Does kicker count as part of a spell's mana cost or an additional cost?

Kicker is an optional additional cost defined under CR 702.33. When you cast a spell with kicker, you first pay its normal mana cost (or alternative cost), and then separately choose whether to pay the kicker cost on top of that. The kicker cost is never considered part of the mana cost printed in the upper-right corner of the card.

This distinction matters in several practical ways. Effects that reduce or replace a spell's mana cost (like cost reducers) do not automatically reduce kicker costs unless they specifically refer to additional costs or kicker. Conversely, effects that care about a spell's mana cost or converted mana cost/mana value do not count any kicker paid.

The mana value of a spell on the stack does include additional costs paid only if the effect specifically says so — normally mana value is calculated from the mana cost in the upper-right corner (CR 202.3), not from kicker or other additional costs actually paid.

Concrete example: You cast Growth Spiral — wait, better example: you cast Fertilid with its kicker cost of {2}{G}. The spell's mana cost is {2}{G} and the kicker is an additional {2}{G}. An effect that says 'spells cost {1} less to cast' reduces the base mana cost, not the kicker, unless it explicitly mentions additional costs.

CR 702.33a defines kicker as an optional additional cost. CR 601.2b instructs players to choose to pay optional additional costs during casting. CR 202.3 confirms mana value is derived from the mana cost only, not additional costs paid.