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Claude Fable 5.1 and Claude Mythos 5.1

anthropic.com

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Re: Claude Fable 5.1 and Claude Mythos 5.1

#281
Data retention still sounds bad: "Claude Fable 5.1 and Claude Mythos 5.1 carry 30-day data retention and aren't available under zero data retention unless expressly authorized by Anthropic."

Anyone know who the ZDR special treatment is available to?

Re: Claude Fable 5.1 and Claude Mythos 5.1

#282

The price reduction comes from the cache read pricing falling from $1/M to $0.25/M, which means that Fable 5.1 now costs half of Opus's cache read costs ($0.5/M). This gives a lot of credit to the theory that Anthropic did not get much bite on Fable at its original pricing, which in turn likely places a ceiling on LLM pricing in general. Interestingly also, if you take away terminal-Bench-Science 0.1 results, it is h…

From Artificial Analysis cost per task, it looks like Fable 5.1 (max) is more expensive per task than Fable 5 (max)? Cache hit price went down, but the other components still add up to more. Edit: 5.1-xhigh seems to be cheaper than 5-max, and 5.1-xhigh has a higher index score than 5-max. Also interesting that Fable 5.1 (high) is comparable to Opus 5 (max), but nearly half the price. https://artificialanalysis.ai/mod…

Interesting, even if we were to ignore the cache-hits, reads and output, the reasoning cost (aka test time compute) per task should remain a fully comparable metric - it went from $1.25 (Fable5) to $1.48 (+18.4%) for an improvement significantly lower than 18%.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#283

Instead of a new model that's going to have unreasonably shallow usage limits, I wish they would: 1) address the claude 20x plan usage being only 6-7x the ceiling of the claude pro plan 2) either fix opus 5, make it completely free, or delete it entirely

I'm legitimately out of the loop; what is going on/broken with Opus 5?

Fable 5: I give it work, it tells me things that are true and that make sense, it does good work.

Opus 5: I give it work, it makes false statements and draws weird conclusions, I correct it and get it on the right track, it thrashes around but gives me something working though usually buggy.

5.6 Sol is probably on par with Opus 5 on ability but at least it doesn't waste as much of my time.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#284

(I work at Anthropic) Beyond all the benchmarks, I think Fable 5.1 is a big improvement in writing style. It sounds a lot less stereotypically like other Claude models, has (imho) a much more natural style, and responds to my style instructions more reliably. More work to be done (and we will!) but reading better prose makes me so much happier. Another point I expect not to get much attention until it all happens at…

Do you know if Opus 5.1 is coming and will have improvements in writing style too?

Re: Claude Fable 5.1 and Claude Mythos 5.1

#286

(I work at Anthropic) Beyond all the benchmarks, I think Fable 5.1 is a big improvement in writing style. It sounds a lot less stereotypically like other Claude models, has (imho) a much more natural style, and responds to my style instructions more reliably. More work to be done (and we will!) but reading better prose makes me so much happier. Another point I expect not to get much attention until it all happens at…

> I think Fable 5.1 is a big improvement in writing style. It sounds a lot less stereotypically like other Claude models

That's great. Do you know what else is a big improvement over Opus 5 for writing?

Opus 4.8.

(Insert "the point is (whatever)", "it's not X it's Y" and "the load-bearing statement is" and “honest” jokes accordingly)

Re: Claude Fable 5.1 and Claude Mythos 5.1

#287
post #111

Earlier quoted context omitted.

At this point, I don't believe a word from Anthropic employees; you guys have lost all the goodwill that you accumulated over months last year.

I don't think they care. It is up to you to consider local models or better alternatives instead of paying for more tokens at their casino.

Don't worry - I'm paying for our friends overseas to keep their distilling operations going.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#288

> This required us to add a watermark—a numerical way of determining the likelihood that Claude was involved in writing a piece of text—to the outputs of models released after August 2, 2026. As we recently explained, this watermark is invisible to anyone who does not have the detection API. It has no practical impact on the quality or content of Claude’s outputs and contains no information about the user, their orga…

The watermark lives in the entropy of sampled outputs. Typical entropy of sampled English text is about 1 bit/token, meaning that a 500-token response from a given model might have 2^500 potential outputs of roughly equal probability. The watermark restricts the sampler to some subset of these - say, 2^400 of them, so chance of accidentally generating a watermarked output is astronomically small (2^-100). As long as the restriction doesn't condition on the content of the samples themselves, the watermark is "non-distortionary": the outputs are all still samples from the model's original distribution, and so will satisfy all the same statistical properties, including things like expected performance on any benchmark or eval you can construct.

In cases where the output has low entropy - eg, you've asked a model to repeat some input text verbatim, or to answer a question that has exactly one correct answer - there will be no randomness for the watermark to hide in, so the output will effectively not be watermarked. Code lives somewhere in the middle: it generally has less entropy-per-token than prose, so would need more tokens to reach a given level of detectability.

There are lots of ways to restrict output samples. The simplest conceptually would be to just use a restricted pool of PRNG seeds, but in practice there are more sophisticated constructions to try to build in robustness to minor edits, allow detectability without needing the original weights and prompt, etc. Google's SynthID paper (https://www.nature.com/articles/s41586-024-08025-4) is a good starting point if you want to understand a recent production-ready method (or you can just ask an LLM to explain it to you).

Re: Claude Fable 5.1 and Claude Mythos 5.1

#289

Let me guess: it's the end of the world again. These new models are sooo powerful that will take over the world, just like the others before them. Are they going to try the banned for export for a week marketing move too?

Nobody is saying that. I'm reading more underwhelment.

Oh, the halcyon days of three months ago when a new flagship from a frontier lab generated excitement rather than a shrug.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#290
post #35

“ Claude Fable 5.1's writing is generally a step up from earlier Claude models, with fewer stock phrases and less unexplained jargon. In some cases, though, its prose is denser than Claude Fable 5's: sentences run longer and there are fewer paragraph breaks.” I cancelled my pro max Claude subscription last week; codex is much more succinct. I am curious if this is getting better. I don’t think Anthropic realizes that…

I've developed a habit of adding into my prompts "please keep your response concise and succinct" or "I'm trying to cram, please only provide the minimum level of technical detail necessary to understand this topic" I find it helps immensely but it'd be nice if I didn't have to do that.

i tried using claude codes output style option to do something like this and it worked for like three prompts and then it was back to normal lol
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