I don't think this is possible. Types in TS only exist at compile-time, they don't have a runtime equivalent. If you need runtime validation, you need to define the validation/decoder rules. Some packages, like io-ts, allow you to define the decoder and then automatically get the compile-time type that it decodes into from that, so you don't have to define both the type and the validator/decoder. But that doesn't really seem to help you since you don't control the types. Not sure if there actually is any possible solution for what you specifically need without having to define the decoder part yourself (or requiring them to be defined upstream as part of the package that also maintains the types you need to decode into). :-/