The Artificer from Eberron: Rising from the Last War gets the Flash of Genius feature at 7th level:
When you or another creature you can see within 30 feet of you makes an ability check or a saving throw, you can use your reaction to add your Intelligence modifier to the roll.
You can use this feature a number of times equal to your Intelligence modifier (minimum of once). You regain all expended uses when you finish a long rest.
Would you activate this ability after the creature has made the ability check or saving throw but before the results are known, or would you be able to activate it after the result has been decided?
Many other features such as the Lore bard's Cutting Words, the Battle Master fighter's Precision Attack maneuver, the Wild Magic sorcerer's Bend Luck, and the Divine Soul sorcerer's Favored by the Gods specify when their modifier can be applied, but the artificer's Flash of Genius does not.