September 1, 2026 · 12 min read
Claude Fable 5.1: What Changed, and a Practical Upgrade Checklist
Claude Fable 5.1 is Anthropic's most capable widely available model. This is not a review — it is the checklist we use before switching a client site or agent to it: a 15-minute readiness check, a should-you-upgrade scorecard, the three things that break, and the prompt fixes that get the new behavior working for you.