# pytest.PytestUsageError: Unknown marker 'slow' - maybe you meant 'slow_test'?

- **ID:** `python/pytest-marker-filter-error`
- **Domain:** python
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

A test uses a custom marker (e.g., @pytest.mark.slow) that is not registered in pytest.ini or conftest.py, causing pytest to raise an error when filtering or running tests with that marker.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.10 | active | — | — |

## Workarounds

1. **** (95% success)
   ```
   Register the marker in pytest.ini: [pytest] markers = slow: marks tests as slow, or in conftest.py: def pytest_configure(config): config.addinivalue_line('markers', 'slow: marks tests as slow')
   ```
2. **** (70% success)
   ```
   Use the marker without filtering by running pytest with -k 'not slow' to skip unregistered markers, but registration is recommended.
   ```

## Dead Ends

- **** — Ignoring the marker and running all tests without filtering may work but defeats the purpose of selective test execution. (50% fail)
- **** — Using a similar built-in marker like @pytest.mark.skip may not achieve the desired filtering behavior. (60% fail)