To tell the truth, there is json-schema[1], and a number of implementations of it which work pretty well.

You can use it to validate your JSON inputs, much like you would with XSD (I've used both).

There is OpenAPI, née Swagger [2], which allows you to describe your API in human- and machine-readable form and generate code for it [3].

But validation is but one step. A robust API needs rate-limiting, injection protection (does your language has tainted strings?), easy enough authz/authn story, etc, etc, all different and non-trivial things.

[1]: http://json-schema.org/ [2]: https://en.wikipedia.org/wiki/OpenAPI_Specification [3]: https://openapi-generator.tech/