Last modified: 2014-04-21 18:52:29 UTC
See zero.inc.vcl.erb. Adam and Christian spoke about this.
Note the User-Agent definition is actually not a valid description, and thus bug 56119 has been opened, too.
Prioritization and scheduling of this bug is tracked on Mingle card https://mingle.corp.wikimedia.org/projects/analytics/cards/1231