## TL;DR
Run `python -m pip install pytest-xdist`. The `-n` flag only exists when the xdist plugin is installed; without it, pytest does not recognize `-n auto` at all.

```text
ModuleNotFoundError: No module named 'xdist'
```

## Fix it

1. **Install the plugin.** Run `python -m pip install pytest-xdist`.
   - Expected: `Successfully installed pytest-xdist-... execnet-...`.

2. **Verify.** Run `pytest --version` and confirm `xdist` appears in the plugins list.
   - Expected: `plugins: xdist-3.x.x`.

3. **Use it.** Run `pytest -n auto`.
   - Expected: tests distribute across workers instead of `unrecognized arguments: -n`.

## When this applies
- `pytest -n auto` says `unrecognized arguments`, or something imports `xdist` directly and fails.
- A fresh environment where you installed pytest but forgot xdist.

## When it does not apply
- If `-n` is accepted but workers crash importing your own modules, that is a PYTHONPATH issue in workers, not a missing xdist.

## Tool compatibility
pytest-xdist 3.x with pytest 7/8, Python 3.8+.

## Why it happens
Parallel execution is a plugin, not core pytest. The package name is `pytest-xdist` but the import name is `xdist`.

## Edge cases
- In CI, make sure the test step installs the same requirements file that includes pytest-xdist.