On 7 January 1459 he was appointed as nuncio by Pope Pius II with support from Francesco I Sforza, Duke of Milan.
The council fell short of achieving the goals of Pius II, among other reasons due to the instable balance between England and France at that time that the English crown needed to maintain.
Regarding the second part of his mission to mediate between York and Lancaster, Coppini was unable to achieve much in his limited role as nuncio instead of a full papal legate.
With Henry VI now under control of the Yorkists, Coppini supposedly played a crucial role in arranging a new order in the kingdom, as he reported to Pius II.
In January 1461, Coppini sent fruitless letters of negotiation to Queen Margaret, then called for a crusade against the Lancastrians, promising absolution for those fighting alongside Warwick.
Pius II however named the bishop of Arras as Coppini's successor to address concerns by the new French king Louis XI, who had sided with the Lancastrians in England.