No value from input->post

[eluser]aquary[/eluser]
Isn't the $.ajax data in a json format, with keys and values, or a query string?

Code:

$.ajax({
                    url: '<?php echo base_url('frontpage/autocomplete');?>',
                    dataType: 'json',
                    type: 'POST',
                    data: {fields:$("#tags").val()},
                    success: function(data){
                        add(data);
                    }
                });},