Api-User-Agent:

[1] API:Main page - MediaWiki (2015-07-07 16:23:16 +09:00 版) <https://www.mediawiki.org/wiki/API:Main_page>

If you are calling the API from browser-based JavaScript, you won't be able to influence the User-Agent header: the browser will use its own. To work around this, use the Api-User-Agent header: