The ContextSwitch helper requires the "format" parameter to be set (see Zend_Controller_Action_Helper_ContextSwitch#initContext()).
However, you could cheat and add the format parameter to the request at the top of your init() hook: public function init() { $this->getRequest()->setParam('format', 'json'); /* ... the rest of your code ... */ } -- *Hector Virgen* Sr. Web Developer http://www.virgentech.com