Every "contract" language (e.g. .graphql) I've used so far has it's ups and downs. One that keeps biting us in GQL, for instance, is that it won't let you defi

Every "contract" language (e.g. .graphql) I've used so far has it's ups and downs. One that keeps biting us in GQL, for instance, is that it won't let you define a union type for mutation inputs, so you end up needing N methods like `GetByX(X)`, `GetByY(Y)`, `...` instead of a single `Get(X|Y|...)`. Not to mention support for versioning message types, etc... FWIW: I think proto3 is probably the best I've used, at length and in production. Granted it has its "warts", but the idioms to circumvent them are fairly well-documented and agreed upon, even if they're a bit "ugly" in the syntactical sense. FWIW pt 2: Like yourself, I think, I would not consider JSON to be appropriate as a schema-defining "contract" language in 2022, for a company that plans to be around in 5 years. There are too many better options available.