also makes it worse. The problem I see with the PR is that the ServerSession is not available to the tool's implementation callback. This is necessary so that that tools can send progress events or elicitations, etc. I think it would be ever harder to get the session now, since you can't just pass the server instance down.

Also, still using () -> Server in the Ktor setup methods seems like a footgun that could still result in creating multiple server instances, IMO they should just take Server.

[rnett]

@devcrocod sorry for the ping, but does my description of the problem make sense? You can try to write a tool that sends notifications, or logging, or creates an elicitation, and you'll see it for yourself. Something like #144 (but updated, and using sessions) would solve it and would be my preferred solution.