feat: add experimental namespace for URL elicitation

[felixweinberger]

Summary

• Adds .experimental property to Server and Client classes, mirroring Python SDK pattern
• Moves URL elicitation APIs to experimental namespace: server.experimental.elicitUrl(), server.experimental.createElicitationCompleteNotifier(), client.experimental.setElicitationCompleteHandler()
• Adds @modelcontextprotocol/sdk/experimental subpath export for experimental types

Test plan

• All existing tests pass
• Build succeeds
• Lint passes

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1153

commit: bc255ea

[nbarbettini]

Isn't this going to cause a breaking change in the next release of the MCP spec?

Can we achieve the same result with a docstring on the methods themselves, rather than guaranteeing a future breaking change to the SDK interface?

[felixweinberger]

Isn't this going to cause a breaking change in the next release of the MCP spec?

Can we achieve the same result with a docstring on the methods themselves, rather than guaranteeing a future breaking change to the SDK interface?

Yeah this is not a good change - was experimenting with something and didn't mean to publish it (hence a draft).

Just trying to figure out the right way to make these experimental features (tasks and url elicitation) more obvious to developers using them - eg in the python SDK we're introducing an experimental package to make this very explicit modelcontextprotocol/python-sdk#1645

A docstring would probably not get noticed in the same way as an explicit import or separate package, so still thinking through what the right API is here.