In this paper I investigate the quite long-term relationship of the War Councillor and expert of the Ottoman Empire, Johann Rudolf Schmid (1590–1667) and the renegade spy, Hans Caspar or Huseyn chiaus (?–1660?). The careers of these two persons can be well reconstructed. Both might have been captives in the Ottoman Empire, but while the Swiss Schmid remained Christian, Hans Caspar, who was born in Vienna as Alexander Fischer, converted to Islam and received the name Huseyn. They must have known each other at least since 1629 and Schmid helped Huseyn on multiple occasions, especially after 1647, to become a spy for the Council of War, because until that time he had only been a local spy of the commanders of the Habsburg–Ottoman frontier. The renegade regarded Schmid in his reports as his patron and shared with him reports on not only politically relevant events, but also several of those of his everyday-life. Although he regularly called the diplomat his patron, yet no sources have been obtained so far underlining that Schmid also considered him as a client. Moreover, Hans Caspar was mentioned in the sources most of the time as a renegade, and that is why their relation cannot be regarded as a classical patron-client relation: they were subjects of two different empires. Despite their different courses of life, they could profit from each other, because Hans Caspar received frequent remunerations for his intelligence services. Thus, he became known among the War Councillors, and in line with this, Schmid was enabled to get access to some secret information about the politics of the actual vizier of Buda by the knack of the ingenious reports transmitted to him by Caspar.
This case study offers a better understanding of the functioning of the Habsburg– Ottoman frontier diplomacy in the mid-17th century.