JSON Schema gives you the equivalent of WSDL. There are various libraries that will take a schema and create the strongly typed objects in the language of your choice.
Theoretically, if you have "Code on Demand", your client can ask the server for the code to handle a media type :)
Things like grpc and GraphQL seem to be the descendants of SOAP/WSDL.
GraphQL is quite nice as it kind of has the best of both worlds: a strongly typed schema for data and operations, while being fairly close to a simple JSON/HTTP API.
JSON Schema gives you the equivalent of WSDL. There are various libraries that will take a schema and create the strongly typed objects in the language of your choice.
Theoretically, if you have "Code on Demand", your client can ask the server for the code to handle a media type :)