mcp-server

A fully-featured Haskell library for building Model Context Protocol (MCP) servers.

Features

• Complete MCP Implementation: Designed against the MCP 2024-11-05 server specification
• Type-Safe API: Leverage Haskell's type system for robust MCP servers
• Multiple Abstractions: Both low-level fine-grained control and high-level derived interfaces
• Template Haskell Support: Automatic handler derivation from data types
• Stdio Transport Only: HTTP/SSE coming soon!

Supported MCP Features

• ✅ Prompts: User-controlled prompt templates with arguments
• ✅ Resources: Application-controlled readable resources
• ✅ Tools: Model-controlled callable functions
• ✅ Initialization Flow: Complete protocol lifecycle with version negotiation
• ✅ Error Handling: Comprehensive error types and JSON-RPC error responses

Quick Start

Add the library mcp-server to your cabal file:

build-depends:
  mcp-server

Create a simple module, such as this example below:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}

import MCP.Server
import MCP.Server.Derive

-- Define your data types
data MyPrompt = Recipe { idea :: Text } | Shopping { items :: Text }
data MyResource = Menu | Specials  
data MyTool = Search { query :: Text } | Order { item :: Text }

-- Implement handlers
handlePrompt :: MyPrompt -> IO Content
handlePrompt (Recipe idea) = pure $ ContentText $ "Recipe for " <> idea
handlePrompt (Shopping items) = pure $ ContentText $ "Shopping list: " <> items

handleResource :: MyResource -> IO Content  
handleResource Menu = pure $ ContentText "Today's menu..."
handleResource Specials = pure $ ContentText "Daily specials..."

handleTool :: MyTool -> IO Content
handleTool (Search query) = pure $ ContentText $ "Search results for " <> query
handleTool (Order item) = pure $ ContentText $ "Ordered " <> item

-- Derive handlers automatically
main :: IO ()
main = runMcpServerStdIn serverInfo handlers
  where
    serverInfo = McpServerInfo
      { serverName = "My MCP Server"
      , serverVersion = "1.0.0" 
      , serverInstructions = "A sample MCP server"
      }
    handlers = McpServerHandlers
      { prompts = Just $(derivePromptHandler ''MyPrompt 'handlePrompt)
      , resources = Just $(deriveResourceHandler ''MyResource 'handleResource)  
      , tools = Just $(deriveToolHandler ''MyTool 'handleTool)
      }

Custom Descriptions

You can provide custom descriptions for constructors and fields using the *WithDescription variants:

-- Define descriptions for constructors and fields
descriptions :: [(String, String)]
descriptions = 
  [ ("Recipe", "Generate a recipe for a specific dish")     -- Constructor description
  , ("Search", "Search our menu database")                  -- Constructor description
  , ("idea", "The dish you want a recipe for")              -- Field description
  , ("query", "Search terms to find menu items")            -- Field description
  ]

-- Use in derivation
handlers = McpServerHandlers
  { prompts = Just $(derivePromptHandlerWithDescription ''MyPrompt 'handlePrompt descriptions)
  , tools = Just $(deriveToolHandlerWithDescription ''MyTool 'handleTool descriptions)
  , resources = Just $(deriveResourceHandlerWithDescription ''MyResource 'handleResource descriptions)
  }

Manual Handler Implementation

For fine-grained control, implement handlers manually:

import MCP.Server

-- Manual handler implementation
promptListHandler :: IO [PromptDefinition]
promptGetHandler :: PromptName -> [(ArgumentName, ArgumentValue)] -> IO (Either Error Content)
-- ... implement your custom logic

main :: IO ()
main = runMcpServerStdIn serverInfo handlers
  where
    handlers = McpServerHandlers
      { prompts = Just (promptListHandler, promptGetHandler)
      , resources = Nothing  -- Not supported
      , tools = Nothing      -- Not supported  
      }

Examples

The library includes a few different examples:

• examples/Simple/: Basic key-value store using Template Haskell derivation
• examples/Complete/: Full-featured example with prompts, resources, and tools

Docker Usage

I like to build and publish my MCP servers to Docker - which means that it's much easier to configure assistants such as Claude Desktop to run them.

# Build the image
docker build -t haskell-mcp-server .

# Run different examples
docker run -i --entrypoint="/usr/local/bin/haskell-mcp-server" haskell-mcp-server

And then configure Claude by editing claude_desktop_config.json:

{
    "mcpServers": {
       "haskell-mcp-server-example": {
            "command": "docker",
            "args": [
                "run",
                "-i",
                "--entrypoint=/usr/local/bin/haskell-mcp-server",
                "haskell-mcp-server"
            ]
        }
    }
}

Documentation

• MCP Specification
• API Documentation
• Examples

Contributing

Contributions are welcome! Please see the issue tracker for open issues and feature requests.

Disclaimer - AI Assistance

I am not sure whether there is any stigma associated with this but Claude helped me write a lot of this library. I started with a very specific specification of what I wanted to achieve and worked shoulder-to-shoulder with Claude to implement and refactor the library until I was happy with it. A few of the features such as the Derive functions are a little out of my comfort zone to have manually written, so I appreciated having an expert guide me here - however I do suspect that this implementation may be sub-par and I do intend to refactor and rewrite large pieces of this through regular maintenance.

License

BSD-3-Clause