Got it. 
Your advice make me to think maybe it is the api consumer's responsibility to show the better error message for the end user, and how to correct it.  The api provider make sure if there's an invalid param, just tell the api consumer, 'hey,your validation have some problem,check your code.'

And also I'll try to find is it possible to add a message options for the validation, just like the active_model's validation, and for a very simple resolution.

Thanks a lot @dblock