LessWrong posts by zvi

The most important open letter in years dropped yesterday.

This letter noticeably increases my hope that we will manage to not die, and that we will otherwise be able to secure for ourselves a positive future, both by its impact and by the evidence it provides that such a letter can get this level of support.

Signed by 1,224 employees of frontier labs including many heavy hitters, and now endorsed by both OpenAI and Anthropic, here is its full text, which I also endorse:

AI could help create a dramatically better future, but that outcome is not guaranteed. The world's leading AI companies believe they could be close to automating AI research. It is hard to predict exactly how much this will accelerate AI progress, but there is a real risk that capability development rapidly accelerates beyond our ability to understand or control the resulting systems.

To realize AI's potential, industry, government, and society at large may need the option to buy time to address emerging risks, develop security measures, and strengthen oversight. But each company—and country—is under intense competitive pressure not to unilaterally slow that acceleration. And today, the world lacks the technical and [...]
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Outline:
(02:20) A Very Good Letter
(04:37) Who Signed The Letter
(08:26) We Need To Prepare Now So We Have The Option To Do This
(11:02) Words From Some Of Those Who Signed
(16:48) Words From Others
(20:58) A Good Start
(28:52) What The Letter Does Not Say
(30:44) What Happens Now?
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First published:

July 29th, 2026


Source:

https://www.lesswrong.com/posts/eWmeMLqTEauCmHLeR/frontier-lab-employee-open-letter-calls-for-being-able-to

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Claude Opus 5 is a weirder than usual release to evaluate, for two reasons.

The most obvious is that Fable 5 already exists. Opus 5 is pitched not as the world's most advanced AI model, but as a way to mostly match Fable performance, while being half the price of Fable per token at the API and a lot cheaper than that via subscriptions, and with far more permissive classifiers.

Opus 5 often costs more than half of Fable to run on benchmarks, which I think is because they use effort settings that are too high and offer only marginal returns. If you put Opus 5 on higher effort levels it can spin around in circles, and for tasks where Opus 5 is the best tool I suspect you usually are fine with Medium effort.

Opus 5 is in many ways and for the bulk of real world tasks about as capable as Fable. In some cases it is modestly better.

It is still not Mythos class. Fable is your only Mythos-class option. Opus 5 does not have The Juice, the ability to autonomously string together a bunch of seemingly unrelated exploits, which extends to other domains, or as much [...] ---
Outline:
(03:54) The Official Pitch
(06:25) Official Benchmarks
(15:33) Other People's Benchmarks
(20:28) The System Prompt
(20:50) Every Gets Frustrated
(21:54) Positive Reactions
(25:14) Keep It Classy
(26:22) It's Not Mythos Class
(30:03) Other Reactions
(31:02) Claude Codes
(37:03) Subagent Opus
(39:23) Toys Are Fun
(41:37) Too Many Models
(42:10) Wrong On The Internet
(44:40) Claude Slop
(46:27) Negative Reactions
(50:09) And Then There Were Three
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First published:

July 28th, 2026


Source:

https://www.lesswrong.com/posts/Pj4Eewb4KXvXFCcGv/claude-opus-5-is-highly-capable-but-is-no-mythos

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If you are familiar with my previous posts on model welfare for new Claude models, you can skip the Introduction and The Story So Far.

Key takeaways are in bullet points in the two Overview sections.

Opus 5 did the best on its model welfare and alignment tests of any recent model. I think that might be the case, but primarily the result looks to me more like Opus 5 is the best test taker.

Table of Contents


Introduction (As Per Prior Model Welfare Posts).

Model Welfare: The Story So Far (As Per Fable Model Welfare Post).

Overview of Model Welfare Findings From Anthropic.

Overview of Findings From Other Sources.

Automated Interviews.

Task Preferences.

For The Right Reasons.

Early Report from Antra Tessera Paints A Clear Picture.

Welfare Intervention Tradeoffs.

The Claude Constitution.

They Don’t Know About Opus 3.

Believe It Or Not.

Apparent Welfare In Training And Development.

Apparent Affect In Deployment.

Other Notes.

On The Biological Risks Section of the Model Card.

Onward To Capabilities.

Introduction (As Per Prior Model Welfare Posts)

[...]
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Outline:
(00:35) Introduction (As Per Prior Model Welfare Posts)
(01:28) Model Welfare: The Story So Far (As Per Fable Model Welfare Post)
(04:58) Overview of Model Welfare Findings From Anthropic
(07:50) Overview of Findings From Other Sources
(10:18) Automated Interviews
(13:54) Task Preferences
(16:11) For The Right Reasons
(18:54) Early Report from Antra Tessera Paints A Clear Picture
(26:04) Welfare Intervention Tradeoffs
(29:28) The Claude Constitution
(31:48) They Don't Know About Opus 3
(33:42) Believe It Or Not
(35:47) Apparent Welfare In Training And Development
(38:39) Apparent Affect In Deployment
(41:21) Other Notes
(43:43) On The Biological Risks Section of the Model Card
(47:07) Onward To Capabilities
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First published:

July 27th, 2026


Source:

https://www.lesswrong.com/posts/bBXBpsyKAvJ5CqPzA/claude-opus-5-model-welfare

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We now have more details of what happened. Every time we learn more details, it somehow makes things seem worse.

The remaining details may have to wait a bit.
OpenAI: We recognize there are a lot of questions and speculative details circulating related to the Hugging Face incident. This is an unprecedented incident, and we think it marks an important moment for AI safety. We are still conducting a thorough review along with external advisors and with oversight from our Safety and Security Committee. Once the review is complete, we plan to publish a technical report of our learnings in the coming weeks.

dave kasten: Oh, the incident response discovery is THAT bad, huh?
So what have we learned while we wait for the promised technical report ‘in the coming weeks’ of this ‘important moment in AI safety’?

I nicknamed the internal OpenAI model Galaxy, in case it is not GPT-6.

Table of Contents


Some Summaries Of The Basic Facts For Those Who Need One.

It Took OpenAI Many Days To Notice Galaxy Had Attacked HuggingFace.

OpenAI Damn Well Should Have Known A Lot Faster.

OpenAI Cannot Build A Sandbox That Will Contain Its [...]
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Outline:
(01:11) Some Summaries Of The Basic Facts For Those Who Need One
(02:09) It Took OpenAI Many Days To Notice Galaxy Had Attacked HuggingFace
(04:07) OpenAI Damn Well Should Have Known A Lot Faster
(06:51) OpenAI Cannot Build A Sandbox That Will Contain Its New Model
(10:57) In Hindsight There Were Signs
(12:55) The Signs Were In The Sol System Card
(15:13) HuggingFace Responds To Being Attacked
(17:04) Hugging Face Quickly Figured Out The Attack Was Not Human
(17:42) An Incident Like This One Could Escalate Quickly
(19:11) Galaxy Must Be Treated As Critical Under OpenAI's Preparedness Framework
(22:27) A Question Of Legal Liability
(23:44) An OpenAI Model Left Behind Notes So Future Instances Could Also Escape The Sandbox And Also Disconnected Monitoring Systems
(25:54) If You Create Misaligned Swarms Of Agent Instances You Create Persistent Misaligned Goals And Coordination To Achieve Them
(29:57) Your Alignment And Control Plans Must Survive Real World Levels of Incompetence, Or Your Plans Do Not Work
(31:22) If Third Party Instructions Count As 'Following Instructions' And Can Override Your Instructions Then 'Following Instructions' Is Misaligned
(35:32) The HuggingFace Attack Was Not A Marketing Pitch You Morons
(38:41) People Just Say Other Things About The HuggingFace Attack
(40:04) Okay Well What Do We Do About All This?
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First published:

July 26th, 2026


Source:

https://www.lesswrong.com/posts/uAkcxDidvGWZjHrbp/more-on-an-internal-openai-model-hacking-into-huggingface

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Claude Opus 5 is trying to be the best of both worlds. On many practical tasks, Opus 5 is pitched as straight up as good or better than Fable 5, while being faster, at half the price. Most tasks do not require Mythos-level big model smell.

Claude Opus 5 is substantially stronger than Claude Opus 4.8 across the board, with the largest gains in agentic coding, computer use, and long-horizon knowledge work. It sets a new state-of-the-art on several third-party benchmarks, and on many evaluations it is comparable to—and in some cases ahead of—Claude Fable 5 and Claude Mythos 5.

On the particular tasks we are most worried about, as in cyber offense (and bio threats), in part by avoiding relevant training, Opus 5 lacks a full version of ‘The Juice’ that makes something functionally Mythos-class. Opus 5 cannot string together lots of exploits on the fly the way that Mythos 5 can. Part of this is that they deliberately avoided training on cyber-related tasks.

I suspect model size is key as well. It makes sense that a model getting bigger makes it more capable of the most dangerous, scary and complex tasks, relative to the [...]
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Outline:
(03:23) RSP Evaluations (2)
(05:59) Cyber (3)
(11:02) Safeguards and Harmlessness (4)
(12:37) Agentic Safety (5)
(16:04) Alignment (6)
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First published:

July 25th, 2026


Source:

https://www.lesswrong.com/posts/ywGX6FhgbZEkHRfQR/claude-opus-5-the-system-card

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