If you cast a spell that has Convoke *and* requires you to sacrifice a creature as an additional cost to cast it - can you tap the creature for convoke that you also are going to sacrifice?
In the order of casting a spell, cost reductions are applied before you pay the costs. Convoke follows the rules for cost reductions, so you tap the creature to reduce the cost, then sacrifice it to pay the cost.
Yes.
You tap the creature for Convoke when determining the actual cost of the spell you are casting, and you do not actually sacrifice it until you are paying the costs to cast it. Refer to 601.2e-g
601.2e The player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be {0}. It can't be reduced to less than {0}. Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes "locked in." If effects would change the total cost after this time, they have no effect.
601.2f If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, "Mana Abilities"). Mana abilities must be activated before costs are paid.
601.2g The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid.
Example: You cast Altar's Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell's total cost is "locked in" before payments are actually made, you pay {B}, not {1}{B}, even though you're sacrificing the Familiar.
601.2g The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid.
Example: You cast Altar's Reap, which costs {1}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell's total cost is "locked in" before payments are actually made, you pay {B}, not {1}{B}, even though you're sacrificing the Familiar.
EDIT: everyone else's answers were more correct, nvm.
Convoke is actually an additional cost which is paid at the same time as you pay any other costs. You declare that you intend to tap the creature for Convoke when you declare any other additional costs, then that is considered when determining the total cost. For example, a spell that would have cost 1G becomes "1, Tap a green creature."
So the answers are correct that you can tap for Convoke and then sacrifice, but for the wrong reason. The true reason is that you can pay the costs in any order once you reach the payment step of the casting process. This is particularly important if you intend to tap a creature for mana, which would happen before paying costs, so you would be unable to tap it for Convoke as well.
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In the order of casting a spell, cost reductions are applied before you pay the costs. Convoke follows the rules for cost reductions, so you tap the creature to reduce the cost, then sacrifice it to pay the cost.
When in doubt, call a judge.
Objectivist here. Hit me up to talk philosophy.
You tap the creature for Convoke when determining the actual cost of the spell you are casting, and you do not actually sacrifice it until you are paying the costs to cast it. Refer to 601.2e-g
EDIT: everyone else's answers were more correct, nvm.
Reckless Waif; It's the red Delver of Secrets.
So the answers are correct that you can tap for Convoke and then sacrifice, but for the wrong reason. The true reason is that you can pay the costs in any order once you reach the payment step of the casting process. This is particularly important if you intend to tap a creature for mana, which would happen before paying costs, so you would be unable to tap it for Convoke as well.
This FAQ answers many of the common questions asked in the MTGS Rulings forum. Take a look!
I'm the editor/content manager of the Magic Rules Tips Blog - Bookmark this site for daily tips about game and tournament rules.
"Abstract concepts of perfect judging run headlong into the realities of how people play the game." - Toby Elliott (papa_funk)
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Remaking Magic - A Podcast for those that love MTG and Game Design
The Dungeon Master's Guide - A Podcast for those that love RPGs and Game Design
Sig-Heroes of the Plane