How does Fist of Suns work with my Commander?

Question

I am working on a deck that features Progenitus as my Commander and started looking at Fist of Suns. Besides just being good because of the large number of high cost spells in the deck, I was curious how this worked with my Commander on every cast beyond the first. And honestly, I'm not entirely sure how to word this question.

For each cast of the Commander, the mana cost is increased by 2 colorless. The Fist of Suns Oracle text says

You can't pay more than one alternative cost for a spell.

Does the additional colorless cost added after the first casting of a Commander count as alternative costs? Can I use Fist of Suns to make the cost of Progenitus WUBRG every time, or just the first time it is cast? I know that it can't be cast for WUBRG plus the additional colorless, since it seems unreasonable to combine mana costs like that.

Answer 1

Fist of Suns only replaces the mana cost. The mana cost is the cost written in the top-right corner of the card [CR 202.1]. You seem to think it's the total cost or the portion of the total cost payable using mana, but that's not the case.

Fist of Suns doesn't replace any additional costs or cost increases, so you still have to pay the Commander tax.

In game terms,

• Fist of Suns provides an alternative cost [CR 117.9].
• The Commander tax is an additional cost [CR 903.10].

total cost to cast the second time
= mana cost or alternative cost + additional cost and cost increases - cost reductions [CR 601.2e]
= alternative cost ({W}{U}{B}{R}{G}) + additional costs ({2})
= {W}{U}{B}{R}{G}{2}