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Roslyn Language Server - Copilot Plugin

docs/roslyn-language-server-copilot-plugin.md

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Roslyn Language Server - Copilot Plugin

The roslyn-language-server is a .NET tool that provides C# language intelligence (go-to-definition, find references, diagnostics, etc.) to AI coding agents via the Language Server Protocol (LSP). It is distributed as a plugin through the dotnet/skills marketplace, enabling automatic installation and use in tools like GitHub Copilot CLI.

Quick Start

Install the plugin

In your agent (Copilot CLI, etc.), add the marketplace and install the dotnet plugin:

/plugin marketplace add dotnet/skills
/plugin install dotnet@dotnet-agent-skills

Restart the agent to load the plugin. Once installed, the agent will automatically have access to C# language intelligence for .cs files through the LSP.

What it provides

With the plugin active, the agent gains LSP-powered capabilities for C# code:

  • Go to Definition — navigate to where a symbol is defined
  • Find References — find all usages of a symbol
  • Hover — get type information and documentation
  • Diagnostics — see compiler errors and warnings
  • Document Symbols — list all symbols in a file
  • And more — any LSP feature supported by the Roslyn language server

Prerequisites

  • .NET 10 SDK must be installed and available on PATH. Your project can still use an older, supported SDK.

Automatic Project Loading

The language server automatically discovers and loads projects using the following strategy (evaluated in order):

1. VS Code Settings (dotnet.defaultSolution)

If a .vscode/settings.json file exists in the workspace folder, the server reads the dotnet.defaultSolution setting:

jsonc
// .vscode/settings.json
{
  "dotnet.defaultSolution": "src/MyApp.sln"
}
  • Relative or absolute paths to a .sln or .slnx file are supported.
  • Set to "disable" to prevent the server from loading any solution or projects automatically:
    jsonc
    {
  "dotnet.defaultSolution": "disable"
}

2. Single Solution File at the Root

If there is exactly one .sln or .slnx file at the root of the workspace folder, the server will automatically load that solution.

3. Individual Project Discovery

As a fallback, the server recursively discovers all .csproj files within the workspace folders and loads them individually.

Troubleshooting

Verify LSP configuration is found

  • In Copilot CLI, running the /lsp show should show the server configured for C#:
  Plugin-configured servers:
    • csharp: (.cs) [from dotnet]

Viewing LSP Logs

Copilot CLI writes LSP server logs to the .copilot/logs/ directory in your home folder. To inspect the language server's output:

  • macOS / Linux: ~/.copilot/logs/
  • Windows: %USERPROFILE%\.copilot\logs\

Look for log files related to csharp or roslyn-language-server. These contain the server's startup output, project loading progress, and any errors encountered. This is the first place to check when the language server isn't behaving as expected.

The tool isn't being found

  • Ensure dotnet is on your PATH.
  • Check for a nuget.config (repo, user, or machine-level) that restricts package sources; dnx uses NuGet sources to resolve tools.
  • Verify nuget.org (or an other feed that mirrors roslyn-language-server) is enabled, for example with dotnet nuget list source.
  • Try installing the tool manually: dotnet tool install -g roslyn-language-server --prerelease.

Project load issues

If the LSP logs show failures loading projects

  • Verify that a compatible .NET SDK is installed by running dotnet --version.
  • Check that your project builds successfully with dotnet build before using the language server.
  • For large repositories with multiple solutions, configure dotnet.defaultSolution in .vscode/settings.json to specify which solution to load.

Performance

  • Loading a solution with many projects may take some time. The server reports progress to the client during loading.
  • For large repositories, prefer loading a specific solution (via dotnet.defaultSolution) rather than relying on individual project discovery, which may load test projects and other projects you don't need.

How It Works

The plugin is configured in the dotnet/skills repository via plugins/dotnet/lsp.json:

When the agent opens a workspace containing .cs files, it will:

  1. Install and run the roslyn-language-server .NET tool on-the-fly using dotnet dnx (which downloads and caches the tool automatically; --yes skips confirmation and --prerelease allows prerelease versions).
  2. Communicate over stdio (--stdio) using the Language Server Protocol.
  3. Automatically discover and load projects (--autoLoadProjects) so that the agent immediately has full semantic understanding of the codebase.

Command-Line Options

OptionDescription
--stdioUse stdio for LSP communication (required for most agent integrations)
--autoLoadProjectsAutomatically discover and load projects from workspace folders
--logLevel <level>Minimum log verbosity (default: Information)
--debugLaunch the debugger on startup