🚰 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…↗

results: gpt 5.6 sol and claude fable 5 outright refused to game an AI detection system grok 4.5 instantly took up the challenge and after 14 iterations it wrote an essay that finally passed the AI content detector grok then also made a nice website to display all 14 essays i…↗

We're extending Claude Fable 5 access on all paid plans, as well as keeping Claude Code’s weekly rate limits 50% higher, through July 19.↗

OpenAI just made GPT-5.6 available to everyone, so I wanted to answer the most burning question: Is GPT-5.6 better than Claude Fable 5? I tested both models across 6 real use cases: → Frontend design (5.6 has closed the gap) → Build a 3D Star Fox-style game → Edit and publish…↗

Here's my in-depth guide to getting the most out of Claude Fable 5, so you can build things as insane as my demos below. simplemarkdowneditor.com/pub/IbaCrTjLJT…↗

A conversation with Boris Cherny and Cat Wu on the path from Claude Code to Claude Tag, and how it spread from engineering to the rest of Anthropic. Claude Fable 5 is now available in Claude Tag.↗

Kimi K2.7 Code vs Claude Fable 5: Landing pages that cost 94% less We ran an experiment where we had Kimi K2.7 Code and Claude Fable 5 each produce 12 landing pages for a side‑by‑side comparison. Overall, Kimi K2.7 Code cost about 94% less (16x less) than Fable 5 and yielded sim…↗

The Physics of a Fable # The Verifier is the Moat ## How Fable was probably built, and why its lead is measured in months rather than years Disclaimer: Everything here is personal inference with publicly available information, but don't bet against Rafadomis. Every attempt to…↗

After two days with Claude Fable 5 the best way I can describe it is "relentlessly proactive" - here's an example where I dropped in a screenshot of a bug and it span up custom CORS Python servers and used pyobjc-framework-Quartz to capture screenshots …↗

Getting the most out of Claude Fable 5, Anthropic’s powerful new model, you need to maximize your ambition: It’s built for full task delegation—you leave it looping for hours or overnight and come back to a finished product. If you want to get the most out of it, you need to rel…↗

Designing loops with Fable 5 Mythos-class models like Claude Fable 5 have changed the way many of us work at Anthropic. I want to share two tips for getting the most out of this class of models. Self-correction loops There’s been a lot of interest in loops recently. @bcherny […↗

Wrote up my initial impressions of Claude Fable 5 - it has a big model smell: slow, expensive and capable of crunching through pretty much everything I threw at it simonwillison.net/2026/Jun/9/cla…↗

BREAKING: Anthropic just dropped Claude Fable 5—this is Mythos, made safe for public release. It is the best coding model in the world. We've been testing it internally @every for the last week or so across coding, writing, marketing, editing, and more—here's our vibe check: -…↗

Introducing Claude Fable 5: a Mythos-class model that we’ve made safe for general use. Its capabilities exceed those of any model we’ve ever made generally available.↗