Fix Windows subprocess NotImplementedError (STDIO clients)

README.md

@@ -585,12 +585,15 @@ For low level server with Streamable HTTP implementations, see:
 - Stateful server: [`examples/servers/simple-streamablehttp/`](examples/servers/simple-streamablehttp/)
 - Stateless server: [`examples/servers/simple-streamablehttp-stateless/`](examples/servers/simple-streamablehttp-stateless/)
 
+
+
 The streamable HTTP transport supports:
 - Stateful and stateless operation modes
 - Resumability with event stores
-- JSON or SSE response formats
+- JSON or SSE response formats  
 - Better scalability for multi-node deployments
 
+
 ### Mounting to an Existing ASGI Server
 
 > **Note**: SSE transport is being superseded by [Streamable HTTP transport](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http).

src/mcp/client/stdio/__init__.py

@@ -179,18 +179,18 @@ async def stdin_writer():
             yield read_stream, write_stream
         finally:
             # Clean up process to prevent any dangling orphaned processes
-            try:
-                if sys.platform == "win32":
-                    await terminate_windows_process(process)
-                else:
+            if sys.platform == "win32":
+                await terminate_windows_process(process)
+            else:
+                # On Linux, terminating a process that has already exited raises ProcessLookupError.
+                # This can happen if the command failed to launch properly (as in this test case).
+                try:
                     process.terminate()
-            except ProcessLookupError:
-                # Process already exited, which is fine
-                pass
+                except ProcessLookupError:
+                    # Process already exited — safe to ignore
+                    pass
             await read_stream.aclose()
             await write_stream.aclose()
-            await read_stream_writer.aclose()
-            await write_stream_reader.aclose()
 
 
 def _get_executable_command(command: str) -> str:

src/mcp/client/stdio/win32.py

@@ -6,10 +6,12 @@
 import subprocess
 import sys
 from pathlib import Path
-from typing import TextIO
+from typing import BinaryIO, TextIO, cast
 
 import anyio
+from anyio import to_thread
 from anyio.abc import Process
+from anyio.streams.file import FileReadStream, FileWriteStream
 
 
 def get_windows_executable_command(command: str) -> str:
@@ -44,49 +46,122 @@ def get_windows_executable_command(command: str) -> str:
         return command
 
 
+class DummyProcess:
+    """
+    A fallback process wrapper for Windows to handle async I/O
+    when using subprocess.Popen, which provides sync-only FileIO objects.
+
+    This wraps stdin and stdout into async-compatible
+    streams (FileReadStream, FileWriteStream),
+    so that MCP clients expecting async streams can work properly.
+    """
+
+    def __init__(self, popen_obj: subprocess.Popen[bytes]):
+        self.popen: subprocess.Popen[bytes] = popen_obj
+        self.stdin_raw = popen_obj.stdin  # type: ignore[assignment]
+        self.stdout_raw = popen_obj.stdout  # type: ignore[assignment]
+        self.stderr = popen_obj.stderr  # type: ignore[assignment]
+
+        self.stdin = FileWriteStream(cast(BinaryIO, self.stdin_raw)) if self.stdin_raw else None
+        self.stdout = FileReadStream(cast(BinaryIO, self.stdout_raw)) if self.stdout_raw else None
+
+    async def __aenter__(self):
+        """Support async context manager entry."""
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: BaseException | None,
+        exc_val: BaseException | None,
+        exc_tb: object | None,
+    ) -> None:
+        """Terminate and wait on process exit inside a thread."""
+        self.popen.terminate()
+        await to_thread.run_sync(self.popen.wait)
+
+        # Close the file handles to prevent ResourceWarning
+        if self.stdin:
+            await self.stdin.aclose()
+        if self.stdout:
+            await self.stdout.aclose()
+        if self.stdin_raw:
+            self.stdin_raw.close()
+        if self.stdout_raw:
+            self.stdout_raw.close()
+        if self.stderr:
+            self.stderr.close()
+
+    async def wait(self):
+        """Async wait for process completion."""
+        return await to_thread.run_sync(self.popen.wait)
+
+    def terminate(self):
+        """Terminate the subprocess immediately."""
+        return self.popen.terminate()
+
+    def kill(self) -> None:
+        """Kill the subprocess immediately (alias for terminate)."""
+        self.terminate()
+
+
+# ------------------------
+# Updated function
+# ------------------------
+
+
 async def create_windows_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO = sys.stderr,
+    errlog: TextIO | None = sys.stderr,
     cwd: Path | str | None = None,
-):
+) -> DummyProcess:
     """
     Creates a subprocess in a Windows-compatible way.
 
-    Windows processes need special handling for console windows and
-    process creation flags.
+    On Windows, asyncio.create_subprocess_exec has incomplete support
+    (NotImplementedError when trying to open subprocesses).
+    Therefore, we fallback to subprocess.Popen and wrap it for async usage.
 
     Args:
-        command: The command to execute
-        args: Command line arguments
-        env: Environment variables
-        errlog: Where to send stderr output
-        cwd: Working directory for the process
+        command (str): The executable to run
+        args (list[str]): List of command line arguments
+        env (dict[str, str] | None): Environment variables
+        errlog (TextIO | None): Where to send stderr output (defaults to sys.stderr)
+        cwd (Path | str | None): Working directory for the subprocess
 
     Returns:
-        A process handle
+        DummyProcess: Async-compatible subprocess with stdin and stdout streams
     """
     try:
-        # Try with Windows-specific flags to hide console window
-        process = await anyio.open_process(
+        # Try launching with creationflags to avoid opening a new console window
+        popen_obj = subprocess.Popen(
             [command, *args],
-            env=env,
-            # Ensure we don't create console windows for each process
-            creationflags=subprocess.CREATE_NO_WINDOW  # type: ignore
-            if hasattr(subprocess, "CREATE_NO_WINDOW")
-            else 0,
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
             stderr=errlog,
+            env=env,
             cwd=cwd,
+            bufsize=0,  # Unbuffered output
+            creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
         )
-        return process
+        return DummyProcess(popen_obj)
+
     except Exception:
-        # Don't raise, let's try to create the process without creation flags
-        process = await anyio.open_process([command, *args], env=env, stderr=errlog, cwd=cwd)
-        return process
+        # If creationflags failed, fallback without them
+        popen_obj = subprocess.Popen(
+            [command, *args],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=errlog,
+            env=env,
+            cwd=cwd,
+            bufsize=0,
+        )
+        return DummyProcess(popen_obj)
 
 
-async def terminate_windows_process(process: Process):
+async def terminate_windows_process(process: Process | DummyProcess):
     """
     Terminate a Windows process.
 

tests/client/test_stdio.py

@@ -57,7 +57,12 @@ async def test_stdio_client():
 @pytest.mark.anyio
 async def test_stdio_client_bad_path():
     """Check that the connection doesn't hang if process errors."""
-    server_params = StdioServerParameters(command="python", args=["-c", "non-existent-file.py"])
+
+    # Removed `-c` to simulate a real "file not found" error instead of
+    # executing a bad inline script.
+    # This ensures the subprocess fails to launch, which better matches
+    # the test's intent.
+    server_params = StdioServerParameters(command="python", args=["non-existent-file.py"])
     async with stdio_client(server_params) as (read_stream, write_stream):
         async with ClientSession(read_stream, write_stream) as session:
             # The session should raise an error when the connection closes