Gearseeker Serpent

R2.10

“The total cost to cast Gearseeker Serpent is locked in before you pay that cost. For example, if you control three artifacts, including one you can sacrifice to add {C} to your mana pool, the total cost of Gearseeker Serpent is {2}{U}{U}. Then you can sacrifice the artifact when you activate mana abilities just before paying the cost.
Once a player has announced that they are casting Gearseeker Serpent, no player may take actions to try to change the number of artifacts its controller controls before the spell’s cost is locked in.
Gearseeker Serpent’s first ability can’t reduce the total cost to cast the spell below {U}{U}.
Once Gearseeker Serpent has been blocked, activating its last ability won’t cause it to become unblocked.”