According to the Nominalia of the Bulgarian khans (Imennik), Kormesiy would have reigned for 28 years and was a descendant of the royal Dulo clan. According to the chronology developed by Moskov, Kormesiy would have reigned 715–721, and the longer period reflected in the Imennik would have indicated the duration of his life or would have included a period of association with his predecessors. Other chronologies date the reign of Kormesiy to 721–738 but cannot be reconciled with the data of the Imennik.

Kormesiy is encountered in relation to the events surrounding the peace treaty between Bulgaria and the Byzantine Empire between 715 and 717 – the chronology has to be argued from the names of the Emperor and patriarch involved – for which our only source is the Byzantine chronicler Theophanes the Confessor. According to Theophanes, the treaty was signed by Kormesiy as ruler of the Bulgars. Kormesiy is not mentioned in any other historical context. The fact that there is no record of wars between Bulgaria and the Byzantine Empire during his reign, however, implies that he sustained the peace between the two countries.