Chinese histories for the Tang dynasty (618–907 CE) record contacts with merchants from "Fulin", the new name used to designate the Byzantine Empire. The first reported diplomatic contact took place in 643 CE during the reigns of Constans II (641–668 CE) and Emperor Taizong of Tang (626–649 CE). The Old Book of Tang, followed by the New Book of Tang, provides the name "Po-to-li" for Constans II, which Hirth conjectured to be a transliteration of Kōnstantinos Pogonatos, or "Constantine the Bearded", giving him the title of a king.

The Tang histories record that Constans II sent an embassy in the 17th year of the Zhenguan regnal period (643 CE), bearing gifts of red glass and green gemstones. Yule points out that Yazdegerd III (r. 632–651 CE), last ruler of the Sasanian Empire, sent diplomats to China to secure aid from Emperor Taizong (considered the suzerain over Ferghana in Central Asia) during the loss of the Persian heartland to the Islamic Rashidun Caliphate, which may also have prompted the Byzantines to send envoys to China amid their recent loss of Syria to the Muslims. Tang Chinese sources also recorded how Sasanian prince Peroz III (636–679 CE) fled to Tang China following the conquest of Persia by the growing Islamic caliphate.