I am currently working on an API which implements an extended Basic resource. One of the optional extensions we have defined for this resource will trigger additional processing if it is included in a POST request. Our current customer has requested that if the validation for this extension fails, then the resource should still be processed as being valid, but the POST response should include a warning message saying that the non-essential data is invalid.
What would be the nicest way of including this warning message in the POST response? My only thought would be to add an OperationOutcome resource with severity = warning as a contained resource, and then add a new extension with a reference to the OperationOutcome resource, but this approach feels a bit messy for my liking. Any other suggestions/input would be greatly appreciated.