C# — LSP setup

Builtin server: csharp — csharp-ls
Extensions: .cs
Install hint: dotnet tool install -g csharp-ls

Install

Requires the .NET SDK. Install the tool globally:

macOS: dotnet tool install -g csharp-ls
Linux: dotnet tool install -g csharp-ls
Windows: dotnet tool install -g csharp-ls

Global .NET tools land in ~/.dotnet/tools — ensure that directory is on PATH (Windows: %USERPROFILE%\.dotnet\tools).

Confirm it resolves:

bash

command -v csharp-ls

Configure

Builtin — usually NO config needed (auto-resolved by extension). Configure only to set priority, init options, override extensions, or disable. Same JSON shape in .codex/lsp-client.json (Codex) AND .opencode/lsp.json (OpenCode/omo):

json

{ "lsp": { "csharp": { "priority": 100 } } }

For builtin ids in a PROJECT config, command is supplied automatically — only set priority/initialization/extensions/disabled/env. A fully custom (non-builtin) server with its own command must go in the USER config (~/.codex/lsp-client.json).

Initialization options (only if commonly needed)

None commonly required. csharp-ls picks up the nearest .sln or .csproj; keep the solution restorable (dotnet restore).

Razor / Blazor

Razor and Blazor files use a separate builtin server:

Builtin server: razor — roslyn-language-server --stdio
Extensions: .razor .cshtml
Install hint: dotnet tool install -g roslyn-language-server --prerelease (requires v5.8.0+; see dotnet/razor)

bash

dotnet tool install -g roslyn-language-server --prerelease
command -v roslyn-language-server

Enable it in a project/user config:

json

{ "lsp": { "razor": { } } }

Alternatives

OmniSharp — legacy C# language server (not builtin). Still works but is being superseded by the Roslyn-based servers; prefer csharp-ls / roslyn-language-server.

Troubleshooting

PATH: csharp-ls / roslyn-language-server on PATH (~/.dotnet/tools); reopen shell after install.
No .NET SDK: install the SDK (not just the runtime) before installing the tool.
No symbols: run dotnet restore; an unrestored solution yields empty results.
Razor needs v5.8.0+: older roslyn-language-server builds lack the --stdio Razor support — install with --prerelease.

Verify

bash

bun ../../scripts/verify-lsp.ts path/to/File.cs