Fix Windows subprocess resolution for .cmd/.bat CLI executables (#338)

* Initial plan

* Fix Windows CLI resolution for .cmd/.bat files using shutil.which()

Co-authored-by: patniko <26906478+patniko@users.noreply.github.com>

* Address code review feedback: move imports to top and use correct patch paths

Co-authored-by: patniko <26906478+patniko@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: patniko <26906478+patniko@users.noreply.github.com>
Co-authored-by: Patrick Nikoletich <patniko@github.com>

Author: Copilot

python/copilot/client.py

@@ -16,6 +16,7 @@
 import inspect
 import os
 import re
+import shutil
 import subprocess
 import threading
 from dataclasses import asdict, is_dataclass
@@ -1035,6 +1036,14 @@ async def _start_cli_server(self) -> None:
             RuntimeError: If the server fails to start or times out.
         """
         cli_path = self.options["cli_path"]
+
+        # Resolve the full path on Windows (handles .cmd/.bat files)
+        # On Windows, subprocess.Popen doesn't use PATHEXT to resolve extensions,
+        # so we need to use shutil.which() to find the actual executable
+        resolved_path = shutil.which(cli_path)
+        if resolved_path:
+            cli_path = resolved_path
+
         args = ["--headless", "--log-level", self.options["log_level"]]
 
         # Add auth-related flags

python/test_client.py

@@ -4,6 +4,8 @@
 This file is for unit tests. Where relevant, prefer to add e2e tests in e2e/*.py instead.
 """
 
+from unittest.mock import MagicMock, patch
+
 import pytest
 
 from copilot import CopilotClient
@@ -136,3 +138,62 @@ def test_use_logged_in_user_with_cli_url_raises(self):
             CopilotClient(
                 {"cli_url": "localhost:8080", "use_logged_in_user": False, "log_level": "error"}
             )
+
+
+class TestCLIPathResolution:
+    """Test that CLI path resolution works correctly, especially on Windows."""
+
+    @pytest.mark.asyncio
+    async def test_cli_path_resolved_with_which(self):
+        """Test that shutil.which() is used to resolve the CLI path."""
+        # Create a mock resolved path
+        mock_resolved_path = "/usr/local/bin/copilot"
+
+        with patch("copilot.client.shutil.which", return_value=mock_resolved_path):
+            with patch("copilot.client.subprocess.Popen") as mock_popen:
+                # Mock the process and its stdout for TCP mode
+                mock_process = MagicMock()
+                mock_process.stdout.readline.return_value = b"listening on port 8080\n"
+                mock_popen.return_value = mock_process
+
+                client = CopilotClient(
+                    {"cli_path": "copilot", "use_stdio": False, "log_level": "error"}
+                )
+
+                try:
+                    await client._start_cli_server()
+
+                    # Verify that subprocess.Popen was called with the resolved path
+                    mock_popen.assert_called_once()
+                    args = mock_popen.call_args[0][0]
+                    assert args[0] == mock_resolved_path
+                finally:
+                    if client._process:
+                        client._process = None
+
+    @pytest.mark.asyncio
+    async def test_cli_path_not_resolved_when_which_returns_none(self):
+        """Test that original path is used when shutil.which() returns None."""
+        original_path = "/custom/path/to/copilot"
+
+        with patch("copilot.client.shutil.which", return_value=None):
+            with patch("copilot.client.subprocess.Popen") as mock_popen:
+                # Mock the process and its stdout for TCP mode
+                mock_process = MagicMock()
+                mock_process.stdout.readline.return_value = b"listening on port 8080\n"
+                mock_popen.return_value = mock_process
+
+                client = CopilotClient(
+                    {"cli_path": original_path, "use_stdio": False, "log_level": "error"}
+                )
+
+                try:
+                    await client._start_cli_server()
+
+                    # Verify that subprocess.Popen was called with the original path
+                    mock_popen.assert_called_once()
+                    args = mock_popen.call_args[0][0]
+                    assert args[0] == original_path
+                finally:
+                    if client._process:
+                        client._process = None