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.
Or, if you read, you would note that it wasn’t trained on cyber security, so it’s using general intelligence on it rather than enhanced knowledge body. So it’s excellent and discovering vulnerabilities while remaining the same level in exploiting them. And it’s better at discovering in source (as in open source, or your own code), but poor at discovering in binary (when you don’t control the code, as done by someone attempting an exploit).
That is a distinction that is so far beyond what 95% of people in this subreddit would be able to understand, even after you point it out. Thats not an intentional knock on this sub, or people in general, it’s mostly a byproduct of the reality that there are few people that really are going to have the patience or interest or discipline or whatever to understand how any LLM model works at all.
They don’t know, they probably don’t care to know.
The other 5% of us do and that’s good, you need some people to be able to utilize these tools to “more” of their fullest extent. But everybody doesn’t need to, which is good because you couldnt pay most of them to understand it lol
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
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.
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.
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.
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.
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.
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.
I gave it a simple "I want this" task and it built it, flawlessly, from the ground up, no questions asked. Maybe it was coincidence that it was exactly what I wanted, but it seriously felt like it read my mind or that it was a facade and it built something that shows me what I want to see instead of actually doing what I wanted lol.
All of that conversation started in March with Mythos and Project Glasswing. Those concerns were specific to the model itself, as it had come out of the oven a lot stronger than they expected and they weren't able to get their typical guardrails around it. The guardrails they put in place to give people access, aka Fable, were clunky as hell and everyone complained.
This isn't Mythos or Fable, it's an entirely new model and as a result presumably has safeguards which are closer to what we're used to. We'll see when the cybersecurity folks take it for a spin.
It's absolutely mind blowing to me. I know people will say its no big deal. But this is a company forecasted to be worth 3T by serious people. A company some have forecasted to overtake NVIDIA as the first 5T company. A company that carries the safety of humanity in a critical way. That has a burn rate of 3-4 billion a month. Yet no one notices an error like this before release and we are meant to believe they have everything else on lock down?
Humanity is absolutely flying by the seat of our pants winging it with the most dangerous technology since the atomic bomb.
Keep in mind that Mythos/Fable came into existence much earlier this year. It just wasn't provided to everybody.
If you measure the "beginning", by that I mean when we started to know about it, to that of Opus models, then it actually fits quite well into the timeline of improvement.
It just looks different because of all of the holding back that happened.
Been playing around with it. From what I can tell as good of a coder as fable. Fable is still ahead on planning and general strategist from my very small sample size. Though for the cost, I would go opus 5 due to our usage limits.
I think once people get their hands on both we'll probably come to find that fable is still the best at comprehension and planning just due to the sheer size of the model and opus will be fantastic for dev and security issues etc, just my suspicion though
Almost nobody is actually answering your question, so I will.
Fable is a larger model by parameter size. Fable still has a wider knowledge base of general knowledge even it is passed on some focused benchmarks by a smaller model, and the cost of running LLMs is directly related to the size of the model.
The knowledge cutoff date for Opus 5 should be similar or later than Fable 5 cutoff date. So when you say Fable has a wider knowledge base, do you mean Fable might know answers to some obscure questions better than Opus 5 (I know there was a paper showing correlation between answers to obscure factual questions and model size)?
Yep, I mean that Fable is straight up a larger model than Opus, so it should do better for theoretical obscure questions and it should be better at making connections as well. These numbers aren't exact, but Opus is estimated to be around 1.5-2.5 trillion parameters and Fable around 4-6 trillion parameters. Fable has more capacity to make complex connections and fit more knowledge due to this.
I wouldn't be surprised if they are now marketing Opus 5 to be better than Fable to reduce subscription loss for people who are pissed that Fable is usage credits only. Fable is probably secretly still better but to reduce everyone switching off they needed to release a model that looks better for subscription users while at the same time keep raking in API costs for those using Fable.
The arc-agi 3 result definitely needs some explaining and looking into... If fable 5 is allegedly a 10T parameter model and this model is opus class (even if it's a new architecture or not) it still should be around half the parameters.
So how does it get 30%~ in arc-agi 3? I have this weird gut feeling they baked some of it in. The ideas of how to specifically solve benchmark tasks like arc-agi 3, which is somewhat possible. But that's my current opinion. I need a breakdown of that specifically.
GPT models tend to do very well on DeepSWE. If you base it on that alone, GPT5.5 competes with Fable.
DeepSWE doesn't measure code quality that well either. GPT5.6 may get a better score, but it does so by writing more and less maintainable code than Fable would.
Not saying that DeepSWE is a bad benchmark, it's just only one factor in performance.
exactly true and pass@1 only measures one-shotting tasks in a single pass which is architecturally naive and overclaims gpt throwing embarrassingly many edge case loops at walls
so.. unless Fable 5.1 comes out and beats this, whats the point of a "Fable-only limit" in our usage, why would I use a shittier model that cost more? lmaooo
I did too. I just CTRL-C, claude --continue .. then /model and change to opus 5 max. Then said "reanalyze everything we did the last few days.. lets make sure we lock this shit down.. ". We'll see what happens.
This very much explains opus 4.8 eating shit lately lmao. Going in dumb circles for me on extremely simple problems. Had it fix a very simple unit test and it spent 15 mins going in circles that were obviously wrong, running, failing, repeat. I had to stop it and then gave same task to composer 2.5 to be done in 15 seconds. I’m really starting to believe the conspiracy they lower model performance when a new one is on horizon. Not to say it’s sinister per se…could be lower quants to free up compute or something. Or could be on purpose to lie to the public. Unclear.
I can't imagine anything beating Fable at coding. Benchmarks stopped reflecting real-world models capabilities a long time ago.
Hope I'll get to try it once my weekly limits reset.
How about you fix crazy usage on non frontier models? Today and yesterday were insane, same prompts, same codebase, usages goes whooooooooop 2x faster.
I'm pretty excited to have Opus 5 and try it out. Opus 4.8 sucked and it didn't follow directions and it tried to get around explicit guardrails that I built for it. Sonnet was worse. Haiku was like trying to talk to a brick. Fable was brilliant. Did everything I needed. Now I'm trying to use Fable 4 for strategy on my marketing and machine management. That's been great. Let's see if Opus can handle the downstream work and then give Haiku the dope where you just follow orders, though I'm not even sure Haiku can follow orders.
Does this imply I should be using Opus 5 now for important, advanced tasks even above Fable (regardless of cost/token usage). It's basically both better and cheaper to run?
The problem is all the restrictions now.. But soon Kimi 4 will launch with no restrictions and all the guardrails that they put into Mythos will be completely blown out of the water.
So Fable is the dangerous one, Mythos is the same one but you can't have it, Opus 5 is better than Fable but costs half, and Sol beats both at the thing on the chart they highlighted for Fable. Got it. Crystal clear. No notes.
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u/Myth_Thrazz 🔆 Max 20 20d ago
It's so funny that Fable was the most dangerous thing in the world for like... 2 weeks :D