Breaking change
also: breaking changes
An update that makes things built on the old version stop working until they are changed too. Renaming something other code relies on is a classic example.
A change to an API, library, data format, or configuration that is incompatible with existing users of it, such as removing a field, renaming an endpoint, or changing a function's inputs. Those users must update to keep working. Semantic versioning signals this with a major version bump.
Your city renumbers every street address overnight. Your house didn't move, but mail, deliveries, and your friends' maps all go to the wrong place until everyone updates.
Agents upgrade libraries and rename things casually. If other code, a mobile app, or another service depends on the old shape, it breaks. Asking "is this a breaking change?" before merging saves real pain.
Upgrading the date library from v2 to v3 includes breaking changes: format() now takes different arguments. I'll need to update 14 places that call it.
Before renaming any API fields, check whether this is a breaking change for the mobile app or the Zapier integration. If it is, propose a way to support both versions for now.
Thinking a change is safe because everything in this codebase still works. Other apps, old app versions, saved data, and outside integrations can depend on the old behavior too.