Code that has never been run is a guess. A sign-up page can look finished and still submit to a route that doesn't exist. The only way to know is to use it.
So in Syvna the developer agent doesn't get to say 'done' until it has opened the preview in headless Chrome, clicked through the flow it just built as a test user, and watched the row land in the database. It reads the console. It reads the build log. If something is off it fixes it before you ever see a message.
The cost is a little time per change. The result is that the reply in the room describes an app that actually works, and the approval card you click is for something an agent has already used.