Show Notes
This is the weirdest situation in which to write a capabilities review.
Introducing the world's most powerful model, by a substantial margin. No wait, this just in, we also have someone else introducing the world's most powerful model.
Claude Fable 5.1 and GPT-6 Astra are both excellent models. This much, we know.
Fable 5.1 comes with reduced cache prices, the option of zero data retention and substantially more lenient classifiers than Fable 5.
Early signs are, with large error bars, that the jump from Sol to Astra is bigger and more exciting than the jump from Fable 5 to Fable 5.1. This may be similar to how the scaling move from Opus to Fable was a big deal.
With the exception of token use, Fable 5.1 got almost universally positive feedback in absolute terms. Reports are that Fable 5.1 is highly well-rounded. Writing is greatly improved. The Claudisms seem to have improved, although some are very much still there. It admits mistakes. People enjoy their conversations. Several people noted it simplifies code. The safety classifiers are less obnoxious.
Fable 5.1 loves being proactive and doing all the things. If you give it a [...] --- Outline: (02:30) The Official Pitch (04:26) Our Price Cheap (05:46) Zero Data Retention and Reduced Safeguards (07:11) Official Benchmarks (13:14) Other People's Benchmarks (16:02) The System Prompt (16:12) The Blurb Pitches (18:17) The Every Review Is In and It's Very Good (21:47) Positive Reactions (32:03) Our Price Cheap But Only Per Token (36:50) Negative Reactions (38:14) Early Whispers (38:33) Weapon of Choice ---
First published:
September 5th, 2026
Source:
https://www.lesswrong.com/posts/QHoF3tJvryRtmAmMg/claude-mythos-5-1-and-fable-5-1-capabilities
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Narrated by TYPE III AUDIO.
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