nn-mcp-stdio -- No nonsense stdio MCP server framework

Build a Model Context Protocol server (protocol 2025-11-25) by writing plain async def handlers and decorating them. You annotate the arguments; the server derives the tool's inputSchema, validates every call strictly, and hands your handler natural keyword arguments. No pydantic, no code generation, no config.

Goals

• A stdio MCP server, specification-conformant.

• Strict validation. Arguments are checked against the generated JSON Schema with jsonschema -- and never coerced. {"n": "3"} for an int is an error, not a silent 3.

• Annotations and docstrings are the source of truth -- one obvious place for each fact, no duplication to keep in sync.

Related MCP server: Easy MCP Server

Non-Goals

• HTTP, websocket (stdio only).

• Dependency injection.

• Storage layers.

Install

pip install nn-mcp-stdio

Runtime dependencies: nn-mcp-types (MCP dataclasses + schema generation), jsonschema (validation), and aiojobs (the handler scheduler).

Implementation overview

The server is a small asyncio pipeline: a single reader parses each stdin line and dispatches by message shape; an aiojobs scheduler runs handlers concurrently and in isolation; each handler enqueues its reply on an outbound queue that a single writer drains to stdout. Logging goes to stderr, so it never corrupts the protocol stream.

A tool wires three pieces of nn-mcp-types together: your handler's signature becomes a synthesised dataclass (parameters become fields, defaults and Annotated metadata carried through); that dataclass becomes the tool's inputSchema (JSON Schema 2020-12); the handler's docstring becomes the tool description and its name the tool name. On tools/call the arguments dict is validated against the schema, reconstructed into the typed dataclass, and the handler is called with natural kwargs.

Example

One server exposing a tool, two fixed resources (a literal and a file), a dynamic resource read on demand, and a URI-template resource:

import asyncio
import json
import pathlib
import typing

from nn_mcp_stdio import Context, Server
from nn_mcp_types import resources
from nn_mcp_types.content import TextResourceContents
from nn_mcp_types.schema import SchemaAnnotation

server = Server(name="demo", version="0.1.0")


@server.tool()
async def crop(
    url: typing.Annotated[str, SchemaAnnotation(description="Image URL")],
    width: typing.Annotated[
        int, SchemaAnnotation(description="Target width (px)", minimum=1)
    ] = 800,
    ctx: Context = None,
) -> str:
    """Crop the image at `url` to `width` pixels."""
    await ctx.info(f"cropping {url} to {width}px", logger="crop")
    return f"cropped {url} to {width}px"


# A literal resource: contents fixed in code, served on every read.
server.add_resource_from_literal(
    resources.Resource(
        uri="config://service",
        name="config",
        title="The service configuration",
        mime_type="application/json",
    ),
    json.dumps({"debug": True}),
)

# A file resource: read lazily on each read, so edits are reflected.
server.add_resource_from_path(
    resources.Resource(uri="file:///README.md", name="readme"),
    pathlib.Path("README.md"),
    describe_contents=lambda path, data: ("text/markdown", TextResourceContents),
)


# A dynamic resource: a reader computes the contents on each read.
@server.resource(
    resources.Resource(
        uri="clock://now",
        name="clock",
        mime_type="text/plain",
    )
)
async def clock() -> str:
    import datetime

    return datetime.datetime.now().isoformat()


# A resource template: a URI shape read on demand. The client expands the
# RFC 6570 template; a read routes back here with `{name}` extracted.
@server.resource_template(
    resources.ResourceTemplate(
        uri_template="greeting://{name}",
        name="greeting",
        mime_type="text/plain",
    )
)
async def greeting(name: str) -> str:
    return f"Hello, {name}!"


asyncio.run(server.run())

Documentation

Guides to using each building block effectively:

• Tools -- annotated handlers, per-argument constraints, structured output, return types, and errors.

• Resources -- dynamic readers, the fixed literal/file registrations, and URI-template resources.

• Context -- logging and progress back to the client mid-call.

• Other handlers -- registering raw requests and notifications with @server.request / @server.notification.