I found this comment: "In V3 the hobby field is explicitly disallowed. This is not backwards compatibly as it breaks previous field that did contain a hobby field" in https://cuelang.org/docs/usecases/datadef/

So if you add a field, you break existing code that doesn't know about the field.

This is wrong. CUE has optional closed schemas marked by a double colon. The V3 entity you’re talking about is explicitly declared to be a closed definition and therefore disallows unknown fields in entities that claim to accord to the V3 type. Not all definitions are closed.

Even for those that you choose to close, it’s a matter of having different code for different definitions. The claim that it just automatically breaks isn’t true even when closed definitions are used.

BTW, this feature speaks to CUE’s intended purpose as a configuration language. It is (or at least can be) nice to ignore unknown fields in transmitted payloads for forwards and backwards compatibility. But if I’m trying to configure some software and misspell a field, I probably want the configuration file to fail validation, not have the software run with an unintended configuration.