The main source is a stele at Karabalgasun, the Uyghur capital in the Orkhon valley, inscribed in three languages: Chinese, Sogdian and Old Turkic. All three faces are damaged and no single face preserves the whole account, so the conversion has to be reconstructed by fitting fragments together. The Chinese face is the best preserved and presents the change as the khagan being instructed by teachers of the religion of light; the Sogdian face supplies detail the Chinese does not.
Chinese dynastic histories add an outside view and they are not friendly. The Tang record notes the Manichaean temples permitted in Chinese cities at Uyghur insistence and treats them as a concession extracted by a difficult ally, and it associates the religion with Sogdian merchants whose dealings the court resented.
Bogu Khagan was killed in 779 by his cousin Tun Bagha Tarkan, in a coup the Chinese sources connect to a plan to invade China that the Uyghur nobility did not want. Manichaeism survived the man who imposed it, which is the strongest evidence that it had taken root beyond the court, and later khagans went on patronising it. Reconstructing the episode means reading a damaged trilingual monument, a hostile foreign chronicle and later religious manuscripts from Turfan against one another, which is the ordinary condition of steppe religious history rather than a special difficulty here.