Does kicker count as part of the spell's mana cost for the purpose of counterspells?

Kicker is an optional additional cost defined under CR 702.33. When you cast a spell with kicker, you may pay the kicker cost on top of the spell's mana cost, but the kicker cost itself is never considered part of the mana cost (CR 202.1, 702.33a).

Counterspells like Counterspell or Negate simply counter the spell on the stack. They do not care how much mana was spent or whether additional costs were paid — the spell is countered entirely, kicked or not (CR 701.5a).

Effects that interact with the mana cost specifically — such as cards that say 'counter unless its controller pays {2}' — also only reference the printed mana cost, not kicker. Kicker is tracked separately on the spell as a flag (CR 702.33e) for effects that care about whether it was kicked, but that flag is irrelevant to most counterspells.

Concrete example: You cast Kor Spiritdancer... actually, consider Cultivate — or better: you cast Verazol, the Split Current for its mana cost of {G}{U} plus kicker {4}, spending 6 mana total. Your opponent casts Counterspell. The counterspell counters Verazol completely — the {4} kicker you paid is simply lost with no refund, because kicker is an additional cost, not the mana cost.

Bottom line: paying kicker makes a spell more expensive to cast but does not change its mana cost, and a counterspell cares only that the spell is on the stack — it counters it fully regardless of any additional costs paid (CR 702.33a, 701.5a).