Dealing with the Happy Path™ in an API is pretty easy: When a client asks for a resource, show them the resource. When they trigger a procedure, let them know if it was triggered OK, and maybe if it completed without a problem.
What to do when something doesn’t go according to plan? Well, that can be tricky.
HTTP status codes are part of the picture, they can define a category of issue, but they are never going to explain the whole story.
Two examples from a carpooling application which had a “simulated savings” endpoint, to let folks know how much they might save picking up a passenger on their daily commute:
This error let the client know the coordinates were too close together, meaning it is not even worth driving let alone trying to pick anyone else up.