Constructor Sentinel

Constructors are small devices that mainly just repair damage to any Forerunner structure they find. The Sentinels will come to their aid if they are attacked, as they have no weapon systems of their own.

Role
Constructors are devices similar to Sentinels in design that mainly repair damage to any Forerunner structure they find, using weak energy beams similar to Sentinel Beams. The beams are lethal if in direct contact with an organism. This is unknown to the Constructor though. They are similar in appearance to the Covenant Plasma Pistol.

The Sentinels will come to their aid if they are attacked, as they are ill-suited for combat, having no weapon systems of their own. However, their beams will damage you if you get caught in the path (on Legendary difficulty their beams can kill in seconds). They also use their beams to lower the shields on pistons, allowing them to move throughout the Sentinel Wall quickly. Constructors are encountered only during the levels Sacred Icon and the Quarantine Zone and are distantly visible on the Halo 3 multiplayer maps, Isolation and Construct, but they can be seen on The Ark (Level) (where the light bridge must be activated) and The Oracle. They first appear in the opening moments of "Sacred Icon", where a swarm of them flies about trying to fix the platform you start on, then moves away further into the Sentinel Wall. They don't really affect anything in game, they show red on your reticule and you may shoot them if you like, although they won't bother you. If you jump in front of one in the beginning of "Sacred Icon", you can get hurt (though very little) by their thin beams.

When they appear in the "Quarantine Zone" level, they appear at the end of the level, when the player fights Flood-controlled Scorpion and Wraith tanks along with many Flood using Covenant Ghosts and a Spectre. Two Sentinel Enforcers fight against the Flood near a downed Human Pelican dropship. During all this chaos, if the player watches one of the large and deep trenches, a large cloud of Constructors fly off into the sky.