John V. A. Fine's The Late Medieval Balkans - A Critical Survey from the Late Twelfth Century to the Ottoman Conquest refers to a so-called "Tati Agreement" between Sigismund of Hungary and Stefan Lazarević regarding the status of Beograd. Sigismund presented himself at the city with an army in 1429 after Đurađ Branković was installed as Despot of Serbia because he did not believe Đurađ would hand over the city. However, I am finding it difficult to find any more resources regarding the Tati Agreement's content. Any help would be greatly appreciated.
Hey!
Not specialised in Medieval History, but happen to be Hungarian and have access to a few sources. Though I only used the simplest of Google searches for this one, but here are to rough translations of two good-quality Hungarian language sources I found.
Firs of all, it is "Tata Agreement" (Tatai szerződés). It was signed in Tata Castle, here's a picture of the building as it still stands.
The first source is the Hungarian Catholic Lexicon (compiled between 1993 - 2014). It writes: "The childless Stefan Lazarević Serbian despot, his nephew, Đurađ Branković and several Serbian nobles visited King Sigismund and made a pact, according to which Branković became a Hungarian aristocrat and member of the Hungarian Royal Council, could purchase a house in Buda, Serbia is guaranteed to him and his male heirs under the suzerainity of the Hungarian Crown and protected with Hungarian forces. The Serbian despot took an oath of loyalty to Sigismund, and promised, that after his death 17 castles will be handed over to Hungary; the despot will help Hungary with its forces if needed. If Branković has no male heirs, his lands will fall upon the King of Hungary. Later, in 1427 based on the treaty, Sigismund occupied Belgrade and the land of Mačva." (rough translation).
The other is from Hungarian history periodical Klió 1994/1. It is a bit longer, so I won't translate it, you can try with DeepL, but it goes into more detail about the reasons Sigismund pursued such a treaty. In short, his plan was creating a buffer zone of vassal states around Hungary against the Ottoman encroachment.
Hope I could help!
