Auto Guard (オートガード Ōto Gādo) is a Device function that automatically activates a defensive spell (usually of Barrier-type) if its master is under attack but cannot respond due to surprise or injury. It is also referred to as Auto Protection (オートプロテクション Ōto Purotekushon) in The MOVIE 1st.
- Nanoha Takamachi's Raising Heart auto-casts Protection.
- Fate Testarossa's Bardiche auto-casts Defenser.
- In The Battle of Aces, Zafira and Material-S (Luciferion) have a special skill called "Auto Guard", which allows them to auto-cast their respective shield defense (Panzerschild and Round Shield). Other characters may also use this skill after Zafira's scenario is cleared.
Subaru Nakajima's Mach Caliber auto-casts Wing Road and Caliber Shot, though whether it is an Auto Guard function is debatable. On the other hand, Mach Caliber displayed the ability to continue fighting even after her master is knocked unconscious.