Looking briefly at the source code, if you get hold of an instance of a BaseLanguageClient, it seems that have

sendRequest

method with flexible params. I think catching these messages should be pretty similar as catching HTTP requests from express (although express is certainly much better documented). IIRC, there was a similar handler on the LSP server side. AFAIK the process of debugging an express-server or lsp-server should be the same (add breakpoint to the server side, initiate from the client as long as the attachment has happened properly). I'd guess that the delegation from extension to lsp to express might be problematic to debug, or at least to establish properly. Anyway, your project and your call how to do it in the end 🙂