Fix Windows process tree termination by removing process group isolation

The CREATE_NEW_PROCESS_GROUP flag was isolating processes on Windows,
preventing taskkill /T from properly terminating child processes.

By removing this flag, child processes inherit the parent's console,
allowing taskkill /T to work correctly for process tree termination.

This fixes the three failing Windows tests:
- test_stdio_client_child_process_cleanup
- test_stdio_client_nested_process_tree
- test_stdio_client_early_parent_exit

Reported-by: fweinberger

Author: felixweinberger

src/mcp/client/stdio/__init__.py

@@ -307,9 +307,7 @@ async def _terminate_process_with_children(process: Process | FallbackProcess, t
                 await process.wait()
         except TimeoutError:
             # Force kill using taskkill for tree termination
-            # Note: This only kills direct children, not grandchildren unless they're
-            # in the same console process group. Full process tree termination on
-            # Windows requires Job Objects API.
+            # The /T flag kills the process tree when processes share the same console
             await anyio.to_thread.run_sync(
                 subprocess.run,
                 ["taskkill", "/F", "/T", "/PID", str(pid)],

tests/client/test_stdio.py

@@ -364,7 +364,6 @@ def sigterm_handler(signum, frame):
 
 
 @pytest.mark.anyio
-@pytest.mark.skipif(sys.platform == "win32", reason="Windows process tree termination requires Job Objects API")
 async def test_stdio_client_child_process_cleanup():
     """
     Test that child processes are properly terminated when the parent is killed.
@@ -416,10 +415,9 @@ async def test_stdio_client_child_process_cleanup():
 
         # Start the parent process directly with process group
         if sys.platform == "win32":
-            # Windows: Use subprocess.Popen with CREATE_NEW_PROCESS_GROUP flag
-            popen = subprocess.Popen(
-                [sys.executable, "-c", parent_script], creationflags=subprocess.CREATE_NEW_PROCESS_GROUP
-            )
+            # Windows: Don't use CREATE_NEW_PROCESS_GROUP as it isolates the process
+            # Instead, let it inherit the parent's console which allows taskkill /T to work
+            popen = subprocess.Popen([sys.executable, "-c", parent_script])
             proc = WindowsProcessWrapper(popen)
         else:
             # Unix: Use start_new_session for process group creation
@@ -469,7 +467,6 @@ async def test_stdio_client_child_process_cleanup():
 
 
 @pytest.mark.anyio
-@pytest.mark.skipif(sys.platform == "win32", reason="Windows process tree termination requires Job Objects API")
 async def test_stdio_client_nested_process_tree():
     """
     Test that a nested process tree (parent → child → grandchild) is properly cleaned up.
@@ -531,10 +528,9 @@ async def test_stdio_client_nested_process_tree():
 
         # Start parent process directly
         if sys.platform == "win32":
-            # Windows: Use subprocess.Popen with CREATE_NEW_PROCESS_GROUP flag
-            popen = subprocess.Popen(
-                [sys.executable, "-c", parent_script], creationflags=subprocess.CREATE_NEW_PROCESS_GROUP
-            )
+            # Windows: Don't use CREATE_NEW_PROCESS_GROUP as it isolates the process
+            # Instead, let it inherit the parent's console which allows taskkill /T to work
+            popen = subprocess.Popen([sys.executable, "-c", parent_script])
             proc = WindowsProcessWrapper(popen)
         else:
             # Unix: Use start_new_session for process group creation
@@ -577,7 +573,6 @@ async def test_stdio_client_nested_process_tree():
 
 
 @pytest.mark.anyio
-@pytest.mark.skipif(sys.platform == "win32", reason="Windows process tree termination requires Job Objects API")
 async def test_stdio_client_early_parent_exit():
     """
     Test that child processes are cleaned up when parent exits during cleanup.
@@ -623,10 +618,9 @@ def handle_term(sig, frame):
 
         # Start the parent process
         if sys.platform == "win32":
-            # Windows: Use subprocess.Popen with CREATE_NEW_PROCESS_GROUP flag
-            popen = subprocess.Popen(
-                [sys.executable, "-c", parent_script], creationflags=subprocess.CREATE_NEW_PROCESS_GROUP
-            )
+            # Windows: Don't use CREATE_NEW_PROCESS_GROUP as it isolates the process
+            # Instead, let it inherit the parent's console which allows taskkill /T to work
+            popen = subprocess.Popen([sys.executable, "-c", parent_script])
             proc = WindowsProcessWrapper(popen)
         else:
             # Unix: Use start_new_session for process group creation