An apparatus includes a shaft and an end effector configured to receive a staple cartridge having staples and a sled. The end effector includes a knife, an anvil jaw, a cartridge jaw, and a blocker coupled to the cartridge jaw. The blocker is vertically translatable relative to the cartridge jaw and the knife between a raised and lowered positions. The blocker in the raised position is configured to directly contact and thereby inhibit distal advancement of the knife, and the blocker in the lowered position is configured to permit distal advancement of the. The blocker is configured to assume the raised position in the absence of an unspent staple cartridge and the lowered position in the presence of an unspent staple cartridge.