After rounding up the open-source and commercial tooling support for OpenAPI Overlays , the next question I keep getting is simpler and more important: what are people actually using overlays for ? An overlay is just a list of actions applied to an OpenAPI document, but that small primitive turns out to solve a surprisingly wide range of problems. Here is what the specification’s release and the implementations explicitly call out, followed by the use cases I think are still underexplored.