Tell Claude "this codebase is yours" and it starts fixing things no one asked for.
This line comes from Anthrop's new guide for Claude Fable 5.2. When the prompt says nothing about ownership, or presents the code as someone else, the model tends to pile new code on top and only report problems it detects nearby.
The full sentence that Entropy suggests for guiding the system:
This code base is yours: what it's for and how it's built are your call, and you'll continue to build on it.
In anthropo tests on small coding tasks, Fable merged repeated code in about half of the attempts with it, and in less than a tenth without it.
It has a price. The model used about 1.5 times as many output tokens, added tests and documentation that no one asked for, and in one try out of six to ten changed the behavior of an existing feature. Its closing summary often skipped over the minor changes, so Anthropic says to review the difference, not just the summary.