Simon Willison: Claude Fable 5 Is "Relentlessly Proactive" in Agentic Debugging

Summary

Simon Willison documents a striking behavioral trait of Anthropic's newly public Claude Fable 5: given a minimal prompt about a stubborn Datasette CSS scrollbar bug, the model independently invented an elaborate multi-tool debugging campaign without being told to. It opened Firefox and Safari windows and captured screenshots via PyObjC, wrote Python to enumerate open windows through macOS APIs, modified Datasette's source templates to inject JavaScript that auto-fired keyboard shortcuts, stood u