An ultrasonic surgical instrument that includes a handle assembly including at least one user input feature； an ultrasonic transducer supported by the handle assembly, a shaft assembly configured to removably couple with the handle assembly, and a mechanical lockout assembly. The shaft assembly includes a distal end portion. The shaft assembly further includes an end effector extending distally from the distal end portion, and an ultrasonic waveguide configured to be acoustically coupled with the ultrasonic transducer. The mechanical lockout assembly is configured to move between at least an unlocked configuration and a locked configuration. In the locked configuration, the handle assembly and the shaft assembly are only partially coupled together physically preventing the user from activating the instrument using the user input feature. In the unlocked configuration, the handle assembly and shaft assembly are completely coupled together enabling the user to activate the instrument using the user input feature.
