HOW-TO: Test this Drupal composer plugin In order to test this plugin, you'll need to get the entire Drupal repo and run the tests there. You'll find the tests under core/tests/Drupal/Tests/Composer/Plugin. You can get the full Drupal repo here: https://www.drupal.org/project/drupal/git-instructions You can find more information about running PHPUnit tests with Drupal here: https://www.drupal.org/node/2116263 Each component in the Drupal\Composer\Plugin namespace has its own annotated test group. You can use this group to run only the tests for this component. Like this: $ ./vendor/bin/phpunit -c core --group Scaffold