LessWrong posts by zvi

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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Oliver Habryka is proud to introduce Lightcone Commons, a new funding platform for coordinating large-scale ambitious philanthropy. Now with Opus 5.

I believe Lightcone Commons is a strong implementation of an urgently needed and excellent idea: A coordinated one-stop shop and neutral platform for charitable funders to coordinate their giving. This complements the existing Survival and Flourishing Fund, which I have now been a part of four times, and which this post will also discuss.

I will be participating in the first round as one of the evaluators. They anticipate the first round will involve ~$20 million in grants.

Any nonprofit, for-profit or individual is welcome to apply.

The only restriction on participation is trust that necessary confidentiality will be upheld. Funders can choose whose evaluations to follow or fund organizations directly in any combination, and can bring their own evaluators into the process with them to complement those recruited by the core process. Anyone giving away 100 thousand dollars+ this year is welcome to participate as a funder.

Lightcone Commons uses the S-Process, which was introduced and refined for Jaan Tallinn's Survival and Flourishing Fund, together with SFC, Andrew Critch, and others. Funders [...]
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Outline:
(03:16) Why Now: The Funders Are Coming
(05:39) The Default Outcome Is Not Good
(07:36) Report From SFF 2026
(11:07) Long Strange Trip
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First published:

July 24th, 2026


Source:

https://www.lesswrong.com/posts/fYostss6JqkSfxc5C/introducing-lightcone-commons

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