michael.faeth

try

class My_Theme_Json_Api {

    public function __construct() {
        add_filter('json_api_encode', array( $this, 'json_api_encode') );
    }

    public function json_api_encode( $data ) {

        if (isset($_GET['exclude_fields'])) {
            // ... parse and unset fields here...
        }

        return $data;
    }
}

new My_Theme_Json_Api();