Prevent stdio connection hang for missing server path. (#401)

Co-authored-by: ihrpr <inna@anthropic.com>

src/mcp/client/stdio/__init__.py

@@ -108,6 +108,7 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
     read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
     write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 
+    try:
         command = _get_executable_command(server.command)
 
         # Open process with stderr piped for capture
@@ -122,6 +123,13 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
             errlog=errlog,
             cwd=server.cwd,
         )
+    except OSError:
+        # Clean up streams if process creation fails
+        await read_stream.aclose()
+        await write_stream.aclose()
+        await read_stream_writer.aclose()
+        await write_stream_reader.aclose()
+        raise
 
     async def stdout_reader():
         assert process.stdout, "Opened process is missing stdout"
@@ -177,12 +185,18 @@ async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stder
             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:
                     process.terminate()
+            except ProcessLookupError:
+                # Process already exited, which is fine
+                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:

tests/client/test_stdio.py

@@ -2,11 +2,17 @@ import shutil
 
 import pytest
 
-from mcp.client.stdio import StdioServerParameters, stdio_client
+from mcp.client.session import ClientSession
+from mcp.client.stdio import (
+    StdioServerParameters,
+    stdio_client,
+)
+from mcp.shared.exceptions import McpError
 from mcp.shared.message import SessionMessage
-from mcp.types import JSONRPCMessage, JSONRPCRequest, JSONRPCResponse
+from mcp.types import CONNECTION_CLOSED, JSONRPCMessage, JSONRPCRequest, JSONRPCResponse
 
 tee: str = shutil.which("tee")  # type: ignore
+python: str = shutil.which("python")  # type: ignore
 
 
 @pytest.mark.anyio
@@ -50,3 +56,43 @@ async def test_stdio_client():
         assert read_messages[1] == JSONRPCMessage(
             root=JSONRPCResponse(jsonrpc="2.0", id=2, result={})
         )
+
+
+@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"]
+    )
+    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
+            with pytest.raises(McpError) as exc_info:
+                await session.initialize()
+
+            # Check that we got a connection closed error
+            assert exc_info.value.error.code == CONNECTION_CLOSED
+            assert "Connection closed" in exc_info.value.error.message
+
+
+@pytest.mark.anyio
+async def test_stdio_client_nonexistent_command():
+    """Test that stdio_client raises an error for non-existent commands."""
+    # Create a server with a non-existent command
+    server_params = StdioServerParameters(
+        command="/path/to/nonexistent/command",
+        args=["--help"],
+    )
+
+    # Should raise an error when trying to start the process
+    with pytest.raises(Exception) as exc_info:
+        async with stdio_client(server_params) as (_, _):
+            pass
+
+    # The error should indicate the command was not found
+    error_message = str(exc_info.value)
+    assert (
+        "nonexistent" in error_message
+        or "not found" in error_message.lower()
+        or "cannot find the file" in error_message.lower()  # Windows error message
+    )