SunNull.RoslynMcpServer 0.2.0

Roslyn MCP Server

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A .NET 11 MCP (Model Context Protocol) server that exposes Roslyn compiler analysis to AI coding agents. Gives any MCP-compatible client (Reasonix, Claude Desktop, Cursor, VS Code Copilot) real-time C# code intelligence — diagnostics, type info, references, code fixes — without running dotnet build.

Why?

Normal AI coding workflow for C#:

write code → dotnet build → see errors → fix → rebuild → ...

With Roslyn MCP Server:

write code → roslyn_csharp_diagnostics → instant feedback (like Visual Studio's red squiggles)

The agent gets compiler-grade analysis in real time, powered by the same Roslyn engine that drives Visual Studio.

18 Tools (C# only)

All tools are prefixed roslyn_csharp_ to make it explicit they only analyze C# files (.cs and .csx). Calling them on non-C# files returns a clear error.

Quick Install

Option 1: dotnet tool (recommended, cross-platform)

The simplest way — one command, auto-added to PATH, no manual binary download:

dotnet tool install -g roslyn-mcp-server

Requires the .NET SDK (the tool runs on your installed runtime). After install, roslyn-mcp-server is on your PATH.

Then register with your MCP client:

Reasonix (reasonix.toml):

[[plugins]]
name    = "roslyn"
command = "roslyn-mcp-server"

Claude Code (.mcp.json):

{
  "mcpServers": {
    "roslyn": { "command": "roslyn-mcp-server" }
  }
}

Upgrade later: dotnet tool update -g roslyn-mcp-server.

Option 2: Install script (pre-built binary, no SDK needed)

Prerequisites

• None! Pre-built binaries are downloaded automatically. No .NET SDK needed.
• Reasonix (or any MCP client)

One-command install (Reasonix)

Linux/macOS:

curl -sL https://raw.githubusercontent.com/SunNull/roslyn-mcp-server/main/install.sh | bash
# 或 clone 后运行：
# ./install.sh                    # 下载预编译二进制 + 注册

Windows:

# 下载 install.bat 并运行，或 clone 后：
install.bat

The script downloads a self-contained binary (no .NET SDK needed) from GitHub Releases, then runs reasonix mcp add roslyn ... to register it.

Fallback: if no pre-built binary matches your platform, the script compiles from source (requires .NET SDK) with --from-source.

Manual install (any MCP client)

1. Download the archive for your platform from Releases
2. Unzip to any directory
3. Add to your MCP client's config:

Reasonix (reasonix.toml):

[[plugins]]
name    = "roslyn"
command = "/path/to/roslyn-mcp-server"

Claude Code (.mcp.json):

{
  "mcpServers": {
    "roslyn": {
      "command": "/path/to/roslyn-mcp-server"
    }
  }
}

How It Works

The server uses a UnifiedWorkspaceHost that transparently picks the right analysis mode — no manual configuration needed:

1. Startup: auto-detects .sln/.slnx/.csproj in the working directory. Found → full semantic analysis active immediately.
2. File query: when the AI queries a .cs file, the host searches upward for its containing project (like git finding .git). Found → loads it and resolves all NuGet references. Not found → falls back to adhoc compilation (syntax + basic types).
3. Explicit control: the AI can call roslyn_csharp_load_project at any time to load or switch projects.

AI calls roslyn_csharp_diagnostics("src/Program.cs")
  │
  ├─ File in loaded project? → full Roslyn analysis (NuGet refs resolved)
  │
  ├─ File not in project? → search upward for .sln/.csproj
  │   ├─ Found → auto-load → full analysis
  │   └─ Not found → adhoc fallback (syntax check)
  │
  AI can also: roslyn_csharp_load_project("/path/to/OtherSolution.sln")

No --workspace flag, no mode switching — the AI just passes file paths.

Architecture

MCP Client (Reasonix / Claude / Cursor)
    │ stdio JSON-RPC (MCP protocol)
    ▼
RoslynMcpServer (.NET 11 console app)
    ├── ModelContextProtocol SDK 1.4.0 (official C# MCP SDK)
    ├── 18 [McpServerTool] methods across 5 groups
    └── Roslyn 4.13 (Microsoft.CodeAnalysis)
        └── UnifiedWorkspaceHost
            ├── Auto-detect .sln/.csproj on startup
            ├── Auto-discover project from .cs file path
            ├── MSBuildWorkspace (full NuGet + project refs)
            ├── Adhoc compilation (standalone file fallback)
            └── WorkspaceWatcher (FileSystemWatcher + incremental recompile)

Project Structure

roslyn-mcp-server/
├── src/
│   ├── RoslynMcpServer/              # MCP entry + tools
│   │   ├── Program.cs                # auto-detect project + stdio transport
│   │   └── Tools/
│   │       ├── DiagnosticsTool.cs    # roslyn_csharp_diagnostics
│   │       ├── SemanticTools.cs      # hover/goto_def/find_ref/completion/sig_help
│   │       ├── StructureTools.cs     # find_symbols/file_members/impls/rename
│   │       ├── CodeFixTools.cs       # get_fixes/apply_fix/format
│   │       └── ProjectManagementTools.cs  # load_project/workspace_info/refs/nuget/status
│   │
│   └── RoslynMcpServer.Core/         # Core (no MCP dependency)
│       ├── Workspace/
│       │   ├── IWorkspaceHost.cs     # unified host interface
│       │   ├── UnifiedWorkspaceHost.cs  # project + adhoc auto-switching
│       │   └── WorkspaceWatcher.cs      # file change monitoring
│       ├── Analysis/
│       │   ├── DiagnosticAnalyzer.cs
│       │   └── PositionMapper.cs     # 1-based ↔ 0-based conversion
│       └── Models/
│           └── DiagnosticResult.cs
│
├── tests/                            # xUnit (17 tests)
├── samples/                          # test fixtures
├── .mcp.json                         # MCP manifest (auto-detect by installers)
├── install.sh / install.bat          # one-command install
└── reasonix.toml                     # Reasonix config template

Tech Stack

Known Limitations

License

MIT