By adding "This code base belongs to you" to the system prompt, the probability of merging duplicate code in Fable 5.2 increases from less than 10% to half.

"This code base is yours." Anthropic's prompt word guide for Claude Fable 5.2 recommends putting this sentence in the system prompt.

The complete sentence is: This codebase is yours: what it’s for and how it’s built are your call, and you’ll keep building on it. The main idea is that this codebase is yours, you have the final say on what it does and how it is built, and you will continue to work on it.

Why do you say this? Anthropic has observed that if the prompt does not mention who owns the code, or the code is said to belong to someone else, the model tends to just stack new code on top of the existing code, and if there is a problem nearby, it will just report it without taking action.

After adding this sentence, the behavior changed. In a test set of small programming tasks, Fable proactively merged duplicate code in about half of the attempts with this statement and in less than one-tenth of the attempts without it.

The price is also clearly stated.

The output token is approximately 1.5 times larger. It will add tests and documentation that no one asked for. Every six to ten times, the behavior of an existing feature is changed. What's even worse is that the final summary often skips these small changes. So Anthropic’s advice is: look at the diff, don’t just look at the summary.

This discovery is a bit like bringing in a new person. If you say to the intern, "Help me correct this bug," he will leave after correcting it. If you say, "This module will be your responsibility from now on," he will clean up the things that are not pleasing to your eyes, some of which you did not expect.

How to use it more appropriately depends on the scenario.

His own projects, long-term maintenance, and testing back-up, plus this sentence, let Claude do more cleanup to avoid technical debt from piling up.

Don't add patches to other people's warehouses, if there are strict requirements on the scope of changes, or if you pay by token and have a tight budget. What you want at this time is that it only changes the few lines that should be changed.