[Adam Harley (Kawauso)]

Doing a quick var_dump of the passed argument, it’s just a string of the tags passed by the user (commas and all) and before checks are made to see if they’re existing tags. I think you’d want to use bb_tag_added which passes $tt_id, $user_id, $topic_id as each tag is added (if you register the action function as taking as many variables).