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

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

1. Build the solution in Release mode
2. Install the generated .vsix from src/RoslynMcpExtension/bin/Release/
3. 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.

Output Pane

The extension logs all activity to a dedicated "Roslyn MCP Extension" pane in the Visual Studio Output window (View > Output, then select "Roslyn MCP Extension" from the dropdown). This includes:

• Server lifecycle: start, stop, process exit, connection status
• Tool invocations: each MCP tool call with name and elapsed time
• RPC events: client connect/disconnect, pipe errors
• Exceptions: any errors during server startup, command execution, or VS interaction
• MCP Server messages: the HTTP server process logs back to the Output pane via RPC

Configuration

In Tools > Options > Roslyn MCP Extension:

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

Session Recovery

The server supports transparent session migration. If the server is restarted (e.g. via Stop/Start commands or a solution switch), MCP clients with stale session IDs are automatically migrated to a new session — no client restart required.

Connecting MCP Clients

Claude Desktop

Add to your claude_desktop_config.json:

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

VS Code (GitHub Copilot)

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

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

Any MCP Client

The server exposes two transports:

• Streamable HTTP: http://localhost:5050/mcp (recommended for modern MCP clients)
• Legacy SSE: http://localhost:5050/sse (for backward compatibility with older clients)

Dead Code Analysis

roslyn_find_dead_code reports potentially unused methods, fields, and types from the live Visual Studio workspace. It uses Roslyn semantic references plus additional heuristics for framework-driven and runtime-driven code paths that do not always appear as normal source references.

The analysis is intentionally conservative and already filters several common false-positive patterns:

• Test code: xUnit, NUnit, and MSTest attributes such as Fact, Theory, Test, TestCase, TestMethod, DataTestMethod, setup/cleanup attributes, and types that contain or inherit test methods
• Interface contracts: both explicit and implicit interface implementations
• XAML usage: event handlers, code-behind types, attached dependency properties, and parameterless constructors for controls and windows instantiated from .xaml — including base classes whose derived types are XAML-activated
• Visual Studio / MEF composition: Export, Import, ImportingConstructor, and Visual Studio package types decorated with PackageRegistrationAttribute
• MCP tool entry points: methods and types decorated with McpServerToolAttribute or McpServerToolTypeAttribute, which are invoked dynamically by the MCP framework
• Generated and interop code: common generated files, compiler-generated members, and marshaling / StructLayout fields
• Extension patterns: static extension containers, classic this extension methods, and newer C# extension(...) { } blocks
• Inherited test base classes: abstract base types whose derived classes are test containers

Dead-code detection can never be perfect, especially for reflection-heavy or externally activated code, so results should still be reviewed before deletion.

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

Go to the definition of the symbol at line 15, column 10 in C:\MyProject\src\OrderService.cs

List all symbols in C:\MyProject\src\UserService.cs

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

What is the symbol at line 25, column 8 in C:\MyProject\src\OrderService.cs?

Find dead code in the active workspace

Find dead code including public members

How It Differs from Other Roslyn MCP Servers

License

MIT