A testing head for functionality testing a device under test comprises a plurality of contact probes, each contact probe having a rod-like body having a preset length less than 5000 μm extending between a first and a second end, the second end being a contact tip and an opening extending all over its length and defining a plurality of arms, parallel to each other, separated by the opening and connected to the end portions of the contact probe, and an auxiliary guide, arranged transverse to the body and provided with suitable guide holes, the contact probes sliding through each of them, the auxiliary guide defining a gap including one end of the opening being a critical portion of the body and a zone more prone to breakings in the body undergoing low or even no bending stresses in the gap with respect to the rest of the body.