Back-end-of-line compatible 400°C thermally robust perpendicular spin-orbit torque (p-SOT) cells with reduced MgO short fails are demonstrated by the etch-stop-on-MgO process. The stop-on-MgO cell features the SOT channel continuity and no metal redeposition at MgO sidewall after ion beam etching. To the best of our knowledge, the endurance as high as 1010 cycles using the field-free spin-transfer torque (STT) assisted SOT writing is achieved for the first time. The SOT switching current density can be reduced by increasing the STT current density to save write energy. The stop-on-MgO cell does not degrade the cell switching speed, since the switching always starts from the inner free layer and the domain propagation at the extended free layer does not affect junction resistance, as shown by micromagnetic simulation. The simulation also reveals that the thermal stability factor of stop-on-MgO cells is enhanced by the extended free layer, which suffers less from the interference of pinned layer edge stray field.