An apparatus includes a body assembly, a shaft, an end effector, and a cartridge. The end effector includes a jaw, a pivotable anvil, and a knife member. The cartridge includes a deck, a buttress assembly, and a folding sled. A first portion of the buttress assembly is positioned on top of the deck. A second portion of the buttress assembly is positioned within the longitudinal channel. The folding sled is configured to translate within the longitudinal channel to move the second portion of the buttress assembly to a position that is above the deck in response to actuation of the knife member. The second portion of the buttress assembly is thereby secured against edges of tissue severed by the knife member.