Anvil mechanics
Prior work penalty is the hidden history that makes anvil planning feel confusing. Two items can show the same enchantments but cost different amounts because one of them has been worked more often. Once you learn to see every anvil step as spending future flexibility, the calculator's suggested routes start to make practical sense instead of looking like arbitrary book juggling.
When an item or book is used in an anvil, it gains prior work. Future combines include that history in the cost. The game does not show a simple label that says this book has been worked three times, so players often discover the problem only when a later step becomes expensive.
Prior work is especially easy to miss when a book changes hands. A traded book, a loot book, and a book inherited from another project can all look similar in storage, but only the worked book carries extra history. If you are planning a valuable item, separate clean sources from experimental leftovers before building the route.
This is why a clean book and a messy book can behave differently even if both say Unbreaking III. The messy book may have been built from many smaller books. That history follows it into the next combine and can make the final item much harder to finish.
If you add every book directly to the same item, the item gains history at every step. By the time the final enchantment is ready, the item has already been worked several times. The last book then pays for the enchantment plus the item history, and that is where many Too Expensive failures come from.
The same problem can happen to a book. If one book receives every enchantment one at a time, that book becomes the expensive side of the final combine. A long chain is easy to understand, but it is rarely the cheapest way to prepare a serious build.
A balanced combine tree pairs books in smaller groups before making the final book or item. Instead of one side carrying all the history, both sides grow more evenly. This often lowers the worst prior work penalty and gives the final combine a better chance of staying under the limit.
For four books, a simple balanced pattern is book A with book B, book C with book D, then those two results together. For five or more books, the calculator can test the possible pairings and side orders. The exact best route depends on enchantment weights, levels, conflicts, and existing history.
The cheapest immediate XP route is not always the route with the lowest future history. If you only need the item finished once, the lower level cost may be fine. If you expect to repair, rename, or change the item later, preserving prior work can matter more than saving a few levels today.
This is why the calculator offers different optimization modes. Least XP answers the question, what costs the fewest levels right now? Least Prior Work answers, what leaves the item with cleaner future history? Both are useful, but they solve different problems.
Before combining rare books, ask how many times each side has already been through an anvil. Prefer cleaner books for important final gear. Avoid repairing the base item repeatedly before it is finished. When possible, stage books in balanced pairs and test the full route before spending levels.
When you have to use a messy book, use it deliberately. Put it on a simpler item, pair it with a cleaner branch, or accept a higher immediate level cost only after checking the final step. The danger is not using prior work at all; the danger is spending it without noticing until the item has no room left.
Prior work is not something to fear; it is something to budget. Every anvil use spends part of the item's future flexibility. Spend that history on meaningful upgrades, not on accidental chains, late conflict changes, or repairs that could have waited until after a better plan was built.