The API contract gets enforced on the wire

Your OpenAPI spec says amount is an integer. On Tuesday a refactor started returning it as a string, the tests didn't cover that path, and the first thing to notice was a customer's parser at 2am.

Now a hook sits on the API's responses. Every reply reaches the client as before: but on the way out, the script validates a sample against the published schema. First mismatch fires a notification with the path, the field, and the offending value.

The spec stops being documentation and starts being enforced: not in CI, where your tests live, but on the wire, where your customers do.

Schema drift pages you before it pages your customers.