What's the correct way for an LSP server to know whether a command is available on the client?

I'm implementing some refactors where the server needs some input from the user. In order to get input from the user I'm going to need to trigger a command on the client that can ask the user for input (for example, by having a command in the client extension that can call window.showInputBox or some equiv).

However, I can't find a way for the client to advertise (or server to know) which commands are supported on the client.

What's the expected way to handle this? Although I can document the commands that a client needs to implement for specific functionality in the server, that's not very compatible - if I wanted to add new requirements in the future, I'd need to know if the connected client supports them.