Let Client take StdioServerParameters directly (#3321)

tests/client/test_client.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import contextvars
+import sys
 from collections.abc import AsyncIterator, Iterator
 from contextlib import asynccontextmanager, contextmanager
 from unittest.mock import patch
@@ -35,7 +36,7 @@ from mcp_types import (
 from mcp_types.version import LATEST_HANDSHAKE_VERSION
 from pydantic import FileUrl
 
-from mcp import MCPDeprecationWarning, MCPError
+from mcp import MCPDeprecationWarning, MCPError, StdioServerParameters
 from mcp.client._memory import InMemoryTransport
 from mcp.client._transport import TransportStreams
 from mcp.client.client import Client
@@ -414,6 +415,24 @@ async def test_client_uses_transport_directly(app: MCPServer):
         )
 
 
+async def test_client_with_stdio_parameters_launches_the_server_as_a_subprocess() -> None:
+    """SDK-defined: `Client` routes a `StdioServerParameters` through `stdio_client`, so entering it
+    spawns the command and negotiates over the child's stdin/stdout. The process boundary is the
+    behaviour, hence a real child interpreter running a one-line `MCPServer`."""
+    params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", "from mcp.server import MCPServer; MCPServer('stdio-demo').run()"],
+    )
+    # Wider than the standard 5: a cold interpreter start plus `import mcp.server` in the child takes
+    # seconds on a loaded Windows runner, and exit may wait out stdio_client's terminate/kill
+    # escalation (PROCESS_TERMINATION_TIMEOUT + FORCE_KILL_TIMEOUT + reap, ~6s) if the child is slow.
+    with anyio.fail_after(20):
+        async with Client(params) as client:
+            assert client.server_info is not None
+            assert client.server_info.name == "stdio-demo"
+            assert (await client.list_tools()).tools == []
+
+
 _TEST_CONTEXTVAR = contextvars.ContextVar("test_var", default="initial")
 
 

tests/docs_src/test_client_transports.py

@@ -6,7 +6,7 @@ import pytest
 
 from docs_src.client_transports import tutorial001, tutorial004
 from mcp import Client
-from mcp.client.stdio import get_default_environment, stdio_client
+from mcp.client.stdio import get_default_environment
 from mcp.client.streamable_http import streamable_http_client
 
 # See test_index.py for why this is a per-module mark and not a conftest hook.
@@ -41,9 +41,9 @@ async def test_streamable_http_configuration_lives_on_the_httpx_client() -> None
     assert list(inspect.signature(streamable_http_client).parameters) == ["url", "http_client", "terminate_on_close"]
 
 
-async def test_stdio_parameters_are_wrapped_by_stdio_client() -> None:
-    """tutorial004: `stdio_client(params)` is the transport, and `Client` takes it like any other."""
-    client = Client(stdio_client(tutorial004.server))
+async def test_stdio_parameters_go_straight_to_client() -> None:
+    """tutorial004: `Client` takes the `StdioServerParameters` directly, and nothing is spawned until you enter it."""
+    client = Client(tutorial004.server)
     with pytest.raises(RuntimeError, match="Client must be used within an async context manager"):
         client.session