How to clear pseudoconstant cache?
You can manually clear caches with civicrm_api3('AnyEntity', 'getfields', array('cache_clear' =&gt; 1)); (use any valid entity, it doesn't matter which).

However, if you are using the api to create tags, and this does not automatically clear the cached list of tags, I would consider it a bug. If you are able to reproduce it in the latest version please look in the issue tracker for an open bug report, and if there isn't one, please open one.