(Posting from Discord for tracking purposes.)

The server fails to start if there's no root directory provided in the initialization parameters:

ruff failed
  Cause: No workspace or root URI was given in the LSP initialization parameters. The server cannot start.

We default to the current working directory if that's not provided in ruff-lsp: https://github.com/astral-sh/ruff-lsp/blob/c410d6b44c1b40a41d254d13e33535bd198935e0/ruff_lsp/server.py#L1656-L1664

/cc @snowsignal