Constructor Sentinel

The Constructor is a specialized Sentinel whose main function is to repair damaged Forerunner structures. Aggressors will come to their aid if they are attacked, as they have no weapon systems of their own to defend themselves.

Operation
Constructors are drones programmed to repair any damaged Forerunner structure they can find, using low-tuned energy beams similar to Sentinel beams. These beams are fired from either the top half or from the bottom half of the Constructor. The beams can also be used to lower the shields on pistons, allowing them to access the tunnel networks built within the Sentinel Wall. Although weak, they can harm an organism if they come in contact with it.

Constructors can also play the role of a security grid. When a Constructor is attacked, a signal is automatically sent to Sentinel launchers to begin deploying Aggressors. For this reason, they can form an excellent security perimeter. With no weapons systems, the Constructors rely on Aggressors when threatened.

Halo 2
They make their first appearance in the Halo 2 level Sacred Icon, exactly beside where Thel 'Vadamee stands after he was dropped off from his Phantom. They were repairing the destroyed edge and left after a while, or if the Arbiter approaches them. Attacking them will result in being attacked by aggressors.

In the level Quarantine Zone, they appear at the end of the level when the player fights Flood-controlled Scorpion and Wraiths along with many flood combat forms using Covenant Ghosts and a Spectre, and two Enforcers fight against the Flood near a downed UNSC Pelican dropship. If the player watches one of the large and deep trenches, a large cloud of Constructors can be seen flying off into the sky.

Halo 3
In the campaign, they appear in the level The Ark, where they are seen inside Forerunner structures including inside the wall to the Cartographer and the map room itself.

They can be also found in the multiplayer maps of Isolation and Construct, where they can be seen flying around in the distance.

Trivia

 * On Isolation and Construct the Constructors can be sniped, though their small size and constant movement make this difficult.
 * In Halo 2 and Halo 3, even though Constructors do not attack you, your weapon reticule will turn red when passed over them.
 * A player can get up close and even touch a Constructor on Construct. First he/she must overload the map with trip mines. When the map is overloaded the player must go through the hole where the purple lift beams are projected. A player then can easy be touched by a Constructor but cannot be moved. They can be destroyed, mainly with a Spartan Laser or a Flamethrower. Note that even when one is destroyed another will immediately replaces it in the same place. So in other words, you will see the debris from a destroyed Constructor fall from a full and complete one.
 * Constructors appear to be able to compact into a smaller form, as their top and bottom halves appear to latch together. However, this has yet to be seen.

List of appearances

 * Halo 2
 * Halo 3