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C# — cs-direct

Proxies csharp-ls over HTTP. One server per workspace. Handles .cs, .csx.

Install prereq

dotnet tool install -g csharp-ls

The binary lands at ~/.dotnet/tools/csharp-ls. Ensure that directory is in PATH.

Verify: csharp-ls --version (should print csharp-ls, X.Y.Z.W).

Workspace markers (walk-up order)

  1. .slnx (new XML-based solution format, SDK 9+)
  2. .sln
  3. .csproj

Invocation

cs-direct start                                                  # cwd walk-up
cs-direct call textDocument/documentSymbol \
  '{"textDocument":{"uri":"file:///abs/path/to/File.cs"}}'

cs-direct call textDocument/definition \
  '{"textDocument":{"uri":"file:///abs/path/to/File.cs"},
    "position":{"line":25,"character":15}}'

cs-direct call workspace/symbol '{"query":"IUserService"}'

The rootUri-at-init fix

csharp-ls binds rootUri at the initialize handshake and cannot change it. Starting csharp-ls from a cwd outside the .sln/.csproj ancestor means the server loads an empty workspace and every query returns "no symbols found" — a hard usability bottleneck for agentic clients switching between multiple .NET projects.

cs-direct solves this by spawning ONE csharp-ls PER WORKSPACE. Each workspace hash slot gets its own server with the correct rootUri. Switching between projects mid-session is free — just pass the workspace arg, the matching slot gets created on first call.

Quirks

  • MSBuild BuildHost pipe failure (.NET 10 macOS): csharp-ls uses MSBuild's out-of-process BuildHost to load projects. On .NET SDK 10.0.x on macOS, the NamedPipeClientStream.ConnectAsync call fails, and cs-direct returns result: null for every query despite a live server. See docs/troubleshooting.md for the workaround (pin to .NET 9.x SDK or wait for a csharp-ls fix). Not a wrapper bug.
  • Cold start: csharp-ls indexes the full solution on first query. 30-120s is normal for mid-size .NET projects. Warm calls are sub-100ms.
  • Multi-solution workspaces: walk-up stops at the FIRST .slnx/.sln/.csproj. For a workspace with nested solutions, pass the intended one explicitly: 
    cs-direct start /abs/path/to/desired-solution-dir
  • initializationOptions.csharp.solutionPathOverride: csharp-ls accepts this to pick a specific solution when multiple exist under rootUri. cs-direct doesn't wire it through currently — open an issue if you need it.

Timing

  • Cold: 30-120s (MSBuild solution load, NuGet restore if needed)
  • Warm: ~70ms per call

State directory

~/.cache/cs-direct/<workspace-hash>/{pid,port,workspace,log}

Inspect log if cold start hangs past 180s (the coordinator's own timeout) — MSBuild issues surface there.

Roslyn LS scaffold (experimental, not yet wired)

bin/cs-roslyn-direct exists as a parallel wrapper targeting Microsoft Roslyn Language Server (the binary shipped with the VS Code C# Dev Kit at ~/.vscode/extensions/ms-dotnettools.csharp-*/.roslyn/Microsoft.CodeAnalysis.LanguageServer) instead of csharp-ls. Goal: ~10× cold-start improvement on documentSymbol for mid-size .NET projects. Currently NOT installed by scripts/install.sh because the integration is blocked.

Blocker

Roslyn LS issues a server→client workspace/configuration request during OnInitializedAsync. bin/lsp-stdio-proxy.js + bin/adapters/lsp-stdio.js + bin/tool-server-proxy.js are uni-directional (client→server only). Without a response, Roslyn LS hits Contract.Fail in DidChangeConfigurationNotificationHandler.cs:128 and SIGABRTs. csharp-ls works because it does NOT issue reverse-RPC.

Resumption path

Extend bin/adapters/lsp-stdio.js to detect server-initiated requests (incoming JSON-RPC with method+id but no result/error). Inject canned responses for known reverse-RPCs:

  • workspace/configuration → return [{}] per item in params.items (empty config object array)
  • client/registerCapability → return null (accept all dynamic registrations)
  • window/workDoneProgress/create → return null

Alternative: forward server-initiated requests to a side-channel HTTP endpoint so the wrapper or upstream caller can respond.

After the proxy supports reverse-RPC, wire cs-roslyn-direct into scripts/install.sh as the recommended C# wrapper, deprecate cs-direct. Benchmark target: csharp-ls measured at ~2.18s on documentSymbol warm-call against a single-file project; Roslyn LS expected to land sub-300ms based on VS Code C# Dev Kit characteristics.