What was the original codename for Assassin's Creed during development?

The original development codename for Assassin's Creed was Prince of Persia: Assassins.

The project began at Ubisoft Montreal as a planned spin-off related to the Prince of Persia series. Its design focused on an Assassin character serving a prince, but the concept grew beyond the planned spin-off and became a separate franchise.

The final game retained ideas associated with the early concept, including acrobatic movement, historical environments, stealth, and a powerful hidden organization. Ubisoft released the finished Assassin's Creed in 2007, with Altaïr Ibn-La'Ahad as its historical protagonist.

Project Altaïr is sometimes used informally when discussing the game's development or its early protagonist, but it was not the original codename. The Hidden Ones is the earlier organization depicted in Assassin's Creed Origins, so it also belongs to the series' fiction rather than the game's development history.