At the 2014 RailsConf DHH declared system testing would be added to Rails. Three years later, Rails 5.1 makes good on that promise by introducing a new testing framework: ActionDispatch::SystemTestCase. The feature brings system testing to Rails with zero application configuration by adding Capybara integration. After a demonstration of the new framework, we'll walk through what's uniquely involved with building OSS features & how the architecture follows the Rails Doctrine. We'll take a rare look at what it takes to build a major feature for Rails, including goals, design decisions, & roadblocks.