Improved error reporting for custom CLI args with missing path

src/_pytest/config/argparsing.py

@@ -443,6 +443,18 @@ def __init__(
     def error(self, message: str) -> NoReturn:
         """Transform argparse error message into UsageError."""
         msg = f"{self.prog}: error: {message}"
+        if "unrecognized arguments:" in message:
+            file_or_dir_args = getattr(self._parser, "extra_info", {}).get(
+                "file_or_dir", []
+            )
+            if file_or_dir_args:
+                from pathlib import Path
+
+                missing_paths = [
+                    str(p) for p in file_or_dir_args if not Path(str(p)).exists()
+                ]
+                if missing_paths:
+                    msg += "\nNote: The specified path(s) do not exist, so custom CLI options from conftest.py may not be available."
 
         if hasattr(self._parser, "_config_source_hint"):
             msg = f"{msg} ({self._parser._config_source_hint})"

testing/test_main.py

@@ -271,6 +271,19 @@ def test_absolute_paths_are_resolved_correctly(self, invocation_path: Path) -> N
             module_name=None,
         )
 
+    def test_custom_cli_arg_with_missing_path(pytester: Pytester):
+        """Test that a helpful error message is shown when a custom CLI argument is used with a non-existent path."""
+        pytester.makeconftest(
+            """
+            def pytest_addoption(parser):
+                parser.addoption("--potato", default="")
+            """
+        )
+        result = pytester.runpytest("file_does_not_exist.py", "--potato=yum")
+        assert result.ret != 0
+        assert "unrecognized arguments: --potato=yum" in result.stderr.str()
+        assert "Note: The specified path(s) do not exist" in result.stderr.str()
+
 
 def test_module_full_path_without_drive(pytester: Pytester) -> None:
     """Collect and run test using full path except for the drive letter (#7628).