During the reign of the Ottoman Sultan Bayezid I (1389–1402), who had conquered most of the other beyliks of Anatolia, İsfendiyar was able to establish good relations with the other Oghuz Turkic clans.

It is believed that the emergence of Kadı Burhaneddin's short lived but powerful Turkmen state in the Central Anatolia was one of the main factors for Ottoman-Candar cooperation.

[2] In 1416, his son Kazım, with Ottoman support, revolted which forced İsfendiyar Bey to abandon all territory south of the Ilgaz Mountains (i.e. Çankırı).

The death of Mehmed I and the two revolts during the early years of the new sultan Murad II gave İsfendiyar a chance to regain his losses.

According to the terms of the treaty signed circa 1423, İsfendiyar Bey was to abandon his gains but was permitted to keep the territories of Kastamonu and Sinop.