For the Artificer's Flash of Genius feature, do you activate the ability before or after the results of the roll?

Question

The Artificer from Eberron: Rising from the Last War gets the Flash of Genius feature at 7th level (p. 57):

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.

Answer 1

As you have noted, it's unclear. That means it's DM's call.

There are a large number of points in the rules of 5e that are not entirely clear. It is explicitly intended that the DM adjudicate those things at their table. It's just one of the features of the game system - they're actively encouraging table adjudications and houserules.

That having been said...

This is in a published book. We can make a reasonable assumption that the wording is as intended... and the wording doesn't say. If we look at the rules for Bardic Inspiration, we see the following (emphasis mine):

Once within the next 10 minutes, the creature can roll the die and add the number rolled to one ability check, attack roll, or saving throw it makes. The creature can wait until after it rolls the d20 before deciding to use the Bardic Inspiration die, but must decide before the DM says whether the roll succeeds or fails. Once the Bardic Inspiration die is rolled, it is lost. A creature can have only one Bardic Inspiration die at a time.

The wording here states outright that in this particular case, the player must decide before the DM says whether the roll succeeds or fails. By implication, then, that's not inherent in the fact that it's giving a bonus to the roll.

As such, it should be possible to give a bonus to a roll after success or failure has been determined - and, indeed, the Divine Soul sorcerer's Favored by the Gods feature (XGtE, p. 50) does this - and Flash of Genius leaves activation in the hands of the player.

Thus, it would make sense that the player could wait until after success or failure had been declared before deciding whether or not to use Flash of Genius.

eidt: and, we now have new information... sort of. The Sage Advice Compendium offers a not-entirely-clear clarification to go with the above.

You use Flash of Genius immediately after the triggering d20 is rolled and before any of the effects of the roll are applied. Unless a rule tells you otherwise, a reaction occurs immediately after its trigger

So... it's after the roll, but before the effects are applied. Yes, that seems to cover both of the previously described options. Admittedly, it does tend to nudge it closer to the "probably before success/failure is declared" side of things, but to my eyes it's still firmly within DM adjudication land.

Answer 2

Another important aspect of the ability is that it requires you to use your reaction. Reactions have their own specific rules on timing.

Typical combatants rely on the opportunity attack and the Ready action for most of their reactions in a fight. Various spells and features give a creature more reaction options, and sometimes the timing of a reaction can be difficult to adjudicate. Use this rule of thumb: follow whatever timing is specified in the reaction’s description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

As for the ability itself, it specifies:

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.

Note the wording. "Makes an ability check" not, "about to make an ability check" or such. So you use your reaction after the result. Also since the timing isn't as clearly specified as say bardic inspiration. Then you would fall back to the "timing is unclear" and it would occur after the trigger finishes.

Answer 3

The system nominally handles abilities like this by giving the player the option add the bonus modifier to their roll while calculating their result. (After they roll their D20) This modifier is added BEFORE the DM announces the result of the roll being a failure or success. The alternative option is to use your reaction to use Flash of Genius and add it to the final calculation of his D20 roll before he rolls the dice.

To support this I'd reference Bardic Inspiration which states:

Once within the next 10 minutes, the creature can roll the die and add the number rolled to one ability check, Attack roll, or saving throw it makes. The creature can wait until after it rolls The D20 before deciding to use the Bardic Inspiration die, but must decide before the DM says whether the roll succeeds or fails. Once the Bardic Inspiration die is rolled, it is lost. A creature can have only one Bardic Inspiration die at a time.

This gives the player the option of rolling the Inspiration die after they make their D20 roll or before they make it. But is done before the DM has announced the outcome of the player's ability check/saving throw.