Very thin and very flexible. The blade must follow the curvature of a fish skeleton closely to minimize waste, and that pronounced flex is the defining requirement: a fillet knife bends along the contour of the bones rather than fighting it. For precision fish prep in a kitchen, a yanagiba or sujihiki is the better tool. The fillet knife is built instead for field or production work, where durability and corrosion tolerance matter more than refined edge performance.
Primary tasks: filleting fish, including skinning, boneless portioning, and following the rib cage in a single stroke from head to tail. It also handles supreming citrus (cutting out the segments free of membrane), since the flexible blade follows curves naturally.
Ideal steel: the blade must flex without taking a A permanent bend the steel keeps after being flexed, instead of springing back straight., which puts a premium on toughness and elasticity over maximum hardness. 440A dominates marine applications for its corrosion tolerance in saltwater, while CPM-154 or ATS-34 serve the higher-quality end.
Limitations: any heavy work, vegetables, or anything that needs a rigid blade. For boned-out meat rather than fish, reach for a boning knife instead. See care for keeping a flexible blade rust-free and construction for how blade flex relates to thickness and grind.