MCP C# SDK

The official C# SDK for the Model Context Protocol, enabling .NET applications, services, and libraries to implement and interact with MCP clients and servers.

Note

This repo is still in preview, breaking changes can be introduced without prior notice.

About MCP

The Model Context Protocol (MCP) is an open protocol that standardizes how applications provide context to Large Language Models (LLMs). It enables secure integration between LLMs and various data sources and tools.

For more information about MCP:

• Official Documentation
• Protocol Specification
• GitHub Organization

Getting Started (Client)

To get started writing a client, the McpClientFactory.CreateAsync method is used to instantiate and connect an IMcpClient to a server, with details about the client and server specified in McpClientOptions and McpServerConfig objects. Once you have an IMcpClient, you can interact with it, such as to enumerate all available tools and invoke tools.

McpClientOptions options = new()
{
    ClientInfo = new() { Name = "TestClient", Version = "1.0.0" }
};
	
McpServerConfig config = new()
{
    Id = "everything",
    Name = "Everything",
    TransportType = TransportTypes.StdIo,
    TransportOptions = new()
    {
        ["command"] = "npx",
        ["arguments"] = "-y @modelcontextprotocol/server-everything",
    }
};
		
var client = await McpClientFactory.CreateAsync(config, options);

// Print the list of tools available from the server.
await foreach (var tool in client.ListToolsAsync())
{
    Console.WriteLine($"{tool.Name} ({tool.Description})");
}

// Execute a tool (this would normally be driven by LLM tool invocations).
var result = await client.CallToolAsync(
    "echo",
    new() { ["message"] = "Hello MCP!" },
    CancellationToken.None);

// echo always returns one and only one text content object
Console.WriteLine(result.Content.First(c => c.Type == "text").Text);

You can find samples demonstrating how to use ModelContextProtocol with an LLM SDK in the samples directory, and also refer to the IntegrationTests project for more examples. Additional examples and documentation will be added as in the near future.

Clients can connect to any MCP server, not just ones created using this library. The protocol is designed to be server-agnostic, so you can use this library to connect to any compliant server.

Tools can be exposed easily as AIFunction instances so that they are immediately usable with IChatClients.

// Get available functions.
IList<AIFunction> tools = await client.GetAIFunctionsAsync();

// Call the chat client using the tools.
IChatClient chatClient = ...;
var response = await chatClient.GetResponseAsync(
    "your prompt here",
    new() 
    {
        Tools = [.. tools],
    });

Getting Started (Server)

Here is an example of how to create an MCP server and register all tools from the current application. It includes a simple echo tool as an example (this is included in the same file here for easy of copy and paste, but it needn't be in the same file... the employed overload of WithTools examines the current assembly for classes with the McpToolType attribute, and registers all methods with the McpTool attribute as tools.)

using ModelContextProtocol;
using ModelContextProtocol.Server;
using Microsoft.Extensions.Hosting;
using System.ComponentModel;

var builder = Host.CreateEmptyApplicationBuilder(settings: null);
builder.Services
    .AddMcpServer()
    .WithStdioServerTransport()
    .WithTools();
await builder.Build().RunAsync();

[McpToolType]
public static class EchoTool
{
    [McpTool, Description("Echoes the message back to the client.")]
    public static string Echo(string message) => $"hello {message}";
}

More control is also available, with fine-grained control over configuring the server and how it should handle client requests. For example:

using ModelContextProtocol.Protocol.Transport;
using ModelContextProtocol.Protocol.Types;
using ModelContextProtocol.Server;
using Microsoft.Extensions.Logging.Abstractions;

McpServerOptions options = new()
{
    ServerInfo = new() { Name = "MyServer", Version = "1.0.0" },
    Capabilities = new() 
    {
        Tools = new()
        {
            ListToolsHandler = async (request, cancellationToken) =>
            {
                return new ListToolsResult()
                {
                    Tools =
                    [
                        new Tool()
                        {
                            Name = "echo",
                            Description = "Echoes the input back to the client.",
                            InputSchema = new JsonSchema()
                            {
                                Type = "object",
                                Properties = new Dictionary<string, JsonSchemaProperty>()
                                {
                                    ["message"] = new JsonSchemaProperty() { Type = "string", Description = "The input to echo back." }
                                }
                            },
                        }
                    ]
                };
            },

            CallToolHandler = async (request, cancellationToken) =>
            {
                if (request.Params?.Name == "echo")
                {
                    if (request.Params.Arguments?.TryGetValue("message", out var message) is not true)
                    {
                        throw new McpServerException("Missing required argument 'message'");
                    }

                    return new CallToolResponse()
                    {
                        Content = [new Content() { Text = $"Echo: {message}", Type = "text" }]
                    };
                }

                throw new McpServerException($"Unknown tool: '{request.Params?.Name}'");
            },
        }
    },
};

await using IMcpServer server = McpServerFactory.Create(new StdioServerTransport("MyServer"), options);

await server.StartAsync();

// Run until process is stopped by the client (parent process)
await Task.Delay(Timeout.Infinite);

License

This project is licensed under the MIT License.