CSharperMcp

An MCP (Model Context Protocol) server that gives LLMs IDE-like semantic understanding of C# code. Stop grepping files and hallucinating code structure—use Roslyn and ILSpy to provide real compiler diagnostics, symbol navigation, find references, decompiled DLLs, and code actions.

Why This Exists

Current AI coding tools (Claude Code, Cursor) lack C# language server integration. Without it, LLMs grep files and brute-force reverse engineer code structure. This is slow, misses semantic context, and completely fails for code in DLLs. This server fixes that by wrapping Roslyn APIs and ICSharpCode.Decompiler.

Features

• Workspace Management: Load and analyze C# solutions and projects
• Diagnostics: Get compiler errors, warnings, and analyzer messages
• Symbol Intelligence: Navigate definitions, find references, inspect types
• DLL Introspection: Decompile and explore third-party assemblies and NuGet packages
• Code Actions: Apply refactorings and quick fixes

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Installation

The server is distributed as a .NET global tool via NuGet. Use dnx (dotnet execute) to run it without explicit installation:

# Run directly via dnx (no installation needed)
dnx CSharperMcp

# Or install globally
dotnet tool install --global CSharperMcp

# Run after installing
csharper-mcp

Requirements

• .NET 10.0 SDK or later
• MSBuild (automatically located via Microsoft.Build.Locator)

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Configuration

Using with Claude Code

Add to your project's .mcp.json or your global MCP configuration:

{
  "mcpServers": {
    "csharp-er-mcp": {
      "type": "stdio",
      "command": "dnx",
      "args": ["--yes", "CSharperMcp", "--workspace-from-cwd"]
    }
  }
}

Recommended: Use --workspace-from-cwd which automatically uses the current working directory. This allows the tool to be installed globally and work across all your C# projects without per-project configuration.

Alternative: If you need to specify an explicit workspace path, use --workspace /some/folder:

{
  "mcpServers": {
    "csharp-er-mcp": {
      "type": "stdio",
      "command": "dnx",
      "args": ["--yes", "CSharperMcp", "--workspace", "/path/to/your/csharp/project"]
    }
  }
}

Note: Variable expansion like "${workspaceFolder}" doesn't work reliably with Claude Code's global tool installations, which is why --workspace-from-cwd is recommended.

Using with Cursor

Recommended: Use --workspace-from-cwd for global installation across all projects:

{
  "mcpServers": {
    "csharp-er-mcp": {
      "command": "dnx",
      "args": ["--yes", "CSharperMcp", "--workspace-from-cwd"]
    }
  }
}

Alternative: If you need to specify an explicit workspace path, you can use ${workspaceFolder} variable expansion (this works in Cursor):

{
  "mcpServers": {
    "csharp-er-mcp": {
      "command": "dnx",
      "args": ["--yes", "CSharperMcp", "--workspace", "${workspaceFolder}"]
    }
  }
}

Other MCP Clients

dnx --yes CSharperMcp --workspace /path/to/your/project

Workspace Auto-initialization

The server supports two ways to automatically initialize a workspace on startup:

--workspace <path> - Specify an explicit path to initialize:

dnx --yes CSharperMcp --workspace /path/to/your/project

--workspace-from-cwd - Use the current working directory:

dnx --yes CSharperMcp --workspace-from-cwd

Both options will:

1. Look for .sln files (prefer one matching directory name, else largest)
2. Fall back to .csproj files if no solution found
3. Load the workspace and make tools available immediately
4. Disable the initialize_workspace tool (restart the server to change workspaces)

Without either parameter: You must call the initialize_workspace tool manually after connecting.

Advanced Configuration with YAML

You can customize server behavior and tool descriptions using YAML configuration files. The server supports hierarchical configuration merging:

Configuration file locations (later files override earlier):

1. ~/.config/csharp-er-mcp.yml - User global preferences (optional)
2. <workspace>/.config/csharp-er-mcp.yml - Project-specific config (optional)

Example configuration:

# Server configuration (optional - has defaults)
mcp:
  serverInstructions: "Custom instructions for the MCP server..."

# Tool descriptions (optional)
tools:
  initialize_workspace:
    description: "Custom description here..."
  get_diagnostics:
  some_tool_to_hide:
    isEnabled: false # Tool will be filtered out from MCP client

Features:

• Override server instructions sent to MCP clients
• Customize individual tool descriptions
• Disable specific tools via isEnabled: false
• Configuration merging: user config → project config → command-line overrides

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Available MCP Tools

Foundation Tools

initialize_workspace

Load a C# solution or project from a path.

Input:

{
  "path": "/absolute/path/to/solution/or/project/or/directory"
}

Returns: Project count, solution path, any load errors.

Note: Disabled when --workspace or --workspace-from-cwd is provided.

---

get_diagnostics

Get compiler errors, warnings, and analyzer messages.

Input:

{
  "file": "/path/to/file.cs", // Optional: filter by file
  "startLine": 10, // Optional: filter by line range
  "endLine": 50, // Optional: filter by line range
  "severity": "Error" // Optional: Error|Warning|Info|Hidden
}

Returns: Array of diagnostics with:

• Diagnostic ID (e.g., CS0103)
• Severity (Error, Warning, Info, Hidden)
• Message
• File path, line, column, end line, end column
• hasFix boolean (whether a code action can fix this)

No arguments: Returns diagnostics for entire workspace.

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Symbol Intelligence Tools (Coming Soon)

get_symbol_info

Get type information at a location or by fully qualified name.

find_symbol_usages

Find all references to a symbol across the workspace.

get_definition_location

Navigate to the definition of a symbol (in workspace or DLL metadata).

get_type_members

Get full type definition with all members (decompiled for DLL types).

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Code Action Tools (Coming Soon)

get_code_actions

Get available refactorings and quick fixes at a location.

apply_code_action

Execute a code action (with optional preview before applying).

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Search & Navigation Tools (Coming Soon)

search_symbols

Find symbols by name pattern (supports camelCase matching).

get_document_symbols

Get hierarchical outline of a file's symbols.

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Extension Methods (Special Feature, Coming Soon)

get_extension_methods

Find all extension methods available for a given type (scans all referenced assemblies).

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Usage Examples

See CLAUDE.md for comprehensive examples showing how to:

• Initialize workspace and fix compiler errors
• Navigate symbols and find usages
• Introspect NuGet package code (decompiled DLLs)
• Apply refactorings with preview
• Discover extension methods

---

Development

Prerequisites

• .NET 10.0 SDK
• MSBuild (automatically located)

Build Commands

# Restore dependencies
dotnet restore

# Build everything
dotnet build

# Run the server locally
dotnet run --project src/CSharperMcp.Server

# Run all tests
dotnet test

# Run only unit tests (fast)
dotnet test tests/CSharperMcp.Server.UnitTests

# Run only integration tests (slower, multi-process)
dotnet test tests/CSharperMcp.Server.IntegrationTests

Building the NuGet Package

# Build the package
dotnet pack src/CSharperMcp.Server/CSharperMcp.Server.csproj -c Release -o ./artifacts

# Test locally before publishing
dotnet tool install --global --add-source ./artifacts CSharperMcp

See NUGET-PUBLISH.md for publishing instructions.

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Project Structure

csharp-er-mcp/
├── Directory.Build.props              # Shared build configuration
├── Directory.Packages.props           # Central package version management
├── CSharperMcp.sln                    # Solution file
├── src/
│   └── CSharperMcp.Server/            # Main MCP server console app
│       ├── Program.cs                 # Entry point (MSBuildLocator.RegisterDefaults())
│       ├── Server/Tools/              # MCP tool implementations
│       ├── Workspace/                 # Solution/project loading & management
│       ├── Services/                  # Roslyn & Decompiler service wrappers
│       └── Models/                    # DTOs for tool responses
└── tests/
    ├── CSharperMcp.Server.UnitTests/           # Fast unit tests
    └── CSharperMcp.Server.IntegrationTests/    # Slower integration tests

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Technology Stack

• .NET 10 - Latest C# language features
• ModelContextProtocol 0.6.0 - Official MCP C# SDK
• Microsoft.CodeAnalysis.Workspaces.MSBuild 5.0.0 - Roslyn APIs
• Microsoft.Build.Locator 1.11.2 - Find MSBuild dynamically
• ICSharpCode.Decompiler 9.1.0 - ILSpy engine for DLL introspection
• NUnit 4 - Testing framework
• Moq - Mocking framework
• FluentAssertions - Fluent assertion library

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Development Guidelines

See CLAUDE.md for comprehensive development instructions including:

• Code style and access modifiers (default to internal)
• Central Package Management requirements
• Testing philosophy (unit vs integration)
• Architecture patterns
• MCP tool implementation details

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Support

• Issues: GitHub Issues
• NuGet Package: CSharperMcp on NuGet.org

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License

See LICENSE file for details.