In the SP2013 SP1 fix list you find KB2817429 "Minimal and no metadata are now enabled as supported JSON formats". This suggests an upgrade to OData 3.0 standard and in fact if you make a metadata request you now get
a response which has a DataServiceVersion of 3.0. So far so good.
You would therefore expect a request with an Accept header of "application/json;odata=minimalmetadata" or "application/json;odata=nometadata" or just "application/json" to return lightweight OData3 style JSON. In fact if I do this on Office365 it now works. But if I make the same request against an “on-premise” test farm with SP2013 + SP1 I get XML (Atom) returned instead of JSON (???). Prior to SP1 we got a 406 error. Now it accepts the header but returns XML - also does this if we put garbage in , e.g. odata=foobar.
Using the older "application/json;odata=verbose" still works, of course. XML seems to be the default because if you put garbage in the accept header this is what you get, so it seems like it only recognizes "application/json;odata=verbose".
Is this a known issue? Was JSON Light actually implemented in Service Pack 1?