A cap assembly for a fastener-driving tool having a working piston, disposed within a piston-cylinder chamber, for discharging a fastener from the fastener-driving tool when the working piston is subjected to actuating air, comprising: a cap member; means for securing said cap member upon a fastener-driving tool; a spool valve reciprocally movable within said cap member between a first and second positions corresponding to two modes of operation of the fastener-driving tool, and connected fluidically to the working piston, characterized in that it comprises: a switch mechanism incorporated within said cap member for permitting the fastener-driving tool to be operated within the two different modes of operation, said switch mechanism comprising a two-position switch mechanism wherein, when said two-position switch mechanism is disposed at a first one of two positions, the spool valve is reciprocally movable between the first and second positions, so that the fastener-driving tool can be operated within a fully automatic continuous firing mode of operation, whereas when said two-position switch mechanism is disposed at a second one of said two positions, the spool valve is prevented to reciprocally move between said two positions, so that the fastener-driving tool can be operated within a single-shot mode of operation. a cap member; means for securing said cap member upon a fastener-driving tool; a spool valve reciprocally movable within said cap member between a first and second positions corresponding to two modes of operation of the fastener-driving tool, and connected fluidically to the working piston, characterized in that it comprises: a switch mechanism incorporated within said cap member for permitting the fastener-driving tool to be operated within the two different modes of operation, said switch mechanism comprising a two-position switch mechanism wherein, when said two-position switch mechanism is disposed at a first one of two positions, the spool valve is reciprocally movable between the first and second positions, so that the fastener-driving tool can be operated within a fully automatic continuous firing mode of operation, whereas when said two-position switch mechanism is disposed at a second one of said two positions, the spool valve is prevented to reciprocally move between said two positions, so that the fastener-driving tool can be operated within a single-shot mode of operation. a switch mechanism incorporated within said cap member for permitting the fastener-driving tool to be operated within the two different modes of operation, said switch mechanism comprising a two-position switch mechanism wherein, when said two-position switch mechanism is disposed at a first one of two positions, the spool valve is reciprocally movable between the first and second positions, so that the fastener-driving tool can be operated within a fully automatic continuous firing mode of operation, whereas when said two-position switch mechanism is disposed at a second one of said two positions, the spool valve is prevented to reciprocally move between said two positions, so that the fastener-driving tool can be operated within a single-shot mode of operation.