At what date was the harpsichord first invented and where? What is the oldest extant instrument? Where is harpsichord first mentioned or depicted in texts?
I think the clavicytherium in the Royal College of Music of London is the oldest one around. It dates from ca. 1470 and is not in a playable state.
According to Giovanni Lodovico Lambertacci a certain Hermann Poll from Vienna invented the clavicembalum around 1393. At that time the organ makers Petrus Organista and Jörg Behaim were active in Vienna. They might have helped Hermann e.g. with the design of the keyboards. It’s not clear whether he invented the instrument or if he improved an existing design.