roslyn-mcp

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 a documented two-layer update story for the global tool and the Claude Code plugin.
• Safe install defaults — no ${user_config.*} 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 — pinned to 10.0.100 in global.json (rollForward: latestFeature)

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

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 Preview 6 or later (dnx ships with the SDK).

One-shot smoke test:

dnx Darylmcd.RoslynMcp --yes

The process should start and then appear to hang — that's expected; it's an MCP server waiting for protocol messages on stdin. The --yes flag is mandatory under MCP hosts because there is no TTY for the interactive install-consent prompt.

.mcp.json snippet:

{
  "mcpServers": {
    "roslyn": {
      "type": "stdio",
      "command": "dnx",
      "args": [
        "Darylmcd.RoslynMcp",
        "--source",
        "https://api.nuget.org/v3/index.json",
        "--yes"
      ]
    }
  }
}

Trade-offs vs. the global tool:

• ✅ No PATH pollution; no manual install step.
• ✅ Each cold start resolves to the latest version unless pinned (no dotnet tool update step).
• ⚠️ Cold-start cost on first invocation while the package downloads.
• ⚠️ For reproducible setups, pin the version: add "--version", "1.35.0" to args.

A copy-paste config also lives at docs/mcp-json-examples/dnx.mcp.json.

Option C — Claude Code Plugin

/plugin marketplace add darylmcd/Roslyn-Backed-MCP
/plugin install roslyn-mcp@roslyn-mcp-marketplace

The plugin bundles the MCP server, 32 skills, and safety hooks. For packaging, reinstall, and local plugin-dev details, see docs/setup.md and docs/reinstall.md.

Build And Run From Source

dotnet build RoslynMcp.slnx --nologo
dotnet test RoslynMcp.slnx --nologo
dotnet run --project src/RoslynMcp.Host.Stdio

Per-Client Config

The JSON shape is the same across MCP clients — only the file path differs. Drop one of the docs/mcp-json-examples/ snippets into the right location for your client: