One can still do what you are describing and have the Swagger spec generated. On my platform, I would just specify data types and the interfaces, and have Swash

One can still do what you are describing and have the Swagger spec generated. On my platform, I would just specify data types and the interfaces, and have Swashbuckle parse this and spit out the Swagger spec. No need to hand-code Swagger while creating the contract up front. After this step, one could work at both sides of the contract independently as you describe.