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During one of my matches I noticed I could normal cast Herald of Anguish without spending any mana after using Dispel to counter an opponent Counterspell. I've attached a save that allows to replicate the issue:

- Normally cast Herald of Anguish;
- Dispel the opponent Counterspell;
- Herald of Anguish will appear on the field even if no mana has been spent to cast it.

Improvise ability in general doesn't charge mana for normal casting. (tested in public version)