Improper wire jumper wire

Is it okay to use two different wire sizes on a jumper to a breaker? The breaker is a 20amp with #12 wire on the circuit. There is a jumper wire connected to the #12 wire that is a #14 wire which is connected to the breaker. I believe you need to correct wire size. One electrician told me it was okay and the other said #12 wire should be used on the jumper. Looking for your opinions

should be a #12

The OCPD needs to be smaller or equal to the size listed in 240.4(D) of the NEC. Except for certain installations (not this one) #14 can not be protected at 20 amps.