Can't configure my custom lsp server on NixOS

[wavesinaroom]

Hi!

I'm working on a lsp for a music language called CSound as a personal project. Since I'm working on Nixos I coded a nix expression to create my binary file and access it through a devshell. The expression builds the binary without any issue but I struggle with setting up Helix even when running which cslsp outputs a path to a nix store binary. How can I configure Helix to run this binary?

languages.toml

[[language]]
name = "csound"
scope = "source.csound"
file-types = ["csd", "sco", "orc"]
language-servers = ["cslsp"]

My log:

2026-07-26T01:21:17.506 helix_view::editor [ERROR] Failed to initialize the language servers for `source.csound` - `cslsp` { Language server 'cslsp' not defined }

Where does Helix look up for language servers on NixOS if not in path? Should I put my binary somewhere else in my system?

[farfromexact]

language-servers = ["cslsp"] only references a server name; it does not define the command that Helix should start. Add a language-server definition alongside your language declaration:

[language-server.cslsp]
command = "cslsp"
# args = ["..."]  # only if your server needs arguments

[[language]]
name = "csound"
scope = "source.csound"
file-types = ["csd", "sco", "orc"]
language-servers = ["cslsp"]

You can put this in the project .helix/languages.toml or your user languages.toml. Helix resolves command using the environment of the Helix process, so which cslsp must succeed in the same nix develop/nix-shell environment from which you launch Helix. If you start Helix from the desktop, it may not inherit the dev-shell PATH; in that case either launch it from the shell or use the absolute /nix/store/.../bin/cslsp path.

The error says the language server is ?not defined?, which is different from ?executable not found?: the missing piece is the [language-server.cslsp] table first. The relevant Helix configuration reference is https://docs.helix-editor.com/languages.html.

[wavesinaroom]

Hi @farfromexact! Thanks a lot for taking time. Usually I look up for a official language configuration to add to my languages.toml file so this is my first time. It made total sense to when I tried the missing piece you suggested. Basically adding language-servers isn't enough because that only works as a declaration but there's no implementation so to say.

Have a good one!

[farfromexact]

no problem~ :D