mcp-server

Build Model Context Protocol servers in Haskell from plain data types. Declare your tools, prompts and resources as ADTs, write one handler per type, and the library derives the JSON schemas, argument decoding, validation and wire protocol for you — then serves it over stdio or Streamable HTTP to Claude Code, Codex, Claude Desktop, Cursor and any other MCP client.

{-# LANGUAGE OverloadedStrings, TemplateHaskell #-}

import Data.Text (Text)
import MCP.Server
import MCP.Server.Derive

data Units = Celsius | Fahrenheit

data WeatherTool
    = CurrentWeather { city :: Text, units :: Maybe Units }
    | Forecast       { city :: Text, days :: Int }

handleTool :: ClientContext -> WeatherTool -> IO Content
handleTool _ (CurrentWeather c _) = pure $ ContentText $ "Sunny in " <> c
handleTool _ (Forecast c n)       = pure $ ContentText $ "Forecast for " <> c

$(pure [])  -- end the declaration group so the splice below can see the types

main :: IO ()
main = runMcpServerStdio serverInfo noHandlers
    { tools = Just $(deriveToolHandler ''WeatherTool 'handleTool) }
  where
    serverInfo = McpServerInfo
      { serverName = "weather", serverVersion = "1.0.0"
      , serverInstructions = "Weather lookups" }

That is a complete, working MCP server exposing two tools, current_weather and forecast, each with a JSON schema derived from its constructor’s fields.

What you get for free

The derivation reads your types, so the schema on the wire always matches the handler that receives the arguments. Given this constructor from examples/Complete:

data ShippingSpeed = Standard | Express | Overnight

data Address = Address
    { street  :: Text
    , city    :: Text
    , zipCode :: Maybe Text
    }

data MyTool
    = Checkout { speed :: ShippingSpeed, shipTo :: Address }
    | ...

tools/list returns exactly this (captured from the running example):

{
  "name": "checkout",
  "description": "Checkout",
  "annotations": { "destructiveHint": true },
  "inputSchema": {
    "type": "object",
    "required": ["speed", "shipTo"],
    "properties": {
      "speed":  { "type": "string", "enum": ["standard", "express", "overnight"] },
      "shipTo": {
        "type": "object",
        "required": ["street", "city"],
        "properties": {
          "street":  { "type": "string" },
          "city":    { "type": "string" },
          "zipCode": { "type": "string" }
        }
      }
    }
  }
}

and a tools/call with matching arguments arrives in your handler as a fully decoded Checkout Express (Address "1 Main St" "Springfield" Nothing). Malformed arguments never reach you; the library answers with the appropriate JSON-RPC error. The same machinery works in reverse for typed results (see Structured output).

Beyond the derivation, the library handles:

• Both protocol eras. Legacy revisions 2024-11-05 through 2025-11-25 negotiated via initialize, and the stateless 2026-07-28 revision declared per request in _meta, from one server binary.
• Two transports. stdio, and Streamable HTTP with bearer auth, Origin validation, per-request SSE and a plain WAI application you can embed.
• Long-running tools. Progress notifications, per-request client logging, and cancellation of in-flight requests on both transports.
• Live servers. listChanged and resource-update pushes over subscriptions/listen.
• Conformance fixtures. A language-agnostic corpus of request/response pairs the test suite replays against every protocol era.

Installation

Add mcp-server to your build-depends:

build-depends:
  base, text, mcp-server

Tested against GHC 9.6 through 9.14 in CI. The HTTP transport requires ghc-options: -threaded (Warp needs the threaded runtime).

Connecting to a client

Build your server, then register it with your client of choice. The examples below use the simple-example executable from this repository; substitute your own.

Claude Code

# stdio: everything after -- is the command Claude Code spawns
claude mcp add my-server -- "$(cabal list-bin exe:simple-example)"

# pass environment variables with --env
claude mcp add my-server --env API_KEY=secret -- /path/to/my-server

# Streamable HTTP
claude mcp add --transport http my-server http://localhost:3000/mcp

Verify with claude mcp list or /mcp inside a session. Add --scope project to write a .mcp.json you can commit for your team:

{
  "mcpServers": {
    "my-server": {
      "type": "stdio",
      "command": "/path/to/my-server",
      "env": { "API_KEY": "${API_KEY}" }
    }
  }
}

Cursor and several other clients read the same mcpServers shape.

Codex

codex mcp add my-server --env API_KEY=secret -- /path/to/my-server

Or in ~/.codex/config.toml, which is also where HTTP servers go:

[mcp_servers.my-server]
command = "/path/to/my-server"

[mcp_servers.my-http-server]
url = "http://localhost:3000/mcp"

Claude Desktop

Claude Desktop launches stdio servers from claude_desktop_config.json. A Docker image is a convenient way to ship a Haskell binary to it — the repository’s Dockerfile builds all three examples:

docker build -t haskell-mcp-server .

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

Keep stdout clean

On stdio, stdout carries only JSON-RPC. The library writes nothing else there, and your handlers must not either: log to stderr.

Defining tools

Naming and arguments

Constructor names become snake_case tool names; record fields become named arguments. The generated inputSchema mirrors the field types:

data Color = Red | Green | Blue          -- all-nullary type: string enum
data Filters = Filters                   -- record: nested JSON object
  { tags     :: [Text]                   -- list: JSON array
  , maxCount :: Maybe Int                -- Maybe: optional field
  }

data MyTool
    = SearchItems                        -- "search_items"
      { query   :: Text
      , color   :: Color                 -- "red" | "green" | "blue"
      , filters :: Filters               -- { "tags": [...], "maxCount": ... }
      , limit   :: Maybe Int
      }

Primitive fields (Int, Integer, Double, Float, Bool, Text) are parsed leniently: 42 and "42" are both accepted, since many clients send numbers and booleans as strings.

A constructor may also wrap a single record type, which is unwrapped recursively until a record is found:

data SetValueParams = SetValueParams { key :: Text, value :: Text }

data SimpleTool
    = GetValue { key :: Text }
    | SetValue SetValueParams              -- fields of SetValueParams are the arguments

Positional (unnamed) fields are not supported, because they have no names to put in the schema:

data SimpleTool = GetValue Int | SetValue Int Text   -- ❌ rejected

Results and errors

Simple handlers return Content (or Text). Return a ToolResult for multiple content blocks or to report an execution failure with isError, which the spec prefers over a protocol error so the model can see what went wrong and react:

handleTool :: ClientContext -> MyTool -> IO ToolResult
handleTool _ (SearchItems q _ _ _)
  | T.null q  = pure $ toolError "query must not be empty"
  | otherwise = pure $ toolResult [ContentText ("Results for " <> q)]

Content blocks can carry annotations (audience, priority, lastModified) via the ContentAnnotated wrapper:

ContentAnnotated defaultAnnotations { annotationsPriority = Just 0.9 }
                 (ContentText "important result")

Structured output

Give the derivation a result type and it derives the tool’s outputSchema (same field rules as inputs) and serializes your value into structuredContent, guaranteed to match. Per the spec’s recommendation the JSON is also returned as a text block for clients that predate structured output:

data WeatherReport = WeatherReport
    { temperature :: Int
    , sky         :: Sky          -- enum
    , alerts      :: [Text]
    , humidity    :: Maybe Int    -- omitted when Nothing
    }

handleTool :: ClientContext -> MyTool -> IO (ToolOutput WeatherReport)
handleTool _ (GetWeather city) = pure $ ToolOutput (lookupWeather city)
handleTool _ (BrokenTool _)    = pure $ ToolOutputError "sensor offline"

tools = Just $(deriveToolHandlerWithOutput ''MyTool 'handleTool ''WeatherReport)

ToolOutputWith supplies custom content blocks alongside the structured value; ToolOutputRaw is the escape hatch back to a plain ToolResult.

Descriptions, annotations and icons

Every derive* function has a WithDescription variant taking a flat list of constructor and field descriptions, and a WithOptions variant taking per-constructor DefinitionOptions: description, title, icons, behavioral annotations (which drive client permission UX, such as auto-approving read-only tools) and argument descriptions scoped to that constructor.

descriptions =
  [ ("SearchItems", "Search the catalog")     -- constructor
  , ("query",       "Search terms")           -- field
  ]
tools = Just $(deriveToolHandlerWithDescription ''MyTool 'handleTool descriptions)

tools = Just $(deriveToolHandlerWithOptions ''MyTool 'handleTool
  [ ("SearchItems", defaultDefinitionOptions
      { optDescription = Just "Search the catalog"
      , optToolAnnotations = Just defaultToolAnnotations
          { toolReadOnlyHint = Just True, toolIdempotentHint = Just True }
      , optIcons = [icon "https://example.com/search.png"]
      , optFieldDescriptions = [("query", "Search terms")]
      })
  ])

Defining prompts

Prompts derive the same way. Arguments are string-valued per the spec, so prompt records are limited to primitive and enumeration fields:

data MyPrompt = Recipe { idea :: Text } | Shopping { items :: Text }

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

prompts = Just $(derivePromptHandler ''MyPrompt 'handlePrompt)

Return a PromptResult instead of Content for a description and a multi-message conversation with user and assistant roles.

Defining resources

Nullary constructors become static resources with resource:// URIs; record constructors become resource templates (RFC 6570), one percent-decoded path segment per field:

data MyResource
    = Menu                                            -- resource://menu
    | ProductDetail { sku :: Text }                   -- resource://product_detail/{sku}
    | OrderItem { orderId :: Int, itemName :: Text }  -- resource://order_item/{orderId}/{itemName}

handleResource :: ClientContext -> URI -> MyResource -> IO ResourceContent
handleResource _ uri Menu                = pure $ ResourceText uri "text/plain" "Today's menu..."
handleResource _ uri (ProductDetail sku) = pure $ ResourceText uri "text/plain" ("Details for " <> sku)
handleResource _ uri (OrderItem o i)     = ...

resources         = Just $(deriveResourceHandler ''MyResource 'handleResource)
resourceTemplates = Just $(deriveResourceTemplates ''MyResource)

The read handler matches template URIs such as resource://product_detail/ABC123 and decodes the segments into the constructor’s fields; typed fields like Int are parsed, and a failing segment yields an invalid-params error.

Argument completion

Provide a completions handler to serve completion/complete for prompt arguments and resource-template parameters. The capability is advertised automatically when the handler is present:

handleComplete :: ClientContext -> CompletionRef -> ArgumentName -> Text -> Map Text Text
               -> IO (Either Error CompletionResult)
handleComplete _ (CompletionRefPrompt "recipe") "idea" partial _ =
    pure $ Right $ completionResult $
        filter (T.isPrefixOf partial) ["pancakes", "pasta", "pizza"]
handleComplete _ _ _ _ _ = pure $ Right $ completionResult []

handlers = noHandlers { completions = Just handleComplete, ... }

Assembling the server

Start from noHandlers and record-update the features you provide. Constructing McpServerHandlers directly is discouraged: the library grows new handler slots over time, and a missed field fails at runtime rather than compile time.

handlers = noHandlers
  { prompts           = Just $(derivePromptHandler ''MyPrompt 'handlePrompt)
  , resources         = Just $(deriveResourceHandler ''MyResource 'handleResource)
  , resourceTemplates = Just $(deriveResourceTemplates ''MyResource)
  , tools             = Just $(deriveToolHandler ''MyTool 'handleTool)
  , completions       = Just handleComplete
  }

Two Template Haskell details to know:

• A derive* splice can only see types declared in an earlier declaration group. Either put the types in their own module (as the examples do) or end the group with an empty $(pure []) splice before the main that uses them.
• Every handler receives the per-request ClientContext first. It carries the caller’s bearer token and principal on HTTP, the protocol revision and client identity for modern clients, and the reportProgress and logToClient actions described below.

Manual handlers

The derived handlers are ordinary values, so for full control you can supply your own instead. Prompt arguments arrive as Map Text Text, tool arguments as Map Text Value:

promptListHandler :: ClientContext -> IO [PromptDefinition]
promptGetHandler  :: ClientContext -> PromptName -> Map Text Text -> IO (Either Error PromptResult)

handlers = noHandlers { prompts = Just (promptListHandler, promptGetHandler) }

Transports

stdio

runMcpServerStdio serverInfo handlers serves JSON-RPC over stdin and stdout. runMcpServerStdioWithConfig takes a StdioConfig for verbose request logging on stderr, cacheability hints for modern clients, and a change-notification source.

Streamable HTTP

import MCP.Server.Transport.Http

main = runMcpServerHttp serverInfo handlers            -- localhost:3000/mcp

main = runMcpServerHttpWithConfig defaultHttpConfig
    { httpPort = 8080
    , httpHost = "0.0.0.0"
    , httpEndpoint = "/api/mcp"
    , httpVerbose = True                                -- request/response logging on stderr
    , httpAllowedOrigins = Just ["https://app.example.com"]
    } serverInfo handlers

httpAllowedOrigins is DNS-rebinding protection: requests carrying an Origin outside the list get 403. Nothing disables the check and is only appropriate for servers unreachable from browsers.

Bearer-token authentication is a callback. Return Just principal (any JSON Value, such as a role) to admit the request, or Nothing for 401. The principal reaches handlers as clientPrincipal in the ClientContext; token policy lives entirely in your application.

defaultHttpConfig
  { httpAuthorize = Just $ \mtoken -> case mtoken of
      Just "secret-admin-token" -> pure $ Just (String "admin")
      Just "secret-user-token"  -> pure $ Just (String "user")
      _                         -> pure Nothing
  }

The endpoint accepts POST only. Server-to-client notifications flow over the subscriptions/listen POST response stream rather than the deprecated standalone GET stream, and CORS is enabled for web clients.

Embedding in an existing WAI stack

The MCP endpoint is a plain WAI application, exported as mcpApplication, so it can be mounted inside your own Warp settings, TLS, middleware or router:

import MCP.Server (mcpApplication, defaultHttpConfig)
import qualified Network.Wai.Handler.Warp as Warp

main = Warp.runSettings mySettings $ \req respond ->
    -- route /mcp to the MCP endpoint, everything else to your app
    mcpApplication defaultHttpConfig serverInfo handlers req respond

httpPort and httpHost are ignored when embedding; the endpoint path, Origin validation, bearer auth and streaming all apply as usual.

Long-running tools

Progress and logging

Handlers report progress and send log messages to the calling client through actions on the ClientContext. Both are safe to call unconditionally:

handleTool ctx (ImportData file) = do
    reportProgress ctx 0.0 (Just 1.0) (Just "starting import")
    logToClient ctx LogInfo (String "opening file")
    ...
    reportProgress ctx 1.0 (Just 1.0) Nothing

• reportProgress emits notifications/progress only when the request carried a progressToken. Progress values must increase call over call.
• logToClient emits notifications/message only when the request declared io.modelcontextprotocol/logLevel, as the spec requires, and drops messages below the declared level.

On stdio the notifications interleave before the response. On HTTP, a request that opted in is answered with an SSE stream carrying the notifications followed by the final response; other requests keep the single-JSON response.

Cancellation

In-flight requests can be cancelled, after which the server stops work as soon as practical and sends nothing further for that request:

• stdio: each request runs in its own task, and a notifications/cancelled naming its id cancels that task. Unknown or completed ids are ignored.
• HTTP: closing the response stream is the cancellation signal. SSE responses detect the disconnect within one keep-alive interval. Single-JSON responses only detect it at the final write, so clients wanting cancellable calls should opt into streaming via a progressToken.

Cancellation is delivered as an asynchronous exception, the standard GHC mechanism used by timeout and cancel. Handlers are interruptible wherever they block in IO, and one that acquires resources should release them with bracket or finally:

handleTool ctx (ImportData file) =
    bracket (openFile file ReadMode) hClose $ \h -> do
        ...

Critical sections can be shielded with mask, but keep them short: cancellation waits for them.

Concurrency

Requests are served concurrently on both transports. Handlers touching shared mutable state must synchronize with MVar, STM or similar.

Change notifications

Servers whose tool, prompt or resource lists change at runtime can push change notifications. Create a notifier, hand its source to the transport, and call the notifier when things change:

main = do
    (notifier, source) <- newMcpNotifier
    _ <- forkIO $ appLogic notifier   -- calls notifyToolsListChanged etc.
    runMcpServerStdioWithConfig
        defaultStdioConfig { stdioNotifications = Just source }
        serverInfo handlers

Delivery is transport- and era-aware, and the listChanged and subscribe capabilities are advertised only where delivery is possible:

• Modern clients (2026-07-28) open a subscriptions/listen stream (a long-lived SSE response over HTTP) and receive only the notification types they opted into, tagged with their subscription id, including notifications/resources/updated for watched URIs.
• Legacy stdio clients receive spontaneous untagged notifications once their notifications/initialized arrives.
• Legacy HTTP clients have no delivery channel, so nothing is advertised.

Protocol support

Feature	Legacy (2024-11-05 to 2025-11-25)	Modern (2026-07-28)
Version selection	initialize handshake	per-request _meta, server/discover
Prompts, resources, resource templates, tools	✅	✅
Argument completion	✅	✅
Tool annotations, icons, structured output	✅	✅
Progress and per-request logging	✅	✅
Cancellation	✅	✅
Change notifications	stdio only	subscriptions/listen (stdio and HTTP)
Result typing and cacheability hints	—	resultType, httpCacheHints
HTTP request-metadata headers	—	MCP-Protocol-Version, Mcp-Method, Mcp-Name validated

Design decisions and planned work (input-required results, OAuth resource metadata, pagination, the tasks extension) live as ADRs under specs/, ordered by specs/ROADMAP.md.

Examples

• examples/Simple/: a key-value store with two tools over stdio.
• examples/Complete/: prompts, resources, a resource template, tools with enum, nested and list arguments, isError, annotations, progress and completions.
• examples/HttpSimple/: the key-value store over Streamable HTTP.

cabal run simple-example        # stdio; type JSON-RPC on stdin
cabal run http-simple-example   # http://localhost:3000/mcp

Conformance corpus

The wire-format fixtures under test/golden/ are a language-agnostic MCP conformance corpus: each case is a raw JSON-RPC .request.json and the exact .response.json a reference server answers, per protocol era, enumerated by a manifest.json. Any MCP implementation that reproduces the small reference server described there can replay the requests and diff the responses. Contributions of new cases are welcome.

Documentation

• API documentation on Hackage
• MCP Specification (2026-07-28)
• MCP Specification (2025-11-25, newest legacy revision)

Contributing

Contributions are welcome. See the issue tracker for open issues and feature requests, and RELEASING.md for how versions reach Hackage.

AI assistance

Much of this library was written with Claude, working from a specification I wrote and iterating together until I was happy with the result. I review and maintain all of it, but parts such as the Template Haskell derivation sit outside what I would have written unaided, and I expect to keep refactoring them.

License

BSD-3-Clause