Annabeth's Invisible Cap is a magical item that had the ability to make the person who wears it invisible. As of The Mark of Athena, the magic cap had its magical properties removed after Annabeth Chase got into an argument with Minerva. However, since The Staff of Serapis, the cap got back its magical properties.
The cap is similar to Hades' Helm of Darkness, however it does not have any fear inducing properties, it cannot change its shape, and it does not make the wearer intangible. It has had many life-saving uses, quite notably helping Percy Jackson escape from the Furies on a bus in The Lightning Thief. Annabeth had used the cap to defeat the Laistrygonians, when they attacked Percy and Tyson and to trick Polyphemus in The Sea of Monsters, escape from Mount Saint Helens in The Battle of the Labyrinth and fight alongside Percy in The Last Olympian. So far, Percy is the only known demigod other than Annabeth who has used this cap. Carter also has worn it in The Crown Of Ptolemy.
In battle, or in any circumstance as a demigod, the hat is very useful. When worn, it makes the user and any item they're holding invisible. That in and of itself would make it very powerful, as its usefulness of stealth is an important and useful factor in battle and ingenuity on Annabeth's part makes it invaluable. Unfortunately the cap lost its magic in The Mark of Athena, after Annabeth get in an argument with her mother's Roman form. In The Staff of Serapis, Athena restores the cap's power and sends it to her daughter to help her defeat Serapis. In The Crown of Ptolemy the cap is ineffective against Setne who is able to see through its magic and render it ineffective when Annabeth is wearing it. However, Egyptian magician Carter Kane was later able to use the cap effectively against Setne with it being wielded by a magician rather than a demigod making it an effective weapon against Setne.
- Annabeth Chase – The cap belongs to Annabeth, and was given to her by her mother Athena. The cap helped cause many victories and success in Annabeth's and Percy's quests.
- Percy Jackson – When Annabeth was abducted in The Titan's Curse, Percy kept the cap and used it to eavesdrop on Atlas and Luke. Six months later, he used it to sneak up on Kronos before he could reform, as well as checking in on Nico as he spotted him spying on the Hunters during Phoebe's outbreak of Hives caused by centaur blood, and accidentally startled Nico as he took it off. He also used it briefly in The Lightning Thief and The Battle of the Labyrinth.
- Carter Kane - In order to defeat the evil ghost magician Setne, Annabeth gave Carter her cap, believing that while it didn't work for her against Setne, with a magician wielding it instead of a demigod, it might work. The cap proves effective for Carter who is able to sneak up on Setne and cut off his hand, allowing them to retrieve the Book of Thoth and saving Percy, Annabeth, and Sadie Kane's lives.
- In ancient myths, Athena gave Perseus a helmet that turned him invisible. During the series, Annabeth, a daughter of Athena, has offered to let Percy Jackson use it on multiple occasions.
- In her official picture, Annabeth has the hat on and is seen starting to turn invisible from the bottom up.
- Annabeth does not let anyone else other than Percy and Carter use the hat.
- Percy actually had Annabeth's hat when she went missing in The Titan's Curse.
- The cap seems to have a different level of efficiency depending on the type of wielder: against Setne while wielded by the demigod Annabeth, it was completely ineffective. However, when the magician Carter Kane wielded it against the same opponent, it was extremely effective to the point that even when hosting the goddess Nekhbet, Percy couldn't see Carter even with her enhanced vision.
- The cap works similar to a Swan Cloak (notably used by Samirah al-Abbas) which can also turn the wearer invisible. Though unlike the cap, the cloak can hide up to two people.