If I cast a spell with X in its mana cost, say Green Sun's Zenith, and there are abilities in effect that increase casting cost like Thalia, Guardian of Thraben, I would have to pay G4 if I wanted to cast the spell for X=3
If I cast Green Sun's Zenith and there are abilities that decrease casting cast like Goblin Electromancer, I would have to pay G2 if I wanted to cast the spell for X=3. Cost-reduction effects decreased the cost of X here
Now if I get to cast a spell "without paying its mana cost" by for example revealing it off Mind's Desire, effects that increase casting cost are still in effect: With Thalia, Guardian of Thraben on the battlefield I would still have to pay 1 to cast my revealed Green Sun's Zenith for X=0
Is everything I said above correct and if so, why? This seems strange to me as a cost increase on a "free" spell does come in effect, but a cost decrease on a "free" spell does not.
The thing to keep in mind is that X always = 0 in this case due to a game rule. Decreasing the cost of a spell that costs 0 by 1 still makes it 0. You don't get to change the value of X just because the math works out the same.
Note that a cost decrease still comes into effect if there is something to decrease. Normally, the spell costs nothing. However, if you had Thalia and Goblin Electromancer, the Electromancer would decrease the additional cost imposed by Thalia, thus making it truly "free" again. Also note that this isn't limited to just "free" spells. The exact same thing happens for spells without X in their costs so decreasing a Time Spiral off Mind's Desire by 1 still makes it cost 0 so the same thing applies to X spells because X always = 0 in that case.
Yes it's all correct. The last part is because of this rule:
107.3b. If a player is casting a spell that has an {X} in its mana cost, the value of X isn't defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0. This doesn't apply to effects that only reduce a cost, even if they reduce it to zero. See rule 601, "Casting Spells."
The fact that your Zenith costs 1 because of an additional cost does not make this rule not apply, you're still not paying the card's mana cost or an alternative cost that includes X, so you still can't choose another value for X than 0.
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So basically; whilst cost reduction effects decrease the mana cost of a spell (which doesn't matter if the cost is 0), cost increasing effects add a additional cost that is separate from the mana cost
So basically; whilst cost reduction effects decrease the mana cost of a spell (which doesn't matter if the cost is 0), cost increasing effects add a additional cost that is separate from the mana cost
That makes sense, thanks!
Not exactly. Both cost increasing effects and cost reduction effects affect the total cost to cast a spell. The base cost can either be the mana cost or an alternative cost ("without paying its mana cost" being a form of alternative cost), and then cost increasing and cost reduction effects (in that order so that they can "cancel" each other, as WizardMN pointed out) apply to that cost to calculate the total cost.
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
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If I cast Green Sun's Zenith and there are abilities that decrease casting cast like Goblin Electromancer, I would have to pay G2 if I wanted to cast the spell for X=3. Cost-reduction effects decreased the cost of X here
Now if I get to cast a spell "without paying its mana cost" by for example revealing it off Mind's Desire, effects that increase casting cost are still in effect: With Thalia, Guardian of Thraben on the battlefield I would still have to pay 1 to cast my revealed Green Sun's Zenith for X=0
However, if Goblin Electromancer is on the battlefield and I want to cast Green Sun's Zenith revealed off Mind's Desire, I still have to cast it for x=0 and NOT for X=1
Is everything I said above correct and if so, why? This seems strange to me as a cost increase on a "free" spell does come in effect, but a cost decrease on a "free" spell does not.
The thing to keep in mind is that X always = 0 in this case due to a game rule. Decreasing the cost of a spell that costs 0 by 1 still makes it 0. You don't get to change the value of X just because the math works out the same.
Note that a cost decrease still comes into effect if there is something to decrease. Normally, the spell costs nothing. However, if you had Thalia and Goblin Electromancer, the Electromancer would decrease the additional cost imposed by Thalia, thus making it truly "free" again. Also note that this isn't limited to just "free" spells. The exact same thing happens for spells without X in their costs so decreasing a Time Spiral off Mind's Desire by 1 still makes it cost 0 so the same thing applies to X spells because X always = 0 in that case.
That makes sense, thanks!