Roslyn MCP server

Roslyn-Backed MCP Server

Local-first MCP (Model Context Protocol) server for semantic C# analysis, navigation, validation, and refactoring on real .sln / .slnx / .csproj workspaces. It uses Roslyn and MSBuildWorkspace, runs over stdio, and does not require Visual Studio.

Correct package ID: Darylmcd.RoslynMcp  ·  CLI: roslynmcp  ·  Plugin: roslyn-mcp@roslyn-mcp-marketplace

What It Does

• Loads real C# solutions and projects with session-scoped workspaceIds.
• Exposes semantic navigation, diagnostics, build/test helpers, and preview/apply refactoring workflows over MCP.
• Ships as a .NET global tool, a Claude Code plugin, and a source-buildable stdio host.
• Publishes the authoritative live surface through server_info and roslyn://server/catalog.

Why Roslyn-Backed MCP

• No Visual Studio dependency — runs anywhere the .NET SDK runs (Windows, macOS, Linux, containers, CI).
• Production ops discipline — repeatable CI mirror (just ci), release verification scripts, and documented update paths for the global tool and release-pinned Claude Code plugin.
• Safe install defaults — no ${userconfig.} placeholder substitution that breaks prompt-skipping install flows; the server starts with compiled-in defaults and accepts literal overrides via project-scope .mcp.json.
• Authoritative live surface — every release publishes server_info + roslyn://server/catalog so clients can discover the exact tool/resource/prompt set and support tier (stable vs experimental) without guessing.
• Preview → apply discipline — refactoring tools issue preview tokens with TTLs and a verify step before mutating the workspace, so agents can dry-run multi-file edits.
• Three install paths — pick one: global tool (dotnet tool install), zero-install via dnx (.NET 10), or the Claude Code plugin.

Quick Start

Prerequisites

• .NET 10 SDK — source builds use the 10.0.400 floor in global.json (rollForward: latestFeature); published install paths retain their own package/runtime requirements

Option A — Install As A Global Tool

dotnet tool install -g Darylmcd.RoslynMcp

• Package ID: Darylmcd.RoslynMcp
• CLI command: roslynmcp
• Updates: dotnet tool update -g Darylmcd.RoslynMcp

Configure a filesystem boundary before first use. The global tool ships a binary, not a

config — you write your own .mcp.json, and an unset boundary is fail-closed: every

path-taking tool (workspace_load, edits, symbol lookups) rejects its input, and solution

discovery returns nothing. Set ROSLYNMCPSANCTIONEDROOTS in your client config:

json

{

"mcpServers": {

"roslyn": {

"type": "stdio",

"command": "roslynmcp",

"env": { "ROSLYNMCPSANCTIONEDROOTS": "." }

}

}

}

. resolves against the server process's working directory, which your MCP client chooses —

use an absolute path if you want the boundary pinned regardless of how the server is launched.

See Configuration. The Claude Code plugin and Desktop extension ship this

default already; only hand-written configs need it.

Option B — Zero-Install Via dnx (.NET 10)

dnx is the .NET SDK's npx-equivalent: it resolves a tool package from NuGet on demand, without installing a global shim. Requires .NET 10 SDK 10.0.100 or later (dnx ships with the SDK).

One-shot smoke test:

dnx Darylmcd.RoslynMcp

Shortened. The full README is on GitHub.