The Thief's Cap is a mid to high-ranked light armor headgear in Final Fantasy XII that boosts the wearer's magickal defense by 40 (Zodiac) or 36 (original), HP by 460, and Speed by +4. It can thus be a good headgear for those who use weapons that take the wielder's Speed stat into account: bows and daggers and ninja swords. However, a helm that boosts Strength has a bigger impact on the damage dealt.
In the updated Zodiac versions, the Thief's Cap is not available in shops, but sells for 4,650 gil. It is found in a treasure in Cerobi Steppe's Old Elanise Road (20% chance to appear, 20% chance for gil, 5% the item treasure is the Thief's Cap with the Diamond Armlet equipped), and in Great Crystal's Kabonii Jilaam Avaa (always appears). It is a common steal from the Ahriman in the Trial Mode Stage 29.
The Thief's Cap is a mid to high-ranked light armor hat, giving 40 or 36 magick resist depending on version, 460 HP, and 4 to Speed. In the original version where everyone can wear any equipment piece, the more expensive heavy armor helm, Giant's Helmet, or the mystic hat Gold Hairpin, may be preferable purchases if the player can stretch their funds to them, as the HP boost from light armor is not usually worth prioritizing over heavy armor that boosts Strength for physical attackers, or mystic armor that boosts Magick Power for spellcasters. Though the Thief's Cap boosts Speed, its effect is minimal, and even those whose weapons take the wielder's Speed stat into account benefit more from the Strength-boosting heavy armor helms.
In the Zodiac versions, Machinist, Monk, Archer, and Shikari can equip the Thief's Cap, and so it is only useful to look for if the player has chosen these License Boards for their party. In The Zodiac Age, after fighting Belias, the player can choose two License Boards per party member, and so more party members can choose from the potentially more useful heavy and mystic armor sets.
The cap is much rarer in the Zodiac versions, not available to purchase from Balfonheim and only to be available from treasures; the superior Gigas Hat can be bought after the party has the Treaty-Blade, and so the Thief's Cap can be skipped unless the player finds the treasure for it in the Great Crystal, or they are doing the Trial Mode. It being a steal from Stage 29, the player could get it much earlier than it is available in the main game, if determined.