mcpkit

A Rust SDK for the Model Context Protocol (MCP) that simplifies server development through a unified #[mcp_server] macro.

Features

• Reduced boilerplate via unified #[mcp_server] macro
• Runtime-agnostic async support (Tokio, async-std, smol)
• Type-safe state machines via typestate pattern for connection lifecycle
• Rich error handling with context chains
• Full MCP 2025-11-25 protocol coverage including Tasks
• First-class middleware via Tower-compatible Layer pattern

Why mcpkit?

mcpkit implements MCP 2025-11-25 — the latest protocol specification — while the official rmcp SDK still uses 2024-11-05. This means mcpkit supports features not available in the official SDK:

If you need these features in Rust, mcpkit is currently the most complete implementation.

Quick Start

Add the dependency to your Cargo.toml:

[dependencies]
mcpkit = "0.2"
tokio = { version = "1", features = ["full"] }
serde_json = "1"

Create a simple MCP server:

use mcpkit::prelude::*;
use mcpkit::transport::stdio::StdioTransport;

struct Calculator;

#[mcp_server(name = "calculator", version = "1.0.0")]
impl Calculator {
    #[tool(description = "Add two numbers")]
    async fn add(&self, a: f64, b: f64) -> ToolOutput {
        ToolOutput::text((a + b).to_string())
    }

    #[tool(description = "Multiply two numbers")]
    async fn multiply(&self, a: f64, b: f64) -> ToolOutput {
        ToolOutput::text((a * b).to_string())
    }
}

#[tokio::main]
async fn main() -> Result<(), McpError> {
    // Create transport - you choose the runtime!
    let transport = StdioTransport::new();

    // Build server with registered handlers
    let server = ServerBuilder::new(Calculator)
        .with_tools(Calculator)
        .build();

    // Serve on the transport
    server.serve(transport).await
}

Note: This SDK is runtime-agnostic. You provide the transport, which lets you use Tokio, async-std, smol, or any other async runtime. The examples use Tokio, but the SDK itself doesn't depend on any specific runtime.

Comparison with rmcp

Crate Structure

mcpkit/
├── mcpkit/                     # Facade crate (use this)
├── crates/
│   ├── mcpkit-core/            # Protocol types, traits
│   ├── mcpkit-transport/       # Transport abstractions
│   │   ├── stdio               # Standard I/O transport
│   │   ├── http                # Streamable HTTP transport
│   │   ├── websocket           # WebSocket transport
│   │   └── unix                # Unix domain sockets
│   ├── mcpkit-server/          # Server implementation
│   ├── mcpkit-client/          # Client implementation
│   ├── mcpkit-macros/          # Procedural macros
│   ├── mcpkit-testing/         # Test utilities
│   ├── mcpkit-axum/            # Axum web framework integration
│   └── mcpkit-actix/           # Actix-web framework integration
└── examples/                   # Example servers

Examples

Minimal Server

use mcpkit::prelude::*;

struct MyServer;

#[mcp_server(name = "minimal", version = "1.0.0")]
impl MyServer {
    #[tool(description = "Say hello")]
    async fn hello(&self, name: Option<String>) -> ToolOutput {
        let name = name.unwrap_or_else(|| "World".to_string());
        ToolOutput::text(format!("Hello, {}!", name))
    }
}

With Resources

use mcpkit::prelude::*;

struct ConfigServer;

#[mcp_server(name = "config", version = "1.0.0")]
impl ConfigServer {
    #[resource(
        uri_pattern = "config://app/{key}",
        name = "Configuration",
        mime_type = "application/json"
    )]
    async fn get_config(&self, uri: &str) -> ResourceContents {
        ResourceContents::text(uri, r#"{"debug": true}"#)
    }
}

With Prompts

use mcpkit::prelude::*;

struct PromptServer;

#[mcp_server(name = "prompts", version = "1.0.0")]
impl PromptServer {
    #[prompt(description = "Review code for issues")]
    async fn code_review(&self, code: String, language: Option<String>) -> GetPromptResult {
        let lang = language.unwrap_or_else(|| "unknown".to_string());
        GetPromptResult {
            description: Some("Code review prompt".to_string()),
            messages: vec![
                PromptMessage::user(format!(
                    "Please review the following {} code:\n```{}\n{}```",
                    lang, lang, code
                ))
            ],
        }
    }
}

Transports

The SDK is runtime-agnostic. You choose the transport and the async runtime.

Standard I/O

use mcpkit_transport::stdio::StdioTransport;

let transport = StdioTransport::new();

HTTP (Streamable)

use mcpkit_transport::http::HttpTransport;

let transport = HttpTransport::new(HttpTransportConfig::new("http://localhost:8080"));

WebSocket

use mcpkit_transport::websocket::WebSocketTransport;

let transport = WebSocketTransport::new(WebSocketConfig::new("ws://localhost:9000"));

Unix Domain Socket (Unix only)

#[cfg(unix)]
use mcpkit_transport::unix::UnixTransport;

#[cfg(unix)]
let transport = UnixTransport::new("/tmp/mcp.sock");

Middleware

use mcpkit_transport::stdio::StdioTransport;
use mcpkit_transport::middleware::{LoggingLayer, TimeoutLayer, LayerStack};
use std::time::Duration;
use log::Level;

let transport = StdioTransport::new();
let stack = LayerStack::new(transport)
    .with(LoggingLayer::new(Level::Debug))
    .with(TimeoutLayer::new(Duration::from_secs(30)));

Error Handling

use mcpkit::prelude::*;

fn process() -> Result<(), McpError> {
    let result = something_risky()
        .context("while processing request")?;
    Ok(())
}

Protocol Version

This SDK implements MCP protocol version 2025-11-25.

License

Licensed under either of:

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT License (LICENSE-MIT)

at your option.

Documentation

• Getting Started
• Working with Tools
• Working with Resources
• Working with Prompts
• Error Handling
• Using Middleware
• Transport Options
• Architecture Decision Records

Contributing

Contributions are welcome! Please read our Contributing Guide before submitting a Pull Request.

Security

For security issues, please see our Security Policy.