fix: make Windows stdio pywin32 optional

README.v2.md

@@ -124,6 +124,13 @@ Alternatively, for projects using pip for dependencies:
 pip install "mcp[cli]"
 ```
 
+On Windows, add the `stdio` extra if you want `pywin32`-backed Job Object cleanup for subprocesses spawned through `mcp.client.stdio`:
+
+```bash
+uv add "mcp[cli,stdio]"
+pip install "mcp[cli,stdio]"
+```
+
 ### Running the standalone MCP development tools
 
 To run the mcp command with uv:

docs/installation.md

@@ -24,8 +24,10 @@ The following dependencies are automatically installed:
 - [`pydantic-settings`](https://pypi.org/project/pydantic-settings/): Settings management used in MCPServer.
 - [`uvicorn`](https://pypi.org/project/uvicorn/): ASGI server used to run the HTTP transport endpoints.
 - [`jsonschema`](https://pypi.org/project/jsonschema/): JSON schema validation.
-- [`pywin32`](https://pypi.org/project/pywin32/): Windows specific dependencies for the CLI tools.
 
 This package has the following optional groups:
 
 - `cli`: Installs `typer` and `python-dotenv` for the MCP CLI tools.
+- `stdio`: Installs [`pywin32`](https://pypi.org/project/pywin32/) on Windows for Job Object based subprocess cleanup in `mcp.client.stdio`.
+
+Server-only Windows installs can use the base `mcp` package (or `mcp[cli]`) without pulling in `pywin32`. Add the `stdio` extra only if you want the additional Windows stdio cleanup integration.

pyproject.toml

@@ -36,7 +36,6 @@ dependencies = [
     "pydantic-settings>=2.5.2",
     "uvicorn>=0.31.1; sys_platform != 'emscripten'",
     "jsonschema>=4.20.0",
-    "pywin32>=311; sys_platform == 'win32'",
     "pyjwt[crypto]>=2.10.1",
     "typing-extensions>=4.13.0",
     "typing-inspection>=0.4.1",
@@ -45,6 +44,7 @@ dependencies = [
 [project.optional-dependencies]
 rich = ["rich>=13.9.4"]
 cli = ["typer>=0.16.0", "python-dotenv>=1.0.0"]
+stdio = ["pywin32>=311; sys_platform == 'win32'"]
 ws = ["websockets>=15.0.1"]
 
 [project.scripts]
@@ -56,8 +56,8 @@ required-version = ">=0.9.5"
 
 [dependency-groups]
 dev = [
-    # We add mcp[cli,ws] so `uv sync` considers the extras.
-    "mcp[cli,ws]",
+    # We add mcp[cli,stdio,ws] so `uv sync` considers the extras.
+    "mcp[cli,stdio,ws]",
     "pyright>=1.1.400",
     "pytest>=8.3.4",
     "ruff>=0.8.5",

src/mcp/__init__.py

@@ -1,7 +1,8 @@
+from importlib import import_module
+from typing import TYPE_CHECKING, Any
+
 from .client.client import Client
 from .client.session import ClientSession
-from .client.session_group import ClientSessionGroup
-from .client.stdio import StdioServerParameters, stdio_client
 from .server.session import ServerSession
 from .server.stdio import stdio_server
 from .shared.exceptions import MCPError, UrlElicitationRequiredError
@@ -65,6 +66,10 @@
 )
 from .types import Role as SamplingRole
 
+if TYPE_CHECKING:
+    from .client.session_group import ClientSessionGroup
+    from .client.stdio import StdioServerParameters, stdio_client
+
 __all__ = [
     "CallToolRequest",
     "Client",
@@ -133,3 +138,19 @@
     "stdio_client",
     "stdio_server",
 ]
+
+_LAZY_EXPORTS = {
+    "ClientSessionGroup": (".client.session_group", "ClientSessionGroup"),
+    "StdioServerParameters": (".client.stdio", "StdioServerParameters"),
+    "stdio_client": (".client.stdio", "stdio_client"),
+}
+
+
+def __getattr__(name: str) -> Any:
+    if name not in _LAZY_EXPORTS:
+        raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+    module_name, export_name = _LAZY_EXPORTS[name]
+    value = getattr(import_module(module_name, __name__), export_name)
+    globals()[name] = value
+    return value

src/mcp/os/win32/utilities.py

@@ -17,10 +17,16 @@
 
 # Windows-specific imports for Job Objects
 if sys.platform == "win32":
-    import pywintypes
-    import win32api
-    import win32con
-    import win32job
+    try:
+        import pywintypes
+        import win32api
+        import win32con
+        import win32job
+    except ImportError:
+        win32api = None
+        win32con = None
+        win32job = None
+        pywintypes = None
 else:
     # Type stubs for non-Windows platforms
     win32api = None

tests/issues/test_2233_windows_stdio_imports.py

@@ -0,0 +1,64 @@
+"""Regression tests for issue #2233: stdio imports should stay optional on Windows."""
+
+import os
+import subprocess
+import sys
+import textwrap
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+
+
+def _run_repo_python(script: str) -> subprocess.CompletedProcess[str]:
+    env = os.environ.copy()
+    src_path = str(REPO_ROOT / "src")
+    env["PYTHONPATH"] = src_path if "PYTHONPATH" not in env else os.pathsep.join([src_path, env["PYTHONPATH"]])
+    return subprocess.run(
+        [sys.executable, "-c", script],
+        cwd=REPO_ROOT,
+        env=env,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+
+
+def test_server_stdio_import_does_not_load_client_stdio():
+    script = textwrap.dedent("""
+        import importlib
+        import sys
+
+        importlib.import_module("mcp.server.stdio")
+        assert "mcp.client.stdio" not in sys.modules
+    """)
+
+    result = _run_repo_python(script)
+
+    assert result.returncode == 0, result.stderr
+
+
+def test_root_stdio_exports_handle_missing_pywin32():
+    script = textwrap.dedent("""
+        import builtins
+
+        real_import = builtins.__import__
+        blocked_modules = {"pywintypes", "win32api", "win32con", "win32job"}
+
+        def guarded_import(name, globals=None, locals=None, fromlist=(), level=0):
+            if name.split(".", 1)[0] in blocked_modules:
+                raise ImportError(f"blocked import: {name}")
+            return real_import(name, globals, locals, fromlist, level)
+
+        builtins.__import__ = guarded_import
+        try:
+            from mcp import StdioServerParameters, stdio_client
+
+            assert StdioServerParameters.__name__ == "StdioServerParameters"
+            assert callable(stdio_client)
+        finally:
+            builtins.__import__ = real_import
+    """)
+
+    result = _run_repo_python(script)
+
+    assert result.returncode == 0, result.stderr