Restore anyio.open_process as primary Windows process creation method

Changes create_windows_process to try anyio.open_process first before
falling back to FallbackProcess. This ensures ProactorEventLoop users
get proper async subprocess support while maintaining compatibility
for SelectorEventLoop users (Python 3.13+, Streamlit, etc).

The function now returns Process | FallbackProcess to reflect both
possible return types.

This makes the fallback truly a fallback rather than the default,
improving performance and compatibility for users with properly
configured event loops.

Author: felixweinberger

src/mcp/client/stdio/win32.py

@@ -9,6 +9,7 @@
 from typing import BinaryIO, TextIO, cast
 
 import anyio
+from anyio.abc import Process
 from anyio.streams.file import FileReadStream, FileWriteStream
 
 
@@ -127,13 +128,13 @@ async def create_windows_process(
     env: dict[str, str] | None = None,
     errlog: TextIO | None = sys.stderr,
     cwd: Path | str | None = None,
-) -> FallbackProcess:
+) -> Process | FallbackProcess:
     """
     Creates a subprocess in a Windows-compatible way.
 
-    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.
+    First attempts to use anyio.open_process (works with ProactorEventLoop).
+    Falls back to subprocess.Popen wrapped in FallbackProcess if that fails
+    (typically with SelectorEventLoop in Python 3.13+ or frameworks like Streamlit).
 
     Args:
         command (str): The executable to run
@@ -143,31 +144,52 @@ async def create_windows_process(
         cwd (Path | str | None): Working directory for the subprocess
 
     Returns:
-        FallbackProcess: Async-compatible subprocess with stdin and stdout streams
+        Process | FallbackProcess: Either anyio Process or FallbackProcess wrapper
     """
     try:
-        # Try launching with creationflags to avoid opening a new console window
-        popen_obj = subprocess.Popen(
+        # Try with Windows-specific flags to hide console window
+        process = await anyio.open_process(
             [command, *args],
-            stdin=subprocess.PIPE,
-            stdout=subprocess.PIPE,
-            stderr=errlog,
             env=env,
+            stderr=errlog,
             cwd=cwd,
-            bufsize=0,  # Unbuffered output
-            creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
+            creationflags=subprocess.CREATE_NO_WINDOW  # type: ignore
+            if hasattr(subprocess, "CREATE_NO_WINDOW")
+            else 0,
         )
-        return FallbackProcess(popen_obj)
-
+        return process
+    except NotImplementedError:
+        # Fall back to subprocess.Popen for SelectorEventLoop compatibility
+        try:
+            popen_obj = subprocess.Popen(
+                [command, *args],
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=errlog,
+                env=env,
+                cwd=cwd,
+                bufsize=0,  # Unbuffered output
+                creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
+            )
+            return FallbackProcess(popen_obj)
+        except Exception:
+            # If creationflags failed, try without them
+            popen_obj = subprocess.Popen(
+                [command, *args],
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=errlog,
+                env=env,
+                cwd=cwd,
+                bufsize=0,
+            )
+            return FallbackProcess(popen_obj)
     except Exception:
-        # If creationflags failed, fallback without them
-        popen_obj = subprocess.Popen(
+        # If anyio failed for other reasons, try without creation flags
+        process = await anyio.open_process(
             [command, *args],
-            stdin=subprocess.PIPE,
-            stdout=subprocess.PIPE,
-            stderr=errlog,
             env=env,
+            stderr=errlog,
             cwd=cwd,
-            bufsize=0,
         )
-        return FallbackProcess(popen_obj)
+        return process