JSON isn't un-evolvable. In fact, thrift can serialize to JSON. What makes thrift evolvable in practice is that we don't remove fields and don't add mandatory

JSON isn't un-evolvable. In fact, thrift can serialize to JSON. What makes thrift evolvable in practice is that we don't remove fields and don't add mandatory fields. The same discipline can be applied to JSON definitions. Well thrift also tags all fields with integers, so a consumer with an older schema can parse a record with a newer schema, skipping the new fields. Of course JSON trivially has this property. Maybe the key here is "ad-hoc"; something like JSON-schema is needed.