Annotate stdio_client's return type with TransportStreams

stdio_client now carries the same AsyncGenerator[TransportStreams, None]
annotation as streamable_http_client, so it formally satisfies the
Transport protocol instead of leaking anyio's concrete memory-stream
types. Also adds the missing -> None on FallbackProcess.__init__ and
retypes the test helper to the ReadStream protocol.

Author: maxisbey

src/mcp/client/stdio.py

@@ -12,6 +12,7 @@
 import logging
 import os
 import sys
+from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager, suppress
 from pathlib import Path
 from typing import Literal, TextIO
@@ -23,6 +24,7 @@
 from pydantic import BaseModel, Field
 
 from mcp import types
+from mcp.client._transport import TransportStreams
 from mcp.os.posix.utilities import terminate_posix_process_tree
 from mcp.os.win32.utilities import (
     ServerProcess,
@@ -129,7 +131,9 @@ class StdioServerParameters(BaseModel):
 
 
 @asynccontextmanager
-async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
+async def stdio_client(
+    server: StdioServerParameters, errlog: TextIO = sys.stderr
+) -> AsyncGenerator[TransportStreams, None]:
     """Client transport for stdio: this will connect to a server by spawning a
     process and communicating with it over stdin/stdout.
 

src/mcp/os/win32/utilities.py

@@ -82,7 +82,7 @@ class FallbackProcess:
     so that MCP clients expecting async streams can work properly.
     """
 
-    def __init__(self, popen_obj: subprocess.Popen[bytes]):
+    def __init__(self, popen_obj: subprocess.Popen[bytes]) -> None:
         self.popen: subprocess.Popen[bytes] = popen_obj
         stdin = popen_obj.stdin
         stdout = popen_obj.stdout

tests/client/test_stdio.py

@@ -29,6 +29,7 @@
 from anyio.streams.memory import MemoryObjectReceiveStream
 
 from mcp.client import stdio
+from mcp.client._transport import ReadStream
 from mcp.client.session import ClientSession
 from mcp.client.stdio import (
     _EXIT_POLL_INTERVAL,
@@ -224,7 +225,7 @@ def _line(message: JSONRPCMessage) -> bytes:
     return (message.model_dump_json(by_alias=True, exclude_unset=True) + "\n").encode()
 
 
-async def _next_message(read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]) -> JSONRPCMessage:
+async def _next_message(read_stream: ReadStream[SessionMessage | Exception]) -> JSONRPCMessage:
     received = await read_stream.receive()
     assert isinstance(received, SessionMessage)
     return received.message