The way the tests are setup is wrong. The only reason the fixtures are working in some cases is because what you're describing happens to be a model or controller name. If you describe anything that's not a model/controller name (i.e. Environment) then it doesn't work as it's not including the necessary Rails test modules. You can fix this either by including the necessary Rails test modules into Minitest::Spec or by encapsulating these tests inside ActiveSupport::TestCase classes and having that `extend MiniTest::Spec::DSL` (if need be).