Testing tool to enforce consumers of my API to comply to my requirements

Currently I'm working on a POC where I try to decouple my frontend form my backend development, making the frontend development platform-independent.

What I'd like to achieve is the following:

A service with a REST API is created that has endpoints to render out HTML. Each endpoint has its own criteria as which properties need to be on the HTTP request. I'd like to ensure that the backend developers have to comply to the needs of the service, so they can be assured that whatever HTML they retrieve from it will be fine.

I've looked into pact, but it's not exactly what I need, but it sort of sums up what I'd like to achieve: a kind of contract that can be defined at the level of the service and gets shared with the backend developers, so that they know what parameters to fill in when requesting HTML.

It seems like a bit of a combination between Swagger and Pact.

If any of you have an idea to point me to some kind of testing tool that could provide me with this, I will be more than glad to look into it!

Kind regards