According to the "Koyo Gunkan", Yoshinobu's tutor,Toramasa OBU, his close associate Gengoro (also known as Masakuni) NAGASAKA, and Suo no kami (the governor of Suo Province), Sone secretly plotted in July 1564 to assassinate Shingen, but the plot was exposed beforehand in a secret letter from Toramasa's younger brother, Masakage YAMAGATA, resulting in, in January 1565, the execution of Toramasa and the other two as ring leaders of the rebellion, and the expulsion of eighty cavalry members of Yoshinobu's retainers.
『甲陽軍鑑』に拠ると、永禄7年（1564年）7月に義信の傅役である飯富虎昌、側近の長坂源五郎（昌国）、曽根周防守らが信玄暗殺の密談をしていたが、計画は事前に虎昌の実弟・山県昌景の密書により露見し、永禄8年（1565年）1月、虎昌以下は謀反の首謀者として処刑され、80騎の家臣団は追放処分となった。