Roslyn MCP Extension — Visual Studio Extension

A Visual Studio extension that exposes semantic C# code analysis via the Model Context Protocol (MCP), powered by the live Roslyn workspace inside Visual Studio.

Unlike standalone Roslyn MCP servers that create their own MSBuildWorkspace, this extension uses Visual Studio's actual VisualStudioWorkspace — giving you access to unsaved changes, live diagnostics, and the full compilation state that VS already maintains.

MCP Tools

Tool	Description
`roslyn_validate_file`	Get live diagnostics (errors, warnings, analyzer results) for a C# file
`roslyn_find_references`	Semantic find-all-references using `SymbolFinder` (not text search)
`roslyn_go_to_definition`	Navigate to symbol definition via Roslyn semantic model
`roslyn_get_document_symbols`	List all symbols in a file with types, modifiers, and spans
`roslyn_search_symbols`	Search for symbol declarations across the solution by name
`roslyn_get_symbol_info`	Get detailed type information, base types, interfaces, parameters

Prerequisites

Visual Studio 2022 or 2026
.NET 10.0 SDK (for the server process)

Building

The solution file is at src/RoslynMcpExtension.slnx. Since the VSIX project requires MSBuild, build via Visual Studio or msbuild:

# Full solution (requires Visual Studio / MSBuild)
msbuild src\RoslynMcpExtension.slnx

# Server and Shared projects only (dotnet CLI)
dotnet build src\RoslynMcpExtension.Server\RoslynMcpExtension.Server.csproj

The VSIX project automatically publishes the MCP server process to its output directory.

Installation

Build the solution in Release mode
Install the generated .vsix from src/RoslynMcpExtension/bin/Release/
Restart Visual Studio

Usage

Starting the Server

The server auto-starts when a solution is loaded (configurable in Tools > Options > Roslyn MCP Extension).

You can also manually start/stop via Tools > Start/Stop Roslyn MCP Server.

Configuration

In Tools > Options > Roslyn MCP Server:

Port: HTTP port (default: 5050)
Server Name: Name shown to MCP clients
Auto Start: Start automatically when a solution loads

Connecting MCP Clients

Claude Desktop

Add to your claude_desktop_config.json:

{
  "mcpServers": {
    "roslyn-mcp": {
      "url": "http://localhost:5050/sse"
    }
  }
}

VS Code (GitHub Copilot)

Add to your .vscode/mcp.json or user settings:

{
  "servers": {
    "roslyn-mcp": {
      "url": "http://localhost:5050/sse"
    }
  }
}

Any MCP Client

Connect to http://localhost:5050/sse using HTTP/SSE transport.

Example Prompts

Validate the file C:\MyProject\src\UserService.cs for errors and warnings

Find all references to the method ProcessOrder in C:\MyProject\src\OrderService.cs at line 42, column 20

Search for all symbols named "Repository" in the current solution

How It Differs from Other Roslyn MCP Servers

Feature	This Extension	Others roslyn/mcp servers
Workspace	Live VS `VisualStudioWorkspace`	Standalone `MSBuildWorkspace`
Unsaved changes	✅ Sees current editor state	❌ Only saved files
Find References	✅ Semantic `SymbolFinder`	❌ Text search or separate workspace
Diagnostics	✅ Live from VS compiler	⚠️ Re-compiled separately
Build integration	✅ Uses VS compilation state	❌ Separate compilation
Setup	Install VSIX, no config needed	Configure solution path per project

License

MIT

Roslyn MCP Extension

Sebastien Lebreton