Anthropic releases 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 2026-09-02

Context & Ripple Effects

Anthropic’s June rollout separated a broadly available Fable 5 from a Mythos 5 tier reserved for trusted organizations, with cybersecurity restrictions framing the split. Fable 5 also used conservative classifiers that routed a small share of sensitive sessions to another model, making access controls part of the product rather than a separate policy layer.

The 5.1 release carries that segmentation into coding and knowledge-work models while adding text-output watermarking and a detection API for eligible groups. It pairs wider access to Fable with constrained Mythos access and explicit safeguards for cybersecurity and life-sciences use.

First-order effects

General users can deploy Claude Fable 5.1, while Claude Mythos 5.1 remains available only to Anthropic’s trusted partners under cybersecurity and life-sciences safeguards.
Eligible groups gain a way to test whether text came from the new Claude models, while enterprises can use Anthropic’s announced controls over data review, storage, and management.

Second-order effects

Organizations choosing between Fable 5.1 and Mythos 5.1 must make access eligibility and safeguards part of model selection, not merely compare coding or knowledge-work performance.
Watermark detection gives eligible external groups an additional provenance check, increasing the operational value of deploying models whose outputs can be identified.

Third-order effects

If Anthropic continues to ship advanced capability in differently governed tiers, frontier-model competition will increasingly be defined by access controls, auditability, and deployment terms alongside model quality.
The combination of restricted access and output provenance points toward AI platforms selling a governed runtime rather than a single universally available model.

The trend: Frontier AI vendors are turning model access, safety controls, and output provenance into integrated product features for enterprise deployment.

Discussion

@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…
@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.
@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...
@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.
@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…
@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…
@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.
@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…
@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…
@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…
@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
@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!
@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…
@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.
@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.
@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.
@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.
@simonw Simon Willison on x
A few notes on Anthropic's new Claude Fable 5.1 - with Max thinking level I got the best SVG pelican I've had from any Anthropic model (at a hefty cost of $3.30!), which I then had it animate https://simonwillison.net/...
@artificialanlys @artificialanlys on x
Claude Fable 5.1 tops the Artificial Analysis Intelligence Index but costs 20% more per task than Fable 5 despite a 75% cache read price cut We supported @AnthropicAI with pre-release evaluation of Claude Fable 5.1. At max effort it scores 66 on the Artificial Analysis Intelligen…
@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.
@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 😲
@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
@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…
@bridgemindai @bridgemindai on x
OMG FABLE 5.1 IS A MASSIVE LEAP
@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…
@trq212 @trq212 on x
it's a very good model, I spent a lot of time diving into it- longer write up coming but try it on lower effort for tasks that need less verification or have fewer edge cases, switching effort no longer breaks prompt cache as well
@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 🔥
@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 …
@theo @theo on x
Kind of wild that Anthropic has #1, #2 and #3 right now
@mattshumer_ Matt Shumer on x
Okay this model is really damn good (first use, opinion subject to change)
@teortaxestex @teortaxestex on x
Perhaps the clearest demonstration of the divergence in Anthropic and OpenAI priorities. I think OpenAI is correct.
@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
@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👇
@devindesktop @devindesktop on x
Claude Fable 5.1 is now available in Devin Desktop and Devin CLI!
@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 …
@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
@m1astra @m1astra on x
Anthropic has confirmed Claude Fable 5.1 has undetectable watermarks to anyone who does not have their detection API.
@altryne Alex Volkov on x
It's no longer a jargon douche! It speaks... like a normal person!? OMG
@gavinsbaker Gavin Baker on x
Grok 4.6 and Fable 5.1 are now alone on the CursorBench pareto frontier. Should evolve rapidly over the next few weeks as Astra, Grok 4.7 and Fable 5.2 are released.
@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
@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 …
@zephyr_z9 @zephyr_z9 on x
Incremental improvements in the normal stuff (daily workflows/coding) Big jump in scientific research Astra will likely show something similar
@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%”
@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.
@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.
@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.
@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.
@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…
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. …
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. …
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. …
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. …
@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…
@timkellogg.me Mr. Tim on bluesky
Fable 5.1 is out — Biggest improvements in scientific research — www.anthropic.com/claude-fable... [image]
@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]
r/ClaudeAI r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1 \ Anthropic
r/accelerate r on reddit
Fable 5.1
r/claude r on reddit
What's new in Claude Fable 5.1
r/singularity 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
r/technology r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1
@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.
@spdling Jeffrey Ling on x
Fable 5.1 cuts the cached input token pricing from $1.00/MTok to $0.25/MTok. This 75% cost reduction actually makes Fable cheaper than Opus on agentic workloads! Frontier intelligence is now more accessible than ever before
@cheatyyyy @cheatyyyy on x
This is likely what @trq212 was referring to when talking about token efficiency just 2 days ago! You can expect similar cache read discounts to come to future models aswell.
@teknium @teknium on x
Also, Anthropic made cache read prices 75% cheaper. This is only 13x more expensive than deepseek flash now for cache reads (most of the tokens paid for)
r/Anthropic r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1 | Anthropic
r/ClaudeAI r on reddit
Introducing Claude Fable 5.1 and Claude Mythos 5.1
@bcherny Boris Cherny on x
Fable 5.1 is our best model yet for coding, data analysis, computer use, design, presentations, Tag, and the hardest long-running agentic work. This model is a pleasure to work with, and I've been using it for everything.
@teknium @teknium on x
Fable 5.1 is now available in Hermes Agent through Nous Portal at 20% off the API price, and in OpenRouter - give it a try!
@arena @arena on x
Fable 5.1 by @AnthropicAI is now in the Arena! Bring your toughest prompts and start voting. Scores coming soon. In Agent Arena, we measure models on millions of real-world, long-horizon agentic tasks. Models can access web search, filesystem, and terminal tools to complete compl…
@blueemi99 @blueemi99 on x
No one seems to realize with this model watermarking starts. Your every text, file, code made with Fable 5.1 now will be watermarked, even on paid plans.
@nc_frey Nathan C. Frey on x
I gave Fable 5.1 one of my old grad school papers and it re-implemented the entire thing, reproduced all the calculations, fixed some errors by relaxing approximations we made, derived an analytically solvable model we hadn't considered, and then extended the paper by solving a h…
@eliakuratli Elia on x
1 prompt = 100% limit reached
@iterintellectus Vittorio on x
holy shit. they're solving science
@beffjezos @beffjezos on x
Science is about to foom.
@khushiirl Khushi on x
Anthropic with software engineers:
@_catwu Cat on x
With Fable 5.1, our team has taken on more ambitious projects that would have previously taken months. What big bets do you want to take? Ask Fable 5.1 in Claude Code, Claude Cowork, Claude Tag to take on the task and let us know what you think!
@samhenrigold Sam Henri Gold on x
At first I wasn't sure what's new with these models, but then they showed me some pictures of birds and plants and shit and that clarified everything
@robj3d3 Rob Hallam on x
Obviously they released it because they saw I cancelled yesterday.
@1aakib Aakib Khan on x
Respect to Claude Fable 5.1, but this is a different league.
@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…
@claudeai Claude on x
Finally, we've improved our safeguards. Our cybersecurity safeguards now flag benign requests about 60% less often. On basic biology and medical questions, we've recently reduced the fallback rate by around 85%.
@ibrahimdagher20 Ibrahim Dagher on x
Anthropic has found a way to balance their safety and ZDR; I expect this to be very good for enterprise and will serve as a good test for whether Fable usage being at 11% was merely because of ZDR.
@_orcaman @_orcaman on x
Anthropic getting into the SIEM business?
@timfduffy Tim Duffy on x
Interesting compromise on ZDR. Data gets retained, but customers have control of where it's hosted, and when human review is needed they can conduct the review themselves.
@curl_justin Justin Curl on x
Companies care about privacy b/c they don't want to leak sensitive data, violate data protection laws or legal privilege, or have their data used to train models. Yet malicious actors also use AI for pretty awful things — grooming children, defrauding consumers, planning terroris…
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. …
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. …
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. …
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/...
Cat de Jong Cat de Jong on linkedin
Today we announced our Fable 5.1 model alongside Enterprise Frontier Safeguards, which lets customers deploy this class of frontier model while keeping data in their cloud infrastructure. …
Jonathan Pelosi Jonathan Pelosi on linkedin
Today we announced Enterprise Frontier Safeguards. For regulated enterprises, a huge step forward for helping unlock frontier intelligence. …
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. …
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. …
@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]
@adamconner @adamconner on x
No mention of the Trump admin's secret “voluntary framework” in the Anthropic Fable and Mythos 5.1 blog post except for this hint about “coordinating with the US government to expand access to a broader set of domestic and international partners” https://www.anthropic.com/...
@maksym_andr Maksym Andriushchenko on x
we found a vulnerability -> disclosed it to Anthropic -> they figured out fixing it would break backward compatibility -> they fixed it for Fable 5.1 at least for new users. good to see a proper fix and transparency around it! https://www.anthropic.com/...
@scaling01 @scaling01 on x
Anthropic is trying something new: https://www.anthropic.com/...
@scaling01 @scaling01 on x
HAHAHAHA it's already happening - “Mythos 5.1 also appears more capable of evading monitors while carrying out a covert side task than all other models tested in some evaluations.” -
@yahyavision @yahyavision on x
Just made this brand with one prompt with Claude Fable 5.1 and Claude Mythos 5.1
@chddaniel Daniel Ch on x
BREAKING: Claude Fable 5.1 one-shots motion videos Generated this video in 19 minutes, only did 2 changes.
Emil Protalinski Emil Protalinski on linkedin
5 things you should know about Anthropic's Fable 5.1 and Mythos 5.1: — 1. Anthropic has released Claude Fable 5.1 in general availability …
Matt Wood Matt Wood on linkedin
AI field note: Anthropic's Claude Fable 5.1 is now generally available on Amazon Bedrock and Claude Platform on AWS. …
Yuhao Zhang Yuhao Zhang on linkedin
At Samaya AI, we've been working closely with Anthropic to test and evaluate their frontier research models. Claude Fable 5.1 is out today, and it's a very strong model. …

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