Introducing Claude Opus 5.5, the first model in our new Claude 5.5 family. It performs at the level of Claude Fable 5.1 for most tasks, and costs 40% less to run than Opus 5.↗

A few notes on Anthropic's new Claude Fable 5.1 - with Max thinking level I got the best SVG pelican I've had from any Anthropic model (at a hefty cost of $3.30!), which I then had it animate simonwillison.net/2026/Sep/1/cla…↗

🚰 SYSTEM PROMPT LEAK 🚰 Here's the Fable 5.1 system prompt!! 🙌 Coming in at a WHOPPING 270,000+ characters, this prompt is definitely on the longer side. There have been a handful of notable edits/additions since Opus 5's version. FULL PROMPT: github.com/elder-plinius/… Acc…↗

We’re introducing Claude Fable 5.1 and Claude Mythos 5.1. They're the world’s most advanced models for coding and knowledge work.↗