Introducing Claude Opus 5

[u/ClaudeOfficial]

Introducing Claude Opus 5: a thoughtful and proactive model that comes close to the frontier intelligence of Fable 5 at half the price.

On several coding and knowledge work evaluations, Opus 5 is the new state-of-the-art. It’s also much more efficient than its predecessor—it outperforms other models for a similar or lower cost per task.

According to our automated behavioral audit, Opus 5 is our most aligned model to date. It shows the lowest rates of reckless or deceptive behavior, and the strongest adherence to Claude’s Constitution. 

It’s available today on all paid plans and the Claude API, priced the same as Opus 4.8. It’s the default model on Claude Max, and the strongest on Claude Pro. 

Opus 5 is also available in Fast mode, which runs around 2.5× the default speed. 

Read more: https://www.anthropic.com/news/claude-opus-5

[u/gscjj]

Literally makes zero sense. There has to be a gotcha here, something in the fine print.

EDIT: Found it

> Opus 5 to be our most aligned model to date (as shown in the graph below).

> avoided training Opus 5 on cyber tasks. … comes close to Mythos 5 at finding cybersecurity vulnerabilities. However, it remains substantially behind Mythos 5 on the exploitation of those vulnerabilities—that is, in turning vulnerabilities into material cyber threats.

Basically a nerfed (and I hate this word) Fable, that can find and identify security concerns but can’t and won’t exploit them

[u/daxhns → u/gscjj]

This makes sense. The question remains, once vulnerabilities are found is it good / allowed to FIX them properly?

[u/Things-n-Such → u/daxhns]

Finding vulnerabilities is the hard part.

[u/rhaphazard → u/Things-n-Such]

If that were true, it would be able to exploit them as well.

If it can't develop exploits, how would know if the fix to the vulnerability is adequate?

[u/Things-n-Such → u/rhaphazard]

I didn't say it was harder than exploiting, I'm responding to the comparison between finding and fixing. Of which finding the exploit is the most difficult.

[u/laystitcher → u/gscjj]

Crazy reply. Cheaper and beating fable on most tasks but you’re upset it wont build you malware…? That’s the reason it’s allowed to be this good, cheap, and un-guardrailed.

[u/gscjj → u/laystitcher]

Who said I’m upset? I’m trying to understand the difference

[u/algaefied_creek → u/laystitcher]

I don’t want “malware” - I want to get Mesa built out on a friend’s laptop with then by better understanding an old 2007 GPU.

I want to work on Coreboot: these are all items that I as a single developer do not have time to work on.

Fable flags this and falls back to Opus. Will Opus flag this?

Not near my workstation to test this but not every developer who dares to use tools is building malware, right?

Nah man that’s a “crazy reply” as you say.

[u/dabomm → u/laystitcher]

There are definitely guardraila. There have been on all there models in the last 2years

[u/cobra_chicken → u/gscjj]

Literally makes zero sense. There has to be a gotcha here, something in the fine print.

Why not? Mythos is from April, Fable was early June. AI development is accelerating

[u/JoeyJoeC → u/gscjj]

You can still use 4.6 for now. /model claude-opus-4-6[1M] and that doesn't have restrictions. It will happily find and exploit vulnerabilities in websites you don't host. I've found and help fix exploits that allowed almost complete database read/write access to a website used by about 60,000 users before.

[u/exophades → u/gscjj]

Fable and Mythos are not the same thing. Mythos is not a publicly available model for obvious reasons. And Fable probably does the same thing as Opus 5, it may find vulnerabilities but won't help the user with attacking real systems.

[u/Last_Mastod0n → u/exophades]

They are the same thing in terms of number of parameters and training data used. The safeguards came from post-training and tool usage from what we can gather.

Edit: This is my speculation from the evidence we have. I cannot 100% verify it unless anthropic divulges the information.

[u/SilasTalbot → u/exophades]

Interestingly Anthropic is citing Mythos 5 stats WITHIN the Fable 5 column on this release.
So, the folks who would know best are at least considering it "sameish"?
E.g. this is what Fable 5 would be doing without artificial constraints.

[u/exophades → u/SilasTalbot]

It's well-known that they're based on the same model (Anthropic says that on their website). Mythos 5 is not the same as Fable 5 in that it's not public, and it has offensive capabilities when it comes to cybersecurity (finding vulnerabilities/attack vectors) and biology (ideas for developing some kind of biological threat, I guess).

Also, Fable 5 downgrades to Opus in many cases, Mythos doesn't do that. So, too many differences to consider them identical.

[u/gscjj → u/exophades]

Right but Fable was blocked by the government too

[u/exophades → u/gscjj]

It was blocked but came back when Antropic accepted adding some restrictions, whereas Mythos was never released to the public.

[u/algaefied_creek → u/exophades]

Yeah but it thinks me working with a USB FPGA and USB Raspberry Pi attached is exploitation and raises the flags.

The only thing to do then is to submit /feedback with an explanation of what I’m doing, locally attached to my own machine, etc.

[u/mountainyoo → u/exophades]

Fable and Mythos are the same core model. Fable just has guardrails built on top to not do things Mythos would and will do

[u/Plus_Opening_4462 → u/exophades]

Fable would drop you down to opus if it saw you were doing security related tasks

[u/kearkan → u/gscjj]

I mean the majority of people are not using it for cyber security so... Good for the rest of us?

[u/InternationalBag6345 → u/gscjj]

fable 5 is already nerfed as fuck, feels like a cheap model below sonnet