Two months after Prince Mochihito took up arms in May 1180, Yasunobu sent a messenger to Yoritomo to warn him that there was a plot to attack the MINAMOTO clan members in various provinces and advised him to escape to Oshu in haste, thereby playing an important role in the Yoritomo' uprising ("Azuma Kagami" [The Mirror of the East]).
治承4年（1180年）4月の以仁王の挙兵二ヶ月後、康信は頼朝に使者を送り、諸国に源氏追討の計画が出されているので早く奥州へ逃げるように伝えるなど、頼朝の挙兵に大きな役割を果たした（『吾妻鏡』）。