[vundercind]

It is not at all uncommon to want a spec for an api that’s out of your control. Or to want to define shared data structures at the transport layer which may have “owners” in heterogenous languages. Spec languages are (well, can be—so very many are terrible) very nice in those cases, which are not at all rare. They may well be more common than the “we’re writing a new single-language monolith that is free to control the shape of the data it slings” use case.