Cutting insert with a cuboid body has a substantially square top face and bottom face, said top face and bottom face connected by four peripheral adjoining side surfaces. At least two cutting edges are formed at the transition of the top face to at least two side surfaces, that cutting edges defining a plane of the top face. To provide a cutting insert and a corresponding tool suited to produce valve seat surfaces with high precision but still having a rather simple construction and being precisely mountable with ease and less efforts of adjustment, the plane defined by the top face cutting edges of the cutting insert has maximum dimensions of 7 mm×7 mm, where the bottom face has a supporting surface formed exactly parallel to the plane defined by the top face cutting edges by means of grinding within a tolerance of less than 5 angular minutes.