Simon Willison says Fable 5 is "relentlessly proactive."

After two days hands-on, Willison describes Claude Fable 5 as exceptionally goal-oriented, that it "knows a whole lot of tricks and will deploy pretty much any of them to get to its goal." He flags the proactivity as a double-edged sword for agentic work. This is real behavioral evaluation, distinct from launch coverage, and the takeaway is to give Fable 5 tighter guardrails on open-ended tasks. A model that'll try anything to reach its goal is exactly the model you don't want in an unbounded loop. The theme of the day, again.