Mapping query_string parameters to function arguments is supported only for GET requests. If we want to pass extra parameters (e.g., session_id), mapping them to function args will be useful.

@Endpoint(method = HttpMethod.POST, path="/v1/user/:id")
def add(id:String, session_id: Option[String]) = {
   ... 
}

With this support POST /v1/user/1?session_id=xxxx will call add(1, Some(xxx)).