Saicho and Kukai, who between them established Japan's two major new Buddhist schools of the early Heian period, Tendai and Shingon, began as something closer to contemporaries and occasional collaborators than as rivals. Saicho, senior in official standing at the time, initially borrowed esoteric texts from Kukai and sought instruction from him in areas of Esoteric Buddhist practice that Kukai had studied more thoroughly during his own time in Tang China.
The relationship did not hold. Historical sources document a genuine falling out, driven by a combination of doctrinal disagreement over how esoteric teaching should be transmitted, whether through textual study alone or through the more restricted master to disciple initiation Kukai insisted on, and a personal rupture after one of Saicho's own senior disciples, Taihan, left Saicho's community to study directly and permanently under Kukai instead. Kukai's later refusal to continue lending certain key esoteric texts to Saicho, on the grounds that genuine understanding of them required direct initiation rather than reading, is recorded as a specific point of contention between the two men. This documented conflict between the founders of Tendai and Shingon complicates any picture of early Heian Buddhism as a single unified movement, showing instead two ambitious reformers who cooperated only briefly before their approaches to esoteric transmission proved incompatible.