It's not only a valid usage of accept, it's simply necessary; you can't possibly process a request for a version of a response without assuming the request cont

It's not only a valid usage of accept, it's simply necessary; you can't possibly process a request for a version of a response without assuming the request contains the necessary information said response requires. If you don't expect the Accept header asking for a specific version to affect the processing of the request, I think you really haven't thought it through as it necessarily must since the output response version must be correlated with the input request. If output version 5 requires a different set of input args, then your code for processing the output will have to assume those args present and thus will have to assume the request was formatted correctly for version 5 otherwise you'll have to throw an error saying the request was formatted incorrectly. Yes, you could use a custom header, many do, but then you're stepping outside of the HTTP standard, the Accept header is supposed to define the response format, not some custom one you make up, and version is another facet of the response format and must imply the request format matches the required data of the requested output format.