The YQ-400 monitor droid patrol ship was model of droid starship in the YQ series of droid patrol ships. The vessels were built with light patrol of peaceful star systems in mind, given that the ships were not heavily armed enough to take on most smuggling or pirate vessels in combat. The vessels was also not smart enough to be of any military use. As a patrol ship, the YQ-400's programming would assign it a predefined vector within the assigned system, and the droid would patrol it. Upon encountering any other ship in the vector, a YQ-400 would report the encounter and hail the other vessel, then following and periodically reporting on the ship until given an all clear by the local spaceport control. The droids would only open fire upon other vessels if attacked first, although the YQ-400s were very difficult to locate due to the fact they only made use of passive sensors. While this programming meant attackers always had an advantage over the patrol ships, it meant that there was no risk of them attacking friendly vessels.
Each YQ-400 cost AurebeshSans-Serif credit175,000 new and was armed with two heavy ion cannons and two laser cannons. They required zero crew, but carried twenty year worth of consumables and despite having no cargo capacity. At thirty-six meters in length the ships were relatively small, and were designed with a long cylindrical shape with an engine at the back, although the droids lacked a hyperdrive.
The YQ-400 model was one of just over a dozen models of droid starship designed and produced at The Nerfworks, a shipyard lab funded jointly by the starship manufacturing Corellian Engineering Corporation and the droid design company Loronar Corporation. They, along with the rest of the YQ series, were used most commonly in systems with heavy shipping and commerce, where stationing manned vessels in every approach vector was financially unfeasible but where ship tariffs and docking fees were lucrative enough to make the droid ships worth employing. The YQ-400 was the most popular of the YQ series, and a used YQ-400 was worth a reduced AurebeshSans-Serif credit100,000.