A stapler or like self-powered tool for dispensing and driving fasteners includes a safety mechanism to prevent unintended ejection of a staple or fastener. A movable hook normally obstructs the path of a staple so that the staple is intercepted and stopped as a spring-biased striker ejects the staple from the stapler. The striker preferably includes a notch and an end of the hook extends into the notch so that the safety mechanism can be compact. A button bar is linked to the hook and the button bar normally extends out from the stapler. When the stapler is placed against a working surface, the working surface presses the button bar vertically into the stapler, which causes the hook to rotate out from the striker slot and enables normal stapling operation.