stdio.py

"""
Stdio Server Transport Module

This module provides functionality for creating an stdio-based transport layer
that can be used to communicate with an MCP client through standard input/output
streams.

Example usage:
```
    async def run_server():
        async with stdio_server() as (read_stream, write_stream):
            # read_stream contains incoming JSONRPCMessages from stdin
            # write_stream allows sending JSONRPCMessages to stdout
            server = await create_my_server()
            await server.run(read_stream, write_stream, init_options)

    anyio.run(run_server)
```
"""

import sys
from contextlib import asynccontextmanager
from io import TextIOWrapper

import anyio
import anyio.lowlevel

import mcp.types as types
from mcp.shared.session import (
    ReadStream,
    ReadStreamWriter,
    WriteStream,
    WriteStreamReader,
)
from mcp.types import MessageFrame


@asynccontextmanager
async def stdio_server(
    stdin: anyio.AsyncFile[str] | None = None,
    stdout: anyio.AsyncFile[str] | None = None,
):
    """
    Server transport for stdio: this communicates with an MCP client by reading
    from the current process' stdin and writing to stdout.
    """
    # Purposely not using context managers for these, as we don't want to close
    # standard process handles. Encoding of stdin/stdout as text streams on
    # python is platform-dependent (Windows is particularly problematic), so we
    # re-wrap the underlying binary stream to ensure UTF-8.
    if not stdin:
        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))
    if not stdout:
        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))

    read_stream: ReadStream
    read_stream_writer: ReadStreamWriter

    write_stream: WriteStream
    write_stream_reader: WriteStreamReader

    read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
    write_stream, write_stream_reader = anyio.create_memory_object_stream(0)

    async def stdin_reader():
        try:
            async with read_stream_writer:
                async for line in stdin:
                    try:
                        message = types.JSONRPCMessage.model_validate_json(line)
                    except Exception as exc:
                        await read_stream_writer.send(exc)
                        continue

                    await read_stream_writer.send(
                        MessageFrame(message=message, raw=line)
                    )
        except anyio.ClosedResourceError:
            await anyio.lowlevel.checkpoint()

    async def stdout_writer():
        try:
            async with write_stream_reader:
                async for message in write_stream_reader:
                    # Extract the inner JSONRPCRequest/JSONRPCResponse from MessageFrame
                    json = message.model_dump_json(by_alias=True, exclude_none=True)
                    await stdout.write(json + "\n")
                    await stdout.flush()
        except anyio.ClosedResourceError:
            await anyio.lowlevel.checkpoint()

    async with anyio.create_task_group() as tg:
        tg.start_soon(stdin_reader)
        tg.start_soon(stdout_writer)
        yield read_stream, write_stream