Do you need to roll when using the Staff of Magi's spell absorption?

Question

In the Staff of Magi's description, it is written:

Spell Absorption. While holding the staff, you have advantage on saving throws against spells. In addition, you can use your reaction when another creature casts a spell that targets only you. If you do, the staff absorbs the magic of the spell, canceling its effect and gaining a number of charges equal to the absorbed spell's level. However, if doing so brings the staff's total number of charges above 50, the staff explodes as if you activated its retributive strike (see below).

Does this mean that you just use your reaction and that's it? the spell is canceled?

I'm looking more for a Rule as intended answer because I don't like the idea of my spellcaster player just cancelling my lich's spells.

In worst case scenario I can still just avoid attacking him or bend the rules a little bit, but still.

Answer 1

You don't need to roll. It's a legendary item and that's likely the intent.

The Staff of the Magi(Basic rules, 203) is a Legendary item! This is something incredibly powerful and using up a reaction (which removes other options for a character) is the cost for absorbing. They do that enough times without tracking the risk, and they risk losing the item entirely (and self or party harm as well.)

Balance by spell information

However, the staff doesn't say you get to know what spell is being cast at you. It is up to the player to decide when to use it. The only information they technically have is a spell coming at them. It could be a cantrip, it could be a 9th level spell.

I don't know how you treat spell identification in your game, but that is a potential balance consideration to include at your table.