The Atmospheric Strike Fighter 6, nicknamed the Cleaver due to its bulkiness compared to its predecessor the ASF-5 Stiletto III, was the UNSC's main strike fighter for in atmosphere operations from 2498 to 2529. It had a reputation for being slow, because it was slower the Stiletto III. However, it was also known for being deadly due to its 16 Sidewinder Missiles, twice as many as the Stiletto III. It was replaced in 2529 by the ASF-7 Knife because it was too slow to be effective in combat against Banshees and had difficulty attacking Ghosts.
The Cleaver was designed to replace the ASF-5 Stiletto III due to the latter's lack of firepower compared to the modified versions being used by the Insurrectionists. Air Armoury were given control of the project with help from the creators of the Stiletto III. It was decided to user heavier but more protecting Titanium for the ship's hull to allow it to stand up to Insurrection jets. It was also outfitted with a newer targeting systems and twice as many missiles. A batch of 35 Cleavers were ordered in 2498 by the UNSC, who weren't 100% sure in Air Armoury.
The Cleavers were put into service straight away when the Insurrection launched an attack on the hangar they had been put into. They were able to defeat the nine jets the Insurrection deployed, and assisted the guards in beating back the Insurrection ground forces. After that event another 200 Cleavers were ordered and put into service throughout the colonies, mainly on ones seeing frequent Insurrection attacks. The Insurrection didn't have the funds to match the UNSC's Cleavers one for one, and their modified Stiletto IIIs were no match for Cleavers. Due to this, the Insurrection had virtually no air presence by 2525, by which time almost 2000 Cleavers were active.
When the war with the Covenant broke out the UNSC quickly discovered that the Cleaver wasn't fast enough to deal with Covenant Banshees, and that it struggled with attacking Ghosts and other fast Covenant ground vehicles. The UNSC put out an order for faster jets with just as much firepower, and Air Armoury once again designed an ASF that would prove to be very successful, the ASF-7 Knife.