Once an API ships it doesn’t matter how it should behave—how it actually behaves is the important part. Users depend on the existing behavior, and we need a way to ensure it doesn’t change. Behat is a tool that was built to help design software, but it’s actually a great tool for capturing existing behavior too. We’ve used these tools to gain confidence to refactor 5+-year-old apps by capturing the existing behavior before making changes. I want to share the secrets we learned with you.