A surgical stapler includes a first elongate member, a second elongate member, and a clamp member operable to releasably clamp the first elongate member against the second elongate member. A distal portion of the first elongate member supports an anvil surface having a plurality of staple forming pockets. A distal portion of the second elongate member is configured to receive a staple cartridge. A firing assembly of the surgical stapler is translatable from a first longitudinal position to a second longitudinal position to fire the staple cartridge when the first elongate member is clamped against the second elongate member. A retaining assembly of the surgical stapler includes a first retaining member configured to releasably couple a proximal end of the first elongate member with a proximal end of the second elongate member, and a second retaining member configured to releasably retain the firing assembly in the first longitudinal position.