ZeroMCP — C#

Sandboxed MCP server library for .NET. Register tools, call server.Serve(), done.

Getting started

using ZeroMcp;

var server = new ZeroMcpServer();

server.Tool("hello", new ToolDefinition
{
    Description = "Say hello to someone",
    Input = new Dictionary<string, InputField>
    {
        ["name"] = new InputField(SimpleType.String)
    },
    Execute = async (args, ctx) =>
    {
        var name = args["name"].GetString() ?? "world";
        return $"Hello, {name}!";
    }
});

await server.Serve();

Stdio works immediately. No transport configuration needed.

vs. the official SDK

The official C# SDK (backed by Microsoft) requires server setup, transport configuration, and schema definition. ZeroMCP handles the protocol, transport, and schema generation with a clean async/await API.

In benchmarks, ZeroMCP C# handles 14,013 requests/second over stdio versus the official SDK's 9,776 — 1.4x faster with 34% less memory (33 MB vs 50 MB). Over HTTP (ASP.NET), ZeroMCP serves 4,421 rps versus the official SDK's 2,517 rps.

C# passes all 10 conformance suites and survives 21/22 chaos monkey attacks.

The official SDK has no sandbox. ZeroMCP lets tools declare network, filesystem, and exec permissions.

HTTP / Streamable HTTP

ZeroMCP doesn't own the HTTP layer. You bring your own framework; ZeroMCP gives you an async HandleRequest method that takes a JsonDocument and returns a response dictionary (or null for notifications).

// var response = await server.HandleRequest(request);

ASP.NET Minimal API

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/mcp", async (HttpContext ctx) =>
{
    var doc = await JsonDocument.ParseAsync(ctx.Request.Body);
    var response = await server.HandleRequest(doc);
    if (response == null)
    {
        ctx.Response.StatusCode = 204;
        return;
    }
    ctx.Response.ContentType = "application/json";
    await ctx.Response.WriteAsJsonAsync(response);
});

app.Run("http://0.0.0.0:4242");

Requirements

• .NET 8

Build & run

dotnet run --project Example

Or publish a self-contained binary:

dotnet publish Example -c Release -o ./out
./out/Example

Sandbox

server.Tool("fetch_data", new ToolDefinition
{
    Description = "Fetch from our API",
    Input = new Dictionary<string, InputField>
    {
        ["url"] = new InputField(SimpleType.String)
    },
    Permissions = new ToolPermissions
    {
        Network = new[] { "api.example.com", "*.internal.dev" },
        Fs = FsPermission.None,
        Exec = false
    },
    Execute = async (args, ctx) => { /* ... */ }
});

Project reference

<ProjectReference Include="../ZeroMcp/ZeroMcp.csproj" />

Testing

dotnet test