dpella/mcp reference

Canonical source: https://github.com/dpella/mcp. Two Hackage packages: mcp-types (pure types, minimal deps) and mcp (Servant-based server with HTTP/Stdio transports). MCP protocol version 2025-06-18. Depends on jsonrpc (also a DPella package, not in nixpkgs).

Versions to pin

Use GitHub sources rather than Hackage when the target GHC's all-cabal-hashes snapshot lags behind:

# flake.nix inputs
dpella-mcp.url = "github:dpella/mcp";
dpella-mcp.flake = false;
dpella-jsonrpc.url = "github:dpella/jsonrpc";
dpella-jsonrpc.flake = false;

# haskell-flake
packages.mcp.source       = inputs.dpella-mcp + /mcp-server;
packages.mcp-types.source = inputs.dpella-mcp + /mcp-types;
packages.jsonrpc.source   = inputs.dpella-jsonrpc;

Supported GHC: 9.6–9.12 (via base >=4.18 && <4.22). servant-auth-server 0.4.x and warp 3.4.x are sibling deps.

Required type-family instances

Every user of MCP.Server must provide both:

type instance MCPHandlerState = YourSessionState
type instance MCPHandlerUser  = YourUserPayload  -- for JWT; use () when using simpleHttpApp

Instances must live in a module that gets compiled before initMCPServerState is called.

Three transports

Transport	Auth	Entry point	Notes
HTTP + JWT	servant-auth-server	mcpAPI stateVar via serveWithContext (Proxy @MCPAPI) ctx	Production web
Simple HTTP	none	simpleHttpApp stateVar :: Application	Local/loopback only; pair with Warp.run or Warp.runSettings
Stdio	none	serveStdio stdin stdout initialState	Subprocess integrations

For HTTP use Warp.setBeforeMainLoop to log after the socket is bound — do not print before Warp.run/runSettings, the bind hasn't happened yet and the log lies.

initMCPServerState signature

initMCPServerState
  :: MCPHandlerState                                                  -- initial state
  -> Maybe (MCPHandlerUser -> MCPHandlerState -> IO MCPHandlerState)  -- onInitialize (JWT only)
  -> Maybe (MCPHandlerState -> IO MCPHandlerState)                    -- onFinalize (after each req)
  -> ServerCapabilities                                               -- what to advertise
  -> Implementation                                                   -- name/version/title
  -> Maybe Text                                                       -- free-text instructions
  -> ProcessHandlers                                                  -- the actual handlers
  -> MCPServerState

Wrap the result in an MVar — simpleHttpApp / mcpAPI both take MVar MCPServerState.

ServerCapabilities record

ServerCapabilities
  { logging      :: Maybe LoggingCapability
  , prompts      :: Maybe PromptsCapability    { listChanged :: Maybe Bool }
  , resources    :: Maybe ResourcesCapability  { listChanged, subscribe :: Maybe Bool }
  , tools        :: Maybe ToolsCapability      { listChanged :: Maybe Bool }
  , completions  :: Maybe CompletionsCapability
  , experimental :: Maybe ...
  }

Advertising Just … without providing a matching handler means clients will call it and hit the library's method_not_found. For Phase-1-style stubs, provide an empty-list handler (e.g. listResourcesHandler = Just (\_ -> pure $ ProcessSuccess ListResourcesResult{..})).

ProcessHandlers record

Every field is a Maybe — set only what the server implements:

• listResourcesHandler, readResourceHandler, listResourceTemplatesHandler
• listToolsHandler, callToolHandler — don't set manually; use withToolHandlers :: [ToolHandler] -> ProcessHandlers -> ProcessHandlers which wires both based on a list of ToolHandlers.
• listPromptsHandler, getPromptHandler
• completeHandler
• subscribeHandler, unsubscribeHandler

Start from defaultProcessHandlers (all Nothing) and override.

ToolHandler construction

toolHandler
  :: Text                         -- name
  -> Maybe Text                   -- description
  -> InputSchema                  -- JSON schema for args
  -> (Maybe (Map Text Value)      -- handler body
        -> MCPServerT (ProcessResult CallToolResult))
  -> ToolHandler

InputSchema "object" (Just propsMap) (Just ["req1", "req2"]) is the typical shape. withToolHandlers validates required args before calling the handler.

Returning results: toolTextResult [Text] for plain text; build CallToolResult directly when you need structured output (structuredContent :: Maybe (Map Text Value)).

Logging

mcp_log_level :: Maybe LoggingLevel on MCPServerState — defaults to Just Warning. Set to Just Debug to get one [request] / [response] stdout line per JSON-RPC call (see MCP/Server/HTTP/Internal.hs). The library does not emit anything else on its own; wire your own startup log via Warp's setBeforeMainLoop.

Clients can also change the level at runtime via logging/setLevel if the server advertises logging = Just LoggingCapability.

ProcessResult shape

data ProcessResult a
  = ProcessSuccess a
  | ProcessRPCError Int Text             -- JSON-RPC error (400-ish, 404-ish, …)
  | ProcessServerError Text              -- internal
  | ProcessClientInput Text Value cont   -- request additional input from client (sampling, elicitation)

Return ProcessRPCError 404 ("resource not found: " <> uri) for unknown URIs; the library turns this into a well-formed JSON-RPC error.

DuplicateRecordFields is required

MCP types reuse name, _meta, title, etc. across many records (Implementation, Tool, Prompt, Resource, …). Enable {-# LANGUAGE DuplicateRecordFields #-} at the use site. Field disambiguation sometimes needs a qualified prefix: MCP.name = …, MCP._meta = … where import MCP.Server qualified as MCP.

Emanote-specific conventions (phase 1)

• MCP runs beside the live server via UnliftIO.Async.race_ under the run subcommand only.
• CLI flag: emanote run --mcp-port PORT (top-level --verbose flips library log level to Debug).
• Startup line: [mcp] listening on http://localhost:PORT/mcp via Warp.setBeforeMainLoop.
• Transport: simpleHttpApp (no auth). Authentication deferred per the issue's open question.
• State types: MCPHandlerState = (), MCPHandlerUser = ().
• Handlers advertise resources and tools capabilities; empty inventories today.
• See emanote/src/Emanote/MCP.hs for the canonical shape.

Rollout tracker: srid/emanote#645. Phases 2–5 will add notebook-backed resources, query tools, subscriptions, and optional prompts respectively.

Common pitfalls

1. Version tax: Hackage's mcp moves faster than nixpkgs' all-cabal-hashes. Pin GitHub sources in flake.nix inputs when the Hackage-via-nixpkgs path refuses to build.
2. newMVar ambiguity: importing Control.Concurrent.MVar alongside Relude produces an ambiguous newMVar. Drop the explicit import — Relude's is fine.
3. NamedFieldPuns with qualified imports: ReadResourceParams {uri} only works if uri is unqualified. If you import MCP.Server qualified, either enable NamedFieldPuns and use the unqualified name, or bind fields positionally.
4. Ambiguous _meta: field is shared across a dozen records; with DuplicateRecordFields on, GHC often still demands a qualified prefix (MCP._meta) at construction sites. fourmolu will re-add the qualifier if removed.
5. Don't advertise subscribe = Just True until the server actually implements subscribeHandler / unsubscribeHandler. The library has no built-in defaults for those.