Heroic Inspiration: commit to the reroll

SRD 5.2.1 · Checks

At the table

1. Decide immediately after rolling the die whether to spend Heroic Inspiration.
2. Reroll that die and use the new result.
3. If you gain Inspiration while already holding it, give it to a player character who lacks it or lose the newly gained benefit.

Example decision

A player spends Heroic Inspiration to reroll a damage die showing 5. The reroll shows 2. Which result must be used?

2, because the new roll must be used.

The rule explicitly requires the new roll. It says any die, so this use is not confined to a d20 test, but it does not offer a choice between the old and new results. Decide whether the possible improvement is worth replacing the current roll before spending the resource.

Watch for this mistake

This is a reroll, not Advantage. Do not apply the procedure for keeping the higher of two d20s, and do not accumulate multiple held Inspirations by default.

Ask the table

Are you willing to replace this result with whatever the new roll produces?

Source rule

If you (a player character) have Heroic Inspiration, you can expend it to reroll any die immediately after rolling it, and you must use the new roll. If you gain Heroic Inspiration but already have it, it’s lost unless you give it to a player character who lacks it.

Source: SRD 5.2.1, 183. Online guide.