I feel like that's fine since both things go hand in hand anyway. And if choosing the JSON-format comes with a rather high amount of contract-breaches it might just be easier to switch that instead of fixing the contract.

Most web frameworks do both at the same time to the point where having to write code which enforced a type contract after deserializing is a delabreaker for me. I eant to be able to define my DTOs in one place, once, and have it both deserialize and enforce types/format. Anything else is code smell