[BUG] Omisharp LSP fails to attach

[Houdiee]

Can't get C# lsps to work no matter what. It used to work on stable version but no longer works when I switched to unstable.

Field	Description
Plugin	lsp.servers.omnisharp
Plugin	lsp.servers.csharp_ls
Nixpkgs	unstable
Home Manager	unstable

• I have read the FAQ and my bug is not listed there.

Description

Setting lsp.servers.omnisharp.enable = true or lsp.servers.csharp_ls.enable = true will load the lsp client, but no matter wether or not I have dotnet 8 or dotnet 9 (or both) installed, i get the following error from LspLog:

[ERROR][2025-04-25 13:00:26] ...p/_transport.lua:36	"rpc"	"csharp-ls"	"stderr"	"You must install or update .NET to run this application.\n\nApp: /nix/store/gc9as1mb01l7pjzc0dksfg3smlmsjds8-csharp-ls-0.16.0/lib/csharp-ls/csharp-ls\nArchitecture: x64\nFramework: 'Microsoft.NETCore.App', version '8.0.0' (x64)\n.NET location: /nix/store/hzdqc45clz93vdrhwmdd87zc92r2bwwc-dotnet-sdk-9.0.105/share/dotnet\n\nThe following frameworks were found:\n  9.0.4 at [/nix/store/hzdqc45clz93vdrhwmdd87zc92r2bwwc-dotnet-sdk-9.0.105/share/dotnet/shared/Microsoft.NETCore.App]\n\nLearn more:\nhttps://aka.ms/dotnet/app-launch-failed\n\nTo install missing framework, download:\nhttps://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=8.0.0&arch=x64&rid=linux-x64&os=nixos.25.05\n"
[ERROR][2025-04-25 13:00:27] ...p/_transport.lua:36	"rpc"	"dotnet"	"stderr"	"Could not execute because the specified command or file was not found."
[ERROR][2025-04-25 13:00:27] ...p/_transport.lua:36	"rpc"	"dotnet"	"stderr"	"\n"
[START][2025-04-25 13:05:34] LSP logging initiated
[ERROR][2025-04-25 13:05:34] ...p/_transport.lua:36	"rpc"	"csharp-ls"	"stderr"	"You must install or update .NET to run this application.\n\nApp: /nix/store/gc9as1mb01l7pjzc0dksfg3smlmsjds8-csharp-ls-0.16.0/lib/csharp-ls/csharp-ls\nArchitecture: x64\nFramework: 'Microsoft.NETCore.App', version '8.0.0' (x64)\n.NET location: /nix/store/hzdqc45clz93vdrhwmdd87zc92r2bwwc-dotnet-sdk-9.0.105/share/dotnet\n\nThe following frameworks were found:\n  9.0.4 at [/nix/store/hzdqc45clz93vdrhwmdd87zc92r2bwwc-dotnet-sdk-9.0.105/share/dotnet/shared/Microsoft.NETCore.App]\n"
[ERROR][2025-04-25 13:05:34] ...p/_transport.lua:36	"rpc"	"csharp-ls"	"stderr"	"\nLearn more:\nhttps://aka.ms/dotnet/app-launch-failed\n\nTo install missing framework, download:\nhttps://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=8.0.0&arch=x64&rid=linux-x64&os=nixos.25.05\n"
[ERROR][2025-04-25 13:05:34] ...p/_transport.lua:36	"rpc"	"dotnet"	"stderr"	"Could not execute because the specified command or file was not found.\n"

Minimal, Reproducible Example (MRE)

# nixvim.nix:

  csharp_ls.enable = true;
  omnisharp = {
    enable = true;
    cmd = [
      # "${pkgs.dotnetCorePackages.dotnet_9.sdk}/bin/dotnet" also tried doing this but didnt work
      "dotnet"
      "${pkgs.omnisharp-roslyn}/bin/OmniSharp" 
    ];
  };


# configuration.nix:
  environment.systemPackages = with pkgs; [
    dotnetCorePackages.dotnet_9.sdk
    dotnetCorePackages.dotnet_8.sdk
  ];

Any help would be very much appreciated!

[ddogfoodd]

might be related to #3211

[IMax153]

I'm also experiencing a similar issue with the ESLint language server after a recent nix flake update. The ESLint server is detected by the LSP but never becomes active.

In the LSP logs the only error I see is:

[ERROR][2025-04-25 06:54:21] .../lua/vim/lsp.lua:455	"cannot start eslint due to config error: ...ovim-unwrapped-0.11.0/share/nvim/runtime/lua/vim/lsp.lua:444: cmd: expected expected function or table with executable command, got nil"

And the only configuration I have for the ESLint language server is to enable it:

plugins.lsp.servers.eslint.enable = true;

[MattSturgeon]

might be related to #3211

If you can test disabling performance.combinePlugins, that would help us identify whether this is a duplicate of #3211

[IMax153]

If you can test disabling performance.combinePlugins, that would help us identify whether this is a duplicate of #3211

In my case, setting:

  performance.combinePlugins = {
    enable = false;
  };

does not fix the issue. The ESLint Language Server still fails to attach with the same error I posted above.

[MattSturgeon]

cmd: expected expected function or table with executable command, got nil"

Hm that's odd. Definitely a different issue to the one originally reported here.

Can you inspect the generated lua (run nixvim-print-init in a terminal)?

:checkhealth vim.lsp may have some info too.

[MattSturgeon]

Looks like esline only got support for the new api ~1week ago (neovim/nvim-lspconfig#3731).

Just a guess: if you're setting inputs.nixvim.inputs.nixpkgs.follows then you may be overriding nixvim's nixpkgs instance to an older version with an older nvim-lspconfig?

[IMax153]

you may be overriding nixvim's nixpkgs instance

I feel quite dumb at the moment. I was not overriding Nixvim's nixpkgs via inputs.nixvim.inputs.nixpkgs.follows (I bootstrapped a long time ago from the flake template). However, on closer inspection of my flake.nix, I realize I've had an inherit pkgs; inside my nixvimModule definition which was using nixpkgs-unstable.

Even after fixing that issue, the problem with the ESLint LSP persists.

Checking :checkhealth vim.lsp, I can see that the ESLint LSP is registered, just not attached to any of the buffers I open. On closer inspection, it seems like the reported ESLint LSP configuration does not have anything specified other than capabilities (i.e. no cmd, on_attach, etc).

Inspecting the init.lua via nixvim-print-init shows that eslint is getting added to the list of LSPs to register - this is the only configuration I see:

local __lspServers = {
  -- ... other LSPs
  { name = "eslint" },
  -- ... other LSPs
}

[MattSturgeon]

  environment.systemPackages = with pkgs; [
    dotnetCorePackages.dotnet_9.sdk
    dotnetCorePackages.dotnet_8.sdk
  ];

@Houdiee

Just a guess: I wonder if your issue is related to installing multiple dotnet SDKs? does installing only one help? How about using the dotnetCorePackages.combinePackages function?

[MattSturgeon]

Checking :checkhealth vim.lsp, [...] it seems like the reported ESLint LSP configuration does not have anything specified other than capabilities (i.e. no cmd, on_attach, etc).

This seems to imply nvim-lspconfig isn't defining the defaults, or they're being overridden somewhere (e.g. vim.lsp.config['eslint'] = {} would reset; vim.lsp.config('eslint', {} is the correct way to merge configs).

[Houdiee]

@MattSturgeon
Unfortunately having 2 sdks isn't the issue. I had 1 originally and only resorted to 2 in an attempt to fix this

[MattSturgeon]

Can anyone affected share the output of nixvim-print-init (in most shells you can pipe the output to a file for easy copy-pasting nixvim-print-init > init.lua)

Also (perhaps more importantly) can you share the output of :checkhealth vim.lsp? This will open in a new buffer, so you can either copy with gg "+yG or just :w lsp-health.txt it.

If anyone wants to dig into their rtp and see what plugins/files are loaded, nixvim doesn't offer a good way to do this, however: you can inspect your rtp in vim with :set rtp, this will print a comma-separated list of directories. It may be a long list, most of the directories are likely packages in the /nix/store. I don't have a good suggestion for viewing the content of these directories other than manually spot-checking them, but you can use enew and execute to open the output in a new buffer (:enew|pu=execute('set rtp')).

If you know how to access/inspect your nixvim configuration, you may also want to inspect options like extraPlugins. If you use standalone nixvim, you can read <nvim-package>.config.extraPlugins. If you use a nixvim module, you can read <configuration>.config.programs.nixvim.extraPlugins.

Additionally, verifying the performance.combinePlugins option may be relevant.

---

It seems there may be multiple underlying issues that cause LSPs to not work. For example, @IMax153 seems to not have nvim-lspconfig's server config defaults (so cmd is nil) while @Houdiee seems to have a different issue where vim's LSP is running fine, but dotnet itself is having issues (You must install or update .NET to run this application).

I'll open new sub-issues and cc the relevant users shortly.