Subtle Shift

You've learned how to hide your meddling from the casters of spells you're shifting. If your Arcana check when Shifting a Spell is greater than its caster's Perception DC, you can prevent the caster from learning the effects of the shift you're attempting to apply. On a critical success, they don't become aware that you're applying a shift at all (although the effect of the shift might still give you away).