By 2525 numerous ships of the class were either mothballed or stripped for parts, with a number being reassigned to Naval academies across UNSC territory as training vessels. The Halcyon-class entered service with the UNSC Navy during the late 2450s. Wartime refits conducted in 2530 upgraded the MAC to a Mark III Heavy Coil system and increased the total number of railgun batteries and missile pods to 30 and 48 respectively, also increasing the ship's PDC compliment to 100, granting increased coverage. They were given traditional small ship names used historically by the Royal Navy and served during World War II. Defensive measures include 200 centimeters of Titanium-A battleplate, electromagnetic reactive armor, and an electronic countermeasure suite. The Halcyon class was a class of 21 oil-fired minesweepers (officially, 'fleet minesweeping sloops') built for the British Royal Navy between 19.

Secondary weapon systems include 20 14-inch/45 caliber railgun batteries, 26 anti-ship missile pods, 8 nuclear missiles, and 75 Butress point-defense cannons. The primary armament of the Halcyon-class is single a Mark III Light Coil Magnetic Accelerator Cannon.

The class was also equipped with a Kawanishi Engineering SED-2550X Shaw-Fujikawa Translight Engine for faster-than-light travel. This configuration provided only 10% of modern engine output as of 2552. Shipboard power is provided by a fusion reactor, with six impulse drives for sublight propulsion. The main engine assembly consists of four underpowered C2H reactors, two primary RT-5 series impulse drives and four secondary N1 series impulse drives. The class is well known for its durability, despite suffering from poor acceleration and maneuverability, a Halcyon-class can remain combat effective, even in the event of losing 90% of its armor plating. An Omega-pattern bridge module is slung underneath one of the forward sections and eight launch bays are located on the port and starboard flanks. Both ships encounter the other during routine patrols near Jupiter and then open fire on each other. Halcyon-class cruisers are 1,170 meters (3,838.5 feet) in length, with a superstructure composed of five hexagonal and octagonal sections mounted onto each other. MCRN Donnager (The Expanse) vs Halcyon-class Light Cruiser) Halo.

Halcyon class light cruiser