Anybody that has built HTTP or REST APIs, has had to deal with things like OpenAPI or Swagger. Those are reasonably trivial to implement using ASP.NET Core, and you even get a nice Swagger UI website out of the box. Just like any interface or programmatic contract, there are plenty of aspects that need careful analysis. Think of comprehensive online documentation for your APIs, using HTTP error codes appropriately, and providing clarity on what fields in the JSON are required and which are optional or can be empty. And don't forget versioning. Allowing a developer to view and switch between the supported versions and make it clear what APIs are deprecated isn't as trivial as you may think. In this talk, I'll show you everything that you might need to know about building nicely documented, properly versioned HTTP APIs that make your consumers happy.