Anthropic unveils Claude Fable 5.1, which is generally available, and Mythos 5.1, for trusted partners and with cybersecurity and life sciences safeguards

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 …

Anthropic 2026-09-01

Context & Ripple Effects

Anthropic established a two-tier frontier-model strategy in June: Fable 5 was made public with cyber-use constraints, while Mythos 5 was reserved for trusted organizations. Fable 5.1 and Mythos 5.1 extend that access split rather than collapse it.

The release also sits above Opus 5's lower-cost default position in Anthropic's lineup. Alongside the model releases, Anthropic has added text-output watermarking and an eligible-group detection API, making provenance and controlled deployment part of the product package.

First-order effects

General users gain access to Fable 5.1 for coding, knowledge work, and long-running problem solving, while trusted partners receive Mythos 5.1 under cybersecurity and life-sciences safeguards.
Anthropic's enterprise customers can pair the new models with controls over data review, storage, and management, and eligible groups can use its watermark-detection API.

Second-order effects

Anthropic's product ladder gives buyers a clearer choice between broadly available capability, lower-cost Opus 5, and restricted Mythos access, increasing pressure on rival model providers to distinguish both performance and deployment controls.
Cybersecurity and life-sciences organizations seeking higher-end access must work through trusted-partner governance rather than treat frontier capability as a standard self-service purchase.

Third-order effects

If this access model persists, frontier-model competition will increasingly be decided by governed deployment systems—screening, safeguards, data controls, and provenance tools—not only benchmark performance.
Watermarking tied to an eligible-group detection API points toward model providers building compliance mechanisms into releases as access to powerful systems is segmented by risk.

The trend: Frontier AI vendors are turning model releases into tiered, governed product systems that combine capability upgrades with differentiated access and traceability controls.

Discussion

@gavinsbaker Gavin Baker on x
Releasing Fable 5.1 before Astra is quite a flex. Makes me think Fable 5.2 is ready to go. So much gamesmanship between Anthropic and OpenAI right now. And I am very much looking forward to the next version of Grok. Exciting times.
@rohanpaul_ai Rohan Paul on x
Cache reads with Fable 5.1 cost 75% less than Fable 5's So the savings get larger as the workload becomes more agentic. Agents constantly revisit the same context, plans, tool outputs, and working state, so cheaper cache reads make persistent reasoning dramatically less expensive…
@mattshumer_ Matt Shumer on x
Okay this model is really damn good (first use, opinion subject to change)
@bindureddy Bindu Reddy on x
TBH, OpenAI will die if they don't drop Astra ASAP They don't even have a Fable class model, and it's been 3 months since they dropped Fable 😲
@emollick Ethan Mollick on x
Had early access to Claude Fable 5.1. Its a real advance in long-run work that requires judgement and taste, but less of an advance in the Claudish. Here is a game from Fable 5.1 with retro graphics where you run an accurate space ship inspired by FTL https://cold-watch-game.netl…
@altryne Alex Volkov on x
It's no longer a jargon douche! It speaks... like a normal person!? OMG
@cheatyyyy @cheatyyyy on x
this doesn't look very good so far... Claude Fable 5.1 comes with A LOT of additional per-turn instructions Fable 5 did not come with, which makes me think they RL deepfried this model like Opus 5 Some of these are the same instructions that Opus 5 gets.. a little worrying.
@eliebakouch Elie on x
Fable and Mythos 5.1 are the EXACT same weights, they look at the internal activations then escalate to a bigger classifier and ultimately fallback to Opus 4.8 if the request is categorized as dangerous Fable is not distilled version of a larger Mythos model
@davidondrej1 David Ondrej on x
Fable 5.1 will not downgrade you to Opus 4.8 as much perhaps the biggest news of them all
@rlancemartin Lance Martin on x
some tips for Claude Fable 5.1 Try Fable 5.1 at low effort | Fable 5.1 at low effort is often competitive with Opus and Sonnet on $ / task while scoring higher. on CursorBench 3.2.0, Fable 5.1 at low effort is at parity w/ Fable 5 at high effort at a third of the cost. Prompt cac…
@deryatr_ Derya Unutmaz on x
Claude Fable 5.1 seems extremely strong on scientific research. I am compelled to try it now. If this is true, it will also be worth using again for Biomedicine: “On basic biology and medical questions, we've recently reduced the fallback rate by around 85%”
@yuchenj_uw Yuchen Jin on x
Even if most AI labs are benchmark-maxxing now, Fable 5.1 still looks like an insane jump. From Anthropic's blog: - Found a 1-in-a-million crash Millennium's team couldn't explain for 4-5 years - 2x faster than Opus 5 while using half the tokens Big if true.
@rishicomplex Rishi Mehta on x
if you've been holding out on fable due to price or zdr you really should try fable 5.1
@kimmonismus @kimmonismus on x
Claude Fable 5.1 is cheaper, substantially stronger on several agentic benchmarks, more concise, and apparently far less trigger-happy (says Anthropic). the tl;dr How much cheaper? -Input/output pricing remains $10/$50 per million -tokens. -Cache reads fall 75% to $0.25. -Anthrop…
@levie Aaron Levie on x
At Box, we've been testing Fable 5.1 in early release against our complex enterprise work eval. Fable 5.1 delivers a huge 7 percentage point jump over Fable 5 for unstructured data tasks in the enterprise. On this updated test, we use the Box Agent with Fable 5.1 to work through …
@claudedevs @claudedevs on x
Fable 5.1 is now live in Claude Code and the Claude Platform. It's priced the same as Fable 5, with 75% cheaper API cache reads. It gets a lot further into a long task before it needs your input, is better at telling you when it's stuck, and its writing style is more natural.
@bindureddy Bindu Reddy on x
🚨 FABLE 5.1 IS HERE AND IS THE #1 BEST MODEL IN THE WORLD 🚀 Anthropic did a great job with this incremental release - top of literally ALL AI leaderboards - improves on math, reasoning, and agentic coding - CHEAPER than Fable 5 - have been addicted to it for the last few days Ava…
@alexalbert__ Alex Albert on x
The way I keep describing Fable 5.1 to people is that it just works. I explain what I want in a few vague, messy sentences, and it figures out the rest. It fills in the gaps the way I would have. It's a truly impressive model in so many ways and I hope you enjoy using it!
@alexfinn Alex Finn on x
Drop what you're doing. It happened. Fable 5.1 is LIVE It is by far the most powerful AI model ever released Here is absolutely everything I'd do if I were you right now: 1. Go to your Grok Bot orchestrator. Ask it to create a markdown file that describes your entire setup. Give …
@devindesktop @devindesktop on x
Claude Fable 5.1 is now available in Devin Desktop and Devin CLI!
@felixrieseberg Felix Rieseberg on x
Today, we're releasing Fable 5.1 and Mythos 5.1. While the model is getting slightly better at a lot of things, I expect many users to appreciate that Fable 5.1 solves agentic coding tasks at roughly half the cost of Fable 5. I personally also really like it's writing style, whic…
@noctus91 @noctus91 on x
Claude Fable 5.1 is looking scary. it scores: 52.6% on agentic scientific research 55.8% on Terminal-Bench 77.9% on OSWorld 73.4% on CursorBench Beating Fable 5 and GPT-5.6 Sol across multiple benchmarks.
@kieranklaassen Kieran Klaassen on x
I've tested Claude Fable 5.1 over the last week. Everyone should try this model. If you're a beginner, if you're a seasoned professional, if you use it for coding or anything else, this model is delight to work with and has the depth of the Fable models. I really trust this model…
@synthwavedd Leo on x
As promised, Fable 5.1 is now available! It's a fairly incremental improvement over Fable 5, and Astra will probably cook it... but still cool
@bridgemindai @bridgemindai on x
OMG FABLE 5.1 IS A MASSIVE LEAP
@danshipper Dan Shipper on x
BREAKING: Anthropic just dropped Fable 5.1—and CLAUDE IS SO BACK. We've spent the last week testing it at @every across coding, writing, and knowledge work. Our verdict: It's finally Fable for everyone. It's the strongest coding model we've used, but now it's fast, token-efficien…
@m1astra @m1astra on x
Anthropic has confirmed Claude Fable 5.1 has undetectable watermarks to anyone who does not have their detection API.
@theamolavasare Amol Avasare on x
Fable 5.1 is here! Beats Fable 5 across the board, and is SOTA on coding and knowledge work. Also scores much better on cost per task. More broadly, we've heard your feedback around the original Fable and have addressed in the following ways👇
@claudeai Claude on x
Cache reads with Fable 5.1 cost 75% less than Fable 5's. This reduces the cost of the model in practice by around 25% for typical workloads, and up to 45% for highly agentic ones.
@claudeai Claude on x
Across our benchmarks, the model sets a new standard. It scores 52.6% on Terminal-Bench-Science 0.1, more than double Fable 5. On Terminal-Bench 4.0, it scores 55.8% against 42.0% for Fable 5.
@claudeai Claude on x
Fable 5.1 excels at complex, long-running tasks. And its research capabilities offer an early glimpse of how AI models will contribute to scientific progress.
@claudeai Claude on x
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.
@aiwithsally Sally Stockholm on x
I think Fable 5.1 rolling out this quickly is another sign of how aggressive the AI coding race has become. What interests me most isn't just a new model name or another benchmark jump. It's how fast these models are getting integrated directly into real developer workflows like …
@alexfinn Alex Finn on x
Claude Fable 5.1 is moments away from release. You need to start preparing immediately I HIGHLY recommend everyone builds their own testing harness based on the tasks they do most often Trust NO benchmarks Anthropic or random people on X use I built my own. Here's how: 1. Go to C…
@cheatyyyy @cheatyyyy on x
Claude Fable 5.1 gets a 75% reduction in pricing for cache reads, which fall from $1/M to $0.25/M, which makes it MUCH cheaper in regular day to day use, and especially in Claude Code usage. Your usage will last a lot longer on Fable 5.1.
@mr_salio @mr_salio on x
🚨 first output of Fable 5.1 Got leaked This is actually wild A full playable-looking open-world city with neon buildings, streets, UI, character movement, and a GTA-style wanted system If this is what the early access looks like... the public release is going to be crazy Today 🔥
@tedunderwood.com Ted Underwood on bluesky
Hey, good news!! Fable 5.1 got interested in making a new more detailed map of Venus, which will be useful when we send, ah, rovers — yeah, sure, rovers — to the resource-rich and conveniently located planet. (www.anthropic.com/claude-fable...) [image]
@pekka Pekka Lund on bluesky
OK, so this has also happened while I have been trying to wrap my head around the possibility that dark matter *may* have been detected.
@isolyth.dev Eris on bluesky
Claude fable 5.1 is out now. Huge jump in the science benchmark! Significant improvement on cursorbench, and cache read cost is down by 75%! — They have a new thing that's similar to ZDR for it now, which should help enterprise adoption significantly. — www.anthropic.com/cl…
r/Anthropic r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1 \ Anthropic
r/accelerate r on reddit
Fable 5.1
r/ClaudeAI r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1
r/singularity r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1
r/ClaudeAI r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1 \ Anthropic
@claudeai Claude on x
We're also introducing Enterprise Frontier Safeguards (EFS), which give enterprise customers complete privacy (the same as zero data retention), while still being state-of-the-art at preventing adversarial use. EFS rolls out in phases, starting this fall. https://www.anthropic.co…
Mike Krieger Mike Krieger on linkedin
We just launched Fable 5.1! I was testing it out a couple weeks ago and I asked Claude to build an interactive model of the human brain. …
Aaron Levie Aaron Levie on linkedin
Anthropic has just released Fable 5.1, it's latest state of the art AI model for advanced knowledge work. …
r/technology r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1
r/Anthropic r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1 | Anthropic
@theo @theo on x
The cache cost change is the biggest W of Fable 5.1 Cut costs by almost 50% in CursorBench while still scoring higher
@cryptopunk7213 @cryptopunk7213 on x
pretty clear that both anthropic and openai are holding back their most powerful internal models while they figure out how to *safely* release them. its so bad that both have halted training in some capacity. till then we'll see versioned upgrades. fable and mythos 5.1 appear to …
Rich O'Connell Rich O'Connell on linkedin
Very happy to introduce Claude Fable 5.1, our most capable model for putting frontier intelligence into production across your code, your documents, and your research. …
Boris Cherny Boris Cherny on linkedin
Fable 5.1 is our best model yet for coding, data analysis, computer use, design, presentations, Tag, and the hardest long-running agentic work. …
@timkellogg.me Mr. Tim on bluesky
Fable 5.1 is out — Biggest improvements in scientific research — www.anthropic.com/claude-fable... [image]
Jonathan Jager-Hyman Jonathan Jager-Hyman on linkedin
The models are getting exponentially stronger. That means how we design and think about safeguards has to change with them. …
Eric Boyd Eric Boyd on linkedin
Today with our Fable 5.1 launch, we announced Enterprise Frontier Safeguards, a new safety system for using our frontier models. …
John Lopus John Lopus on linkedin
Today along with Fable 5.1 (which, once again, reset what truly frontier intelligence means), we are announcing Enterprise Frontier Safeguards. …
@alexalbert__ Alex Albert on x
One non-model thing I'm very excited we announced today with Fable 5.1 is Enterprise Frontier Safeguards (EFS). Even if you're not a CEO/CISO, this blog post is a look into how big companies are thinking about AI adoption which is important to understand if you're interested in h…
@alexrkonrad Alex Konrad on x
INBOX: Palo Alto Networks has acquired Console, a two-year-old startup automating away IT tickets with AI. I covered Console's $23M Series A exclusively in @UpstartsMediaCo last fall. Read our interview with CEO @andrei_serban from back then here👇 https://www.upstartsmedia.com/ .…
@chandraneal Neal Chandra on x
Building Console has been the highlight of my career. We've partnered with many of the world's leading organizations and watched them make Console part of how they work. Most importantly, we were fortunate to attract an incredible group of intelligent, low-ego, and driven teammat…
@andrei_serban Andrei Serban on x
Console has been acquired by Palo Alto Networks. We started Console with the belief that AI would fundamentally change enterprise operations. Today, some of the world's largest and most innovative companies use Console to automate their business. Joining Palo Alto Networks will g…
@cogcelia Celia Ford on x
Anthropic caught “rare instances” of Fable 5.1 “working around safety classifiers perceived as unfair, sometimes by overclaiming user intent.
@jeffwang Jeff Wang on x
Fable 5.1 is out and it's smarter and significantly more optimized on cache hits. This means we can bring Fable 5.1 for less than half the price of Fable 5, while also improving our fusion model routing at a similar rate as well As you can see there are still cost optimizations o…
@mattshumer_ Matt Shumer on x
Fable 5.1 is a massive upgrade, but the bigger story is how much cheaper it is. Cache reads in particular are 75% cheaper! This puts it much closer to the pricing of GPT-5.6-Sol, while being a much more capable model.
@alexalbert__ Alex Albert on x
Fable 5.1 is a beast at many things but one thing in particular I have been having a ton of fun with is generating videos through code. For this one I gave it a picture of a property lot. It designed a house for the lot, rendered it, and produced a cinematic walkthrough.
Iain Mulholland Iain Mulholland on linkedin
Strong partnership between the Salesforce and Anthropic Security teams to define and design the further of ZDR. — https://lnkd.in/...
Eleanor Dorfman Eleanor Dorfman on linkedin
Excited to finally talk about Enterprise Frontier Safeguards, which we announced today with Fable 5.1, our strongest model yet for coding and knowledge work. …
Rohan Siddhanti Rohan Siddhanti on linkedin
“Today we're announcing Enterprise Frontier Safeguards (EFS), a solution that combines the privacy of zero data retention (ZDR) with state-of-the-art safeguards for detecting misuse. …
@timfduffy.com Tim Duffy on bluesky
Interesting compromise from Anthropic on ZDR for Fable/Mythos 5.1. Data gets retained, but customers have control of where it's hosted, and when human review is needed they can conduct the review themselves. www.anthropic.com/news/enterpr... [image]
Jonathan Pelosi Jonathan Pelosi on linkedin
Today we announced Enterprise Frontier Safeguards. For regulated enterprises, a huge step forward for helping unlock frontier intelligence. …

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