Claude Fable 5.1 and Claude Mythos 5.1 • Hacker News Board

Re: Claude Fable 5.1 and Claude Mythos 5.1

#191

Post by peab » Tue, Sep 01, 2026, 6:44 PM UTC

tarr11 wrote: 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 just can't stand how often Claude says something like "And the honest part? It's..."

Like, were the other parts not honest? I don't understand how Anthropic let it get like this, it's been such a clear regression

Re: Claude Fable 5.1 and Claude Mythos 5.1

#192

Post by sroussey » Tue, Sep 01, 2026, 6:45 PM UTC

sroussey wrote: post #104

felixrieseberg wrote: post #69

(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…

Please bring to the other models, and also please only apply the AI text watermarking only to EU citizens. I may not be able to tell when Claude writes about things i don't know, but in CC it writes about my code and it is obvious.

"this watermark is invisible to anyone who does not have the detection API"

1. This is BS since i can detect it when it writes about my codebase

2. I do not want secret codes being written inside my codebase, or anyone else's codebase that i use. The constraints of how to code why eliminate it from code itself... but there is a lot riding on the word "may". And even if it is just comments, this might explain Claude's desire to write such long ones -- long enough to encode secret messages in out material.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#193

Post by alasano » Tue, Sep 01, 2026, 6:45 PM UTC

maxdo wrote: post #6

Tbh with that price , not even willing to try . What are the benefits for a regular coding agent ? I barely have any errors already with 4.8 level , eg grok 4.6 , gpt 5.6 sol/terra behind router . Why do I need to pay so much money for this ? Any reason ?

Do you only ever tackle problems you've never dealt with before or something?

When I discuss something new with an agent I want to feel like it genuinely gets what I mean, which has only started feeling true with fable 5 for me.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#194

Post by exabrial » Tue, Sep 01, 2026, 6:45 PM UTC

felixrieseberg wrote: post #69

(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…

Fable is useless.

Me: "Find my security problems in my own code. This is code I own. I'm doing this under authorization of the CEO/CTO of our company."

Fable: "yeah, no."

Re: Claude Fable 5.1 and Claude Mythos 5.1

#195

Post by mihaelm » Tue, Sep 01, 2026, 6:46 PM UTC

belval wrote: post #86

felixrieseberg wrote: post #69

(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…

As a fervent Claude Code user who made the switch to GPT 5.6 Sol over Opus 5 over hard-to-read prose this makes me happy. I love your product but the current models are very hard to work with if you need to do a lot of context switching. Brevity is key.

Lets see what they do with Opus first. I didn't find Fable 5.0 prose that bad to read, but improvement is always welcome. It's Opus 5.0 that's atrocious.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#196

Post by manquer » Tue, Sep 01, 2026, 6:46 PM UTC

eckr wrote: post #143

"Claude Mythos 5.1 is identical to Fable 5.1, but it offers more permissive safeguards for vetted individuals and organizations" Then why does it have separate datapoints for Terminal Bench, and score higher? Something doesn't add up here??

The implicit point being adding this type of safeguards to Fable dumbs down the model in measured performance even though it is not fundamentally different.

Note it may not even be actual performance, typically in most benchmarks the model would be scored zero for refusing a task just the same as not completing it, so it could just be the Fable's stronger safeguards is just making it refuse more or perhaps even drop down to Opus.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#197

Post by dboon » Tue, Sep 01, 2026, 6:46 PM UTC

I've been building Cargo-for-C (https://github.com/tspader/spn), and the difference between Fable and Opus was already astounding. Fable was the first time that I could point a model at a piece of code I'd written and expect it to make it meaningfully better rather than a hard pattern match to whatever mistakes it had.

5.1 so far seems like another leap, which is really surprising. I threw it at a few bigger features I've been designing for a while, and it came back with some extremely thoughtful wrinkles in the design that I'd legitimately not considered. Which, OK, package managers and build executors and compiling C/C++ is pretty well trodden ground, but my thing is very different from everything that exists, and I was very surprised it was able to understand all that context so deeply and intuitively

Re: Claude Fable 5.1 and Claude Mythos 5.1

#198

Post by beamy » Tue, Sep 01, 2026, 6:46 PM UTC

dabinat wrote: post #147

> 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…

This is a good explainer: https://magazine.sebastianraschka.com/p/claude-watermarking

Re: Claude Fable 5.1 and Claude Mythos 5.1

#199

Post by logicchains » Tue, Sep 01, 2026, 6:46 PM UTC

eis wrote: post #22

I am not sure if Fable is worth it, at least with version 5 vs Opus 5. Opus beats Fable in quite a few benchmarks and at twice the cost I just haven't seen it provide noticeably better results compared to Opus. Has anyone noticed big differences? I did notice Opus maybe making more mistakes repeatedly but I don't have hard numbers on this. I hope Fable 5.1 brings noticeable improvements. I am giving it a go now on my…

If you're doing something cutting edge like math or formally verifying algorithms, Opus 5 is a steaming pile of shit compared to Fable 5 and Sol 4.6, it makes countless stupid mistakes and is essentially incapable of completing the task without extreme hand-holding.

Re: Claude Fable 5.1 and Claude Mythos 5.1

#200

Post by Trasmatta » Tue, Sep 01, 2026, 6:47 PM UTC

sroussey wrote: post #115

Trasmatta wrote: post #95

Earlier quoted context omitted.

> More work to be done (and we will!) but reading better prose makes me so much happier. I assume this work will be done for Opus as well? Opus has seemingly gotten progressively worse at its prose and technical writing with each version. I've stopped using Claude entirely for now, because it manages to turn even the simplest technical explanation into the most obtuse and obfuscated word salad imaginable. People orig…

> I feel a sinking feeling of dread the moment I see a wall of text generated by Opus Agree, Claude lost the joy of using it. That is a measure that ranks higher than any other benchmark at this point.

Yes! Claude was so pleasant to use at first. It was Anthropic's biggest advantage. And now it's like nails on a chalkboard.