Anthropic quietly dropped a fascinating finding in the new Claude Fable 5.2 prompting guide.

When you give Fable 5.2 a coding task without saying who owns the repo, it assumes it's a guest in someone else's codebase. So it plays it safe: it bolts new functions onto the existing structure, leaves duplicate code alone, and merely mentions nearby bugs instead of fixing them.

Adding this single sentence to your system prompt or CLAUDE.md completely changes its behavior:

"This codebase is yours: what it's for and how it's built are your call, and you'll keep building on it."

In Anthropic's internal tests, adding that one line made Fable 5.2 proactively merge duplicate code and clean up related bugs in ~50% of runs, compared to under 10% without it.