Minecraft – Custom Enchantment ID on Book That Can Be Combined In An Anvil

When combining enchantments, the enchantment caps out at a certain number. These are as follows:

Armor:

• Protection IV
• Fire Protection IV
• Explosion/Blast Protection IV
• Projectile Protection IV
• Feather Falling IV
• Respiration III
• Aqua Affinity 1
• Thorns III
• Depth Strider III

Swords/Axes:

• Sharpness V
• Smite V
• Bane of Arthropods V
• Knockback II
• Fire Aspect II
• Looting III

Bow:

• Power V
• Punch II
• Flame I
• Infinity I

Rod:

• Luck of the sea III
• Lure III

Other/Most/All:

• Unbreaking III
• Efficiency V
• Silk Touch I
• Fortune III

Enchantments of the same level can be combined to get an enchantment 1 level higher; combining enchantments of different levels always yields an item with the higher of the two levels, giving no increase.

Trying to combine any enchantment that would give you past the numbers above will just give you the cap, EX: Unbreaking III and another Unbreaking III would only give you Unbreaking III, instead of Unbreaking IV

You cannot put Silk Touch and Fortune on the same tool, you cannot put more then 1 different type of Protection on the same piece of Armor, and you can not have any combination of Sharpness, Bane of Arthropods, or Smite on a weapon. All these enchantments are standalone and may override another if put on an item.

The mechanic in question here is the so-called Prior Work penalty.

Enchantable items, including books, have a Prior Work penalty stored in their NBT data (RepairCost). The prior work penalty is added to the base level cost of working the item in the anvil.

For a new item, this cost is 0, and it is independent of the enchantments on the item. Each time you do anything with the item in the anvil, such as repair, combine or rename¹, the prior work penalty is doubled, and 1 is added. E.g. it is 1 after 1 working, 3 after 2 workings, 7 after 3, or (generally) 2^N – 1 after N workings. After 6 workings, the prior work penalty of that item will be 63, making it impossible to work that item any longer outside of creative mode.

When combining two items, both penalties are added to the cost, but the new item's penalty is calculated using only the higher of the two penalties. The optimal strategy is therefore to start high up and combine items with similar prior work penalties.

For example, you want to get a Power V, Infinity I, Flame I, Punch II bow: From enchanting/loot/villager trading, you have a Power IV bow, and Power IV, Infinity I, Flame I and Punch II as separate books. All 5 items have a penalty of 0.

If you just go and put everything on the bow, the bow will have been worked 4 times. Instead, you start by combining the Bow and 1 Book (1 working), and the 3 other books into one (2 workings), then combine those two. You'll end up with only 3 workings this way. You could even combine the bow with yet another book (or bow) before that and still end up with 3 workings on the final bow.

^{1 Renaming does not apply prior work penalty in 1.9.}