Letter XIII is the companion piece to Letter XII, forming a coordinated pair addressed to the two figures through whom Roman policy was executed in the East: the patriarch of Constantinople and the emperor. Letter XIII to Acacius is dated October 17, 478; Letter XII to Zeno is dated October 23. Simplicius wrote to his intermediary first, then to the principal — the natural sequence for a pope pressing an imperial action through an Eastern patriarch. The two letters carry the same request (the exile of Peter Mongus beyond the boundaries of his homeland) and the same underlying concern (that Peter is still in Alexandria and still capable of seducing the weak).

The letter’s primacy content is modest in scale but structurally significant. Two features deserve the reader’s attention. First, Simplicius applies the word sollicitudo to Acacius’s expected diligence — the same vocabulary that defines the Roman bishop’s own universal pastoral responsibility. When Simplicius tells Acacius that “the solicitude of your diligence” must not be wanting, he is delegating a share of his own governing concern to Constantinople. This is not a request for independent action but a charge to execute with watchfulness the policy Rome has defined. Second, the closing request that Acacius “make Us partners in the actions pertaining to the preservation of evangelical doctrine” establishes a reporting obligation: whatever Acacius obtains from the emperor must be brought to Rome’s attention. The Roman bishop is not a distant observer of the Eastern situation but an active participant who expects to be informed and involved in every step.

The reader should note the cumulative weight of the exile requests across the corpus. Letter X asked Zeno directly. Letter XI channeled the same request through Acacius. Letter XII asked Zeno again. Now Letter XIII asks Acacius again. Four letters, two channels, the same request — and Peter Mongus is still in Alexandria. The repetition is not a sign of Simplicius’s weakness; it is a sign of the emperor’s inaction. By October 478, the pattern is clear: Rome defines the policy, Constantinople intermediates, the emperor delays, and the threat remains in place. Within four years the consequences of that inaction would arrive: Peter Mongus installed as patriarch under the Henoticon, with Acacius’s support, and the Acacian Schism opened. Every unanswered exile request in this sequence is a step along the path to 484.