Sentinels are described as
"a killing machine designed for one thing... search and destroy" by
Trinity and Dozer in
The Matrix. Sentinels are also called "squiddies" and "calamari" by redpills for their cephalopod-like appearance. They are the soldiers used almost exclusively against the humans of the story.
Sentinels are highly effective scouts and lethal combatants. A standard unit of the Machines resembling a black-painted mechanized
nautilus or
octopus, these fast and deadly machines appear in huge swarms and are a serious threat to every hovercraft. They possess the ability to float over the surface, as if under water, while each of their many tentacles is tipped with razor-sharp claws and armed with a variety of sensors, limited maintenance/repair devices, and other equipment. The main weapon is located on the underside of the body: it is a powerful laser that is able to cut through the metal hull of any hovercraft.
A Sentinel unit can perform different types of mission like patrolling in small groups, escorting bigger units, or lethal assault. They often search out Rebel ships to swarm them and tear them apart. Once they discover their target, it is almost impossible to escape them. Several Sentinels can tear apart a hovercraft and they are intelligent enough to target communication and weapon systems first. Sentinels are most dangerous when working in squads of three or more. They coordinate their attacks and use their tentacles quite effectively, relying on their laser only to cut through hulls and other obstacles. Rebel ships power down when Sentinels are near so they can use an
EMP to attack them if the ship is spotted. Later in the series, Sentinels appear armed with silverfish-like "tow bombs", which they can launch at a ship while staying out of EMP range. It is unknown why Sentinels, and probably most other machine-warriors, do not have EMP-safe circuitry to immunize themselves against such attacks, despite their knowledge of the same.
Sentinels are frequently used by the Machines to destroy human ships and disrupt their operations (
The Matrix); they are also the main force during the attack on Zion, the human city (
The Matrix Revolutions). They are also able to repair or recharge the enormous drilling Machines (known simply as "Diggers" or "Drillers"). The latter are used to drill through to Zion from the surface, thus avoiding the defenses the Rebels had built into the approaching tunnels, and opening a path for the Sentinels to invade the city.
A different model of Sentinel is seen in the Animatrix short
Matriculated. It possesses a semi-humanoid torso studded with sensors, a tentacle replacing each arm, and a tail instead of legs. It is not to be confused with the Runners, as, unlike them, it has no defined head, hovers like other Sentinels, and (presumably) has no ability to transform.
Another only slightly changed Model was seen in the Animatrix short "The Final Flight of the Osiris"; one that had knife blades instead of claws.
At the end of
The Matrix: Revolutions, the robotic eyes of the Sentinels change from red to green after they make peace with the human rebels.