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- Replace asyncio.timeout with anyio.fail_after for consistency
- Simplify exception handling
- Uncomment Windows-specific test skip condition
- Rewrite tests to focus on verifying no hang occurs (issue #552)
- Use simple Python subprocesses that exit quickly
- Remove complex assertions - test passes if it completes without timeout

Author: felixweinberger

tests/issues/test_552_windows_hang.py

@@ -1,50 +1,64 @@
-import asyncio
+"""Test for issue #552: stdio_client hangs on Windows."""
+
 import sys
+from textwrap import dedent
 
+import anyio
 import pytest
 
 from mcp import ClientSession, StdioServerParameters
 from mcp.client.stdio import stdio_client
 
-# @pytest.mark.skipif(sys.platform != "win32", reason="Windows-specific test")
-@pytest.mark.skipif(sys.version_info < (3, 11), reason="asyncio.timeout in 3.11+")
-@pytest.mark.parametrize(
-    "args",
-    [
-        ["/C", "echo", '{"jsonrpc": "2.0", "id": 1, "result": null}'],
-        ["dfghfgh"],
-        ["/C", "echo"],
-    ],
-)
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Windows-specific test")
 @pytest.mark.anyio
-async def test_windows_process_creation(args):
+async def test_windows_stdio_client_with_session():
     """
-    Test that directly tests the process creation function that was fixed in issue #552.
-    This simpler test verifies that Windows process creation works without hanging.
+    Test the exact scenario from issue #552: Using ClientSession with stdio_client.
+
+    This reproduces the original bug report where stdio_client hangs on Windows 11
+    when used with ClientSession.
     """
-    # Use a simple command that should complete quickly on Windows
+    # Create a minimal MCP server that responds to initialization
+    server_script = dedent("""
+        import json
+        import sys
+
+        # Read initialization request
+        line = sys.stdin.readline()
+
+        # Send initialization response
+        response = {
+            "jsonrpc": "2.0",
+            "id": 1,
+            "result": {
+                "protocolVersion": "1.0",
+                "capabilities": {},
+                "serverInfo": {"name": "test-server", "version": "1.0"}
+            }
+        }
+        print(json.dumps(response))
+        sys.stdout.flush()
+
+        # Exit after a short delay
+        import time
+        time.sleep(0.1)
+        sys.exit(0)
+    """).strip()
+
     params = StdioServerParameters(
-        command="cmd",
-        # Echo a valid JSON-RPC response message that will be parsed correctly
-        args=args,
+        command=sys.executable,
+        args=["-c", server_script],
     )
 
-    # Directly test the fixed function that was causing the hanging issue
-    try:
-        # Set a timeout to prevent hanging
-        async with asyncio.timeout(5):
-            # Test the actual process creation function that was fixed
+    # This is the exact pattern from the bug report
+    with anyio.fail_after(10):
+        try:
             async with stdio_client(params) as (read, write):
-                print("inside client")
-                async with ClientSession(read, write) as c:
-                    print("inside ClientSession")
-                    await c.initialize()
-
-    except asyncio.TimeoutError:
-        pytest.xfail("Process creation timed out, indicating a hang issue")
-    except ProcessLookupError:
-        pytest.xfail("Process creation failed with ProcessLookupError")
-    except Exception as e:
-        assert "ExceptionGroup" in repr(e), f"Unexpected error: {e}"
-        assert "ProcessLookupError" in repr(e), f"Unexpected error: {e}"
-        pytest.xfail(f"Expected error: {e}")
+                async with ClientSession(read, write) as session:
+                    await session.initialize()
+                # Should exit ClientSession without hanging
+            # Should exit stdio_client without hanging
+        except Exception:
+            # Connection errors are expected when process exits
+            pass