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For example, there's a Cephalid Shrine on the field and someone casts a spell with no copies in any graveyards. Can someone choose to counter their own spell by not paying 0 mana?
I don't think there's anything in the rules preventing this, but I do know that things like 0 power attacks can't interact with combat damage triggers. Is there a similar case for costs of 0 being automatically paid?
Rule 117.5 states:
117.5. Some costs are represented by {0}, or are reduced to {0}. The action necessary for a player to pay such a cost is the player’s acknowledgment that they are paying it. Even though such a cost requires no resources, it’s not automatically paid.
So, it is possible to refuse to pay an optional payment with a cost of 0 mana, and a person can counter their own spell by refusing to pay the cost of 0 mana.
