CP2 Wiki

Tenacity is one of four perks that essentially provide a free mastery point at the beginning of the game:

Tenacity provides the Iron man Mastery as soon as the player gets control of their character. Unless specifically omitted from the Mastery Taken trigger, it is always considered as taking a mastery. In Hellwalker, Tenacity does not trigger a retaliation, but in Season 2 Nemesis Episode 6, Tenacity does count towards a retaliation event.

Similar to Compartmentalization, Tenacity doesn't immediately provide the needed research and will take the usual 30 minutes to provide full snare resistance.

In addition to providing a free Mastery, Tenacity also increases the player Handicap by 8, resulting in essentially a 20% health increase for all units owned by the player. This can be extremely useful on tanks such as Dune, Predator, and Pyro, who all have extremely high health pools and have to fight in close combat.