Helix

Register nuten lsp as a language server in languages.toml and format on save. Helix uses the nuten on the PATH, so install nuten first.

[language-server.nuten]
command = "nuten"
args = ["lsp"]

[[language]]
name = "nu"
language-servers = ["nu-lsp", "nuten"]
auto-format = true

Since language-servers replaces Helix's default list, keep nu-lsp, the Nushell language server, in it. nu-lsp does not support formatting, so Nuten does the formatting.

To use a nuten that is not on the PATH, give its path in command.

Nix

With home-manager, write the contents of languages.toml in programs.helix.languages.

{
  programs.helix = {
    enable = true;
    languages = {
      language-server.nuten = {
        command = "nuten";
        args = [ "lsp" ];
      };
      language = [
        {
          name = "nu";
          language-servers = [ "nu-lsp" "nuten" ];
          auto-format = true;
        }
      ];
    };
  };
}