Guo Tu
The fifth-century historian Pei Songzhi pointed out a discrepancy between Zhang He's biography and the biographies of Cao Cao and Yuan Shao about the time when Zhang He defected to Cao Cao's side. According to Cao Cao and Yuan Shao's biographies, Yuan Shao sent Zhang He and Gao Lan () to attack Cao Cao's main camp per Guo Tu's suggestion. They defected to Cao Cao when they learnt that Wuchao was lost, and their defection resulted in Yuan Shao's defeat. Based on these two accounts, Zhang He defected to Cao Cao "before" Yuan Shao's defeat at the Battle of Guandu. On the other hand, Zhang He's biography mentioned that Zhang He defected to Cao Cao "after" Yuan Shao's defeat at Guandu and after Guo Tu slandered him.