In 1185, detained at Manpukuji in Koshigoe, Minamoto no Yoshitsune composed the famous 'Koshigoe Letter' addressed to Oe no Hiromoto (1148–1225), the first head of the Kamakura shogunate's Mandokoro. Oe no Hiromoto was a civil official from Kyoto invited by Yoritomo to design the shogunate's institutions, including the shugo and jito system, and was the most influential administrator in the early shogunate. Yoshitsune pinned his hopes on Hiromoto's intercession, but whether Hiromoto relayed the letter, and whether it moved Yoritomo at all, remains unclear from historical sources. The Koshigoe Letter survives at Manpukuji as a copy and stands as one of the most celebrated petitions in Japanese history.

In 1185, after Yoshitsune destroyed the Taira at Dan-no-ura, Yoritomo barred his brother from entering Kamakura, angered by his unauthorized acceptance of court titles. Detained at Manpukuji, Yoshitsune wrote the celebrated "Koshigoe Letter" to aide Oe no Hiromoto, pleading his loyalty—but Yoritomo's heart remained unmoved, setting Yoshitsune on a path toward his tragic end.