MTG: Does a spell still go off if there isn't enough mana?

Question

This nquestion relates to a specific card I can't remember the name of, so for now I will called it Fred.

So, my opponent has Fred on the table, who has the ability that spells cost (1) less mana to play.

Opponent starts to play a spell, and it costs one less.
In response to his playing that spell, I play a direct damage Instant, destroying Fred. My thought here was that if Fred were destroyed, my opponent won;t have paid enough mana to play his spell.

Now it seems I was wrong, and the spell resolved an yway, but I'm not quite sure how that worked out. I always figured that the whole point of being able to play Instants in response to something so to weaken or stop it from happening, but apparently its happening anyway.

So, does destroying Fred stop a spell if not enough mana was spent?
And if it doesn't, can you explain why not?

Cantra · Answer

I'm seeing two issues here.

The first is 'priority', which is the 'right' to cast a spell or use an ability at the current point in the game.
The second is that you are trying to interrupt an uninterruptable action, which is an illegal move in its own right, not just because you do not have priority to do anything at the time.

So imagine it is my turn, my main phase. As the turn player, priority to cast is mine, and I choose to cast a spell that costs 4 mana. Because of 'Fred', the mana cost is 3. I pay the 3 mana and place the spell on the stack, waiting to resolve. I have no further actions and wish for my spell to resolve, this passes priority to you.

This, here, is the first chance you get to respond. You can cast an Instant if you want to destroy Fred, this will go above my spell on the stack. But even though your spell is resolving first, and poor Fred is being destroyed, I have already cast my spell and spent the mana it needed - it only mattered that he was there when I actually *played* the spell, letting me play it for cheap, not at any point afterwards. Destroying him doesn't force me to have to pay 1 more, the cost was locked in and paid for in order for me to place it on the stack.

Yes, the concept of the stack, with actions going on, and coming off in different orders, is vitally important. Like if I tried to boost a 1/1, then you could respond with a 1 damage spell to kill it first and make my boosting spell fail due to losing its target.
 
But disrupting *costs* in the way you outline is very, very, very hard since mana taps faster than instant speed, costs are paid before responses can be performed (which can include sacrificing something before a spell/ability can destroy it), and you rarely, if ever get a chance to interfere with a cost while it is being paid.

If you wish to read more in the comprehensive rules, you could take a look at this page here,
http://mtgsalvation.gamepedia.com/Casting_spells
The relevant section is 601.2f, and the relevant statement is,

"Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect. "

Whatevers · Answer

"My thought here was that if Fred were destroyed, my opponent won;t have paid enough mana to play his spell. "

Your thought is wrong, the costs were locked in when Fred's controller declared the spell.

There are some spells that work to increase costs, like Mana Leak, but not by killing Fred.

Anonymous · Answer

You can go to hell

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MTG: Does a spell still go off if there isn't enough mana?

3 Answers