Does Arcane Signet count as a mana ability and therefore cannot be responded to?

Arcane Signet has the activated ability '{T}: Add one mana of any color in your commander's color identity.' This ability meets all the criteria for a mana ability under CR 605.1a: it is an activated ability (not a loyalty ability), and it produces mana as part of its effect, and it does not have a target.

Under CR 605.3a, mana abilities do not go on the stack and therefore cannot be responded to. They resolve immediately when activated. This is a fundamental exception to the normal rule that activated abilities use the stack (CR 602.2).

This means no player can cast an instant, activate another ability, or take any other game action 'in response to' Arcane Signet being tapped for mana — there is simply no window to do so.

Concrete example: You tap Arcane Signet to add {G} for your commander's color identity. Your opponent cannot respond with Stifle to counter the ability, because mana abilities never go on the stack and Stifle only counters activated or triggered abilities that are on the stack (CR 605.3b confirms mana abilities can't be countered this way).

Note: If Arcane Signet somehow gained a target (e.g., due to a strange text-changing effect), it would no longer qualify as a mana ability and would then use the stack normally per CR 605.1a.