C# LSP in Claude Code

The roslyn-language-server is a .NET tool that provides rich language features for C# through the Language Server Protocol. It powers editor integrations including the C# extension for Visual Studio Code and C# Dev Kit. It offers several operations that Claude can use to navigate C# code rather than using grep and friends.

The Roslyn LSP supports many operations defined by the language service protocol, but from what I read only these commands are supported by Claude as of v2.1.62.

• goToDefinition: find where a symbol is defined.
• findReferences: find all references to a symbol.
• hover: get documentation and type info for a symbol.
• documentSymbol: list all symbols in a document.
• workspaceSymbol: find implementations of an interface or abstract method.
• goToImplementation: find implementations of an interface or abstract method.
• prepareCallHierarchy: get call hierarchy item at a position.
• incomingCalls: find all callers of a function/method.
• outgoingCalls: find all functions/methods called by a function.

The instructions below describe how I integrated the roslyn-language-server into Claude Code to give Claude smarter tools for working with C# in the console. My understanding is that Claude in VS Code (especially as part of Copilot) shares VS Code's LSP.

Instructions

I ran these on Windows using Claude Code v2.1.62. The language server is cross platform so it should work on osx or linux. There might be ways to simplify this but this worked so I stopped fiddling with it.

1. Set the env variable ENABLE_LSP_TOOL=1 in ~/.claude/settings.json:

{
    "env": { "ENABLE_LSP_TOOL": "1" }
}

2. Run dotnet tool install --global roslyn-language-server --prerelease. This will put roslyn-language-server.cmd (or .sh?) in your global tools directory (for me, ~/.dotnet/tools/).
3. Create a local marketplace folder. Name/location of the directory doesn't matter, but I used ~/.claude-custom-plugins/.
4. Inside your marketplace folder, create a directory .claude-plugin.
5. In the new directory create marketplace.json with the following content. Note that all slashes should be forward slashes. When providing an absolute path start with / (ex: /Users/{{ Your Name }}. Claude was very finicky about the structure of this document. I went through multiple iterations of this and now that it's working I'm scared to touch it.

{
  "$schema": "https://anthropic.com/claude-code/marketplace.schema.json",
  "name": "local",
  "metadata":{
    "description": "Local plugins for development and testing.",
    "version": "1.0.0",
    "license": "MIT"
  },
  "owner": {
    "name": "{{ Your Name }}"
  },
  "plugins": [
    {
      "name": "roslyn-ls",
      "version": "1.0.0",
      "source": "./plugins/roslyn-ls",
      "description": "LSP using roslyn-language-service.",
      "category": "development",
      "author": { "name": "{{ Your Name }}" },
      "tags": [ "csharp" ],
      "lspServers": {
        "roslyn-ls": {
          "command": "{{Path to the .NET tools folder}}/roslyn-language-server.cmd",
          "args": [
            "--stdio",
            "--autoLoadProjects",
            "--logLevel",
            "Information",
            "--extensionLogDirectory",
            "{{ any directory}}/roslyn-language-service/logs"
          ],
          "transport": "stdio",
          "extensionToLanguage": {
            ".cs": "csharp",
            ".csx": "csharp",
            ".cshtml": "csharp"
          },
          "initializationOptions": {},
          "settings": {},
          "startupTimeout" : 120000
        }
      }
    }
  ]
}

6. Inside your custom plugins directory, next to the .claude-plugin folder, create the directories plugins/roslyn-ls/ (this must match the source in the marketplace.json file).
7. Create the JSON files .lsp.json and .claude-plugin/plugin.json. Your final directory structure should look like this:

{{ custom plugins path }}/
|-- .claude-plugin/
|   ʟ marketplace.json
ʟ-- plugins/
    ʟ-- roslyn-ls/
        |-- .lsp.json
        ʟ-- .claude-plugin/
            ʟ plugin.json

8. The content of .claude-plugin/plugin.json should look like the JSON below. I don't know if all these are necessary, but it at least works for me. I also think these values need to match their respective properties in marketplace.json.

{
  "name": "roslyn-ls",
  "description": "LSP using roslyn-language-service.",
  "version": "1.0.0",
  "license": "MIT",
  "author": {
    "name": "{{ Your name }}"
  }
}

9. The content of .lsp.json properties under the csharp peroperty should match the properties in the roslyn-ls object in marketplace.json.

{
  "csharp": 
  {
    "command": "{{Path to the .NET tools folder}}/roslyn-language-server.cmd",
    "args": [
        "--stdio",
        "--autoLoadProjects",
        "--logLevel",
        "Information",
        "--extensionLogDirectory",
        "{{ any directory}}/roslyn-language-service/logs"
    ],
    "transport": "stdio",
    "extensionToLanguage": {
    ".cs": "csharp",
    ".csx": "csharp",
    ".cshtml": "csharp"
    },
    "initializationOptions": {},
    "settings": {},
    "startupTimeout" : 120000
  }
}

10. Start claude and run the command /plugin marketplace add {{ absolute path to custom plugins folder }}. Ex. for me it was: /plugin marketplace add /Users/{{me}}/.claude-custom-plugins.
11. Exit claude and run claude plugin install -s user roslyn-ls.
12. In claude you can verify the LSP is working with these two commands:
    • /plugin -> select the Installed tab, and you should see roslyn-ls Plugin · local · ✔ enabled.
    • Tell claude to run the C# LSP command documentSymbols on {{ path to some C# file }}. You should see a line like ● LSP(operation: "documentSymbol", file: "src\Vendors\IVendor.cs")

You should be all set!