Okay, story time.
If you've worked on a full stack app where the backend is Java/Spring Boot and the frontend is React, you know the drill. Someone changes something on one side of a contract and nobody tells the other side. Weeks later you're playing detective across five files trying to figure out who calls what.
And it's not just REST endpoints. It's the scheduled job that quietly writes to the same table your API touches. It's the service that calls another service, which calls another service. It's the Kafka event your controller publishes that some completely unrelated listener is consuming three modules away. All of that is "the contract" too, it's just invisible unless you go looking for it.
Now add AI coding agents into that picture. They're great at writing code in the file they're looking at. They're not great at knowing that the component they're editing calls an endpoint, which hits a controller, which calls a service, which calls a repository, which is also written to by a scheduled job at 2am, which also fires an event three other services are listening for. Agents see one file at a time. So they'll happily rename a field or change a return shape on one side and leave everything downstream of it completely unaware anything changed.
