Function stdio_client

src/mcp/client/stdio.py:114–215

Spawns an MCP server subprocess and connects to it over stdin/stdout. Raises: OSError: If the server process cannot be spawned. ValueError: If the spawn parameters are invalid (embedded NUL bytes).

(
    server: StdioServerParameters, errlog: TextIO = sys.stderr
)

@asynccontextmanager
async def stdio_client(
    server: StdioServerParameters, errlog: TextIO = sys.stderr
) -> AsyncGenerator[TransportStreams, None]:
    """Spawns an MCP server subprocess and connects to it over stdin/stdout.

    Raises:
        OSError: If the server process cannot be spawned.
        ValueError: If the spawn parameters are invalid (embedded NUL bytes).
    """
    command = _get_executable_command(server.command)

    process = await _create_platform_compatible_process(
        command=command,
        args=server.args,
        env=get_default_environment() | (server.env or {}),
        errlog=errlog,
        cwd=server.cwd,
    )

    # The spawn succeeded; no awaits until the task group is entered, or a
    # cancellation delivered in the gap would leak the live process.
    read_stream_writer, read_stream = anyio.create_memory_object_stream[SessionMessage | Exception](0)
    write_stream, write_stream_reader = anyio.create_memory_object_stream[SessionMessage](0)

    shutting_down = False
    writer_done = anyio.Event()

    async def stdout_reader() -> None:
        assert process.stdout, "Opened process is missing stdout"

        stdout = TextReceiveStream(process.stdout, encoding=server.encoding, errors=server.encoding_error_handler)
        try:
            async with read_stream_writer:
                try:
                    # One line at a time; no read-ahead while a delivery is blocked.
                    buffer = ""
                    async for chunk in stdout:
                        lines = (buffer + chunk).split("\n")
                        buffer = lines.pop()
                        for line in lines:
                            try:
                                await read_stream_writer.send(_parse_line(line))
                            except (anyio.ClosedResourceError, anyio.BrokenResourceError):
                                return  # the session is gone; only the drain below remains
                finally:
                    await _drain_stdout(process)
        except anyio.ClosedResourceError:
            pass  # our own shutdown closed the stdout stream under the read
        except (anyio.BrokenResourceError, ConnectionError):
            # Teardown noise during shutdown, a real failure otherwise; either way
            # the session sees clean closure when the read stream closes.
            if not shutting_down:
                logger.exception("Reading from the MCP server's stdout failed mid-session")

    async def stdin_writer() -> None:
        assert process.stdin, "Opened process is missing stdin"

        try: