API compatibility check

An API compatibility check compares the public interface of a change with a baseline, the last release or the main branch, and reports every change that breaks a caller. Loops read it because a break is cheap to fix before a release and expensive after it.

What it sees

The interface callers depend on: an HTTP API in its interface description, the messages services exchange, the public symbols of a library. It can also check a running API against its own schema. It sees the form of the interface, not its behaviour: a response with the same shape and a different meaning passes.

Reading the signal

• No breaking change: every existing caller still fits the new interface. Additions that callers can ignore do not fail the check.
• Breaking changes: one line per break, naming the endpoint, message or symbol and what changed in it.
• An intended break needs a new major version or an explicit approval, not a silenced check.

A check run against the wrong baseline, or with none, has compared nothing useful. Confirm which release or branch it compared against before trusting a pass.