[web2py] Getting Json data from Spinejs request

[Carl]

I've a function in my default.py controller :

@service.jsonrpc
def get_agent():
    import gluon.contrib.simplejson
    data = gluon.contrib.simplejson.loads(request.body.read())

to get a dict of data sent from the browser I find I have to call 
simplejson.loads(request.body.read()) rather than define parameters in "def 
get_agent():" which is what I've been doing when calling from Pyjamas.

Does anyone know why?