Gift of the Machine God. As a creature of technology, the robot gains many damage and condition resistances and immunities, but they are also susceptible to electrical attacks:

• Automatically gain a single proficiency or feat as needed though it can only have one new ability active at any given time. It does this as part of short rest downloading the knowledge it needs to finish a mission at hand.
• Has advantage on Wisdom (Perception) checks that rely on sight or sound and has double proficiency bonus in the Perception skill as it constantly scans its surroundings for danger.
• Is combat-hardened and has a damage threshold equal to half its CR rounded down, that is, it reduces all individual attacks by the threshold numbers damage.
• All robots have a weakness to lightning-based damage. They have disadvantage on saving throws against attacks that do lightning damage and, on a natural 1 are stunned until the end of its next turn. Lightning damage also bypasses all damage reduction.
• Robots built by the Ancients were built to last. Each has a continuous reanimation protocol active which heals/mends the robot a number of hit points equal to its CR (minimum of 1) per round. This ability ceases to function when the robot is reduced to 0 hit points. The Mk. I has a DR of 3.