arc46 squid currently does not support JSON schemata, but we’re working on it.
What arc46 squid returns as JSON is highly dynamic and depends on various factors including the requested configuration, the parameters of the request as well as the state of your ServiceNow instance.
A future release will provide a query flag that will return a JSON schema matching the request instead of data. (Think ?wsdl flag for WebServices.) This already works as a proof-of-concept, but still needs some work for general availability.