Artificer’s Armor

Your understanding of combat physics and your uncanny ability to adapt on the fly makes you more difficult to hit.  If you are not wearing armor, a magical force protects you that you can enhance and adjust to your situation, and anyone able to see invisible objects will discern a swirling mass of translucent gears and mathematical symbols surrounding you.

Your base AC becomes (10 [or whatever it might be naturally while unarmored] + your Dexterity modifier + your Intelligence modifier), and can only be improved beyond that by using  protective magic other than armor or shields.  The spell ends after 8 hours or if you are rendered unconscious or otherwise unable to exert at least minimal concentration while the spell is active.