The Best AI Model Ever Built Just Landed in Your Claude Account. Don't Let Your Firm Touch It Yet.
Harvey scored it at triple the nearest competitor on complex legal tasks. Google's model scored zero. And there's still a reason to pause before your firm dives in.
Claude Fable 5 Is Here, and It’s Not Just Another Model Update
TL;DR: Anthropic released Claude Fable 5 today, the first model in a new tier that sits above Opus. It leads nearly every benchmark that matters, including legal, and after six hours of hands-on testing I think it’s the best model available right now. Try it before June 22 while it’s included in paid plans. But read the data retention section first, because the rules changed, and legal teams need to know that before anyone at the firm touches it.
I’ve been testing AI models for years, and most “new model” days follow a script. The company posts a chart, the numbers tick up a few points, and by Thursday nobody remembers the announcement.
Today wasn’t that.
Anthropic released Claude Fable 5 this morning, and I’ve spent the last six hours doing nothing but putting it through my actual workload. Writing. Coding. Spreadsheet work. The verdict, at least so far: this is the biggest single jump I’ve felt since I started doing this work.
What Fable 5 actually is
Here’s the short version. Fable 5 is not a new Opus. It’s the first model in a new class entirely, what Anthropic calls Mythos-class, a tier that sits above Opus the way Opus sits above Sonnet.
Anthropic built the underlying model, called Mythos, and found it was so capable in areas like cybersecurity that they initially restricted it to a small group of cyber defense partners working with the US government. Fable 5 is that same model, made safe for the rest of us. When you ask it something in a handful of high-risk areas (offensive security, certain biology and chemistry topics), it quietly hands the question to Opus 4.8 instead. Anthropic says that happens in fewer than 5% of sessions. In six hours of legal, writing, and coding work, I never hit it once.
So when you use Fable 5, you’re using the most capable model Anthropic has ever built, with guardrails on the dangerous parts. That’s the whole product.
The benchmarks, including the one your firm cares about
The numbers are not subtle. On SWE-Bench Pro, the standard test for real-world software engineering, Fable 5 scores 80.3% against Opus 4.8’s 69.2%. An eleven-point jump on a benchmark where models had been grinding out gains of one or two points at a time. Stripe reported that during early testing, Fable 5 completed a migration across a 50-million-line codebase in a single day. Their estimate for a human team doing the same work: over two months.
But here’s the result I want you to sit with. Harvey, the legal AI company, published its own evaluation today. On their Legal Agent Benchmark, which measures end-to-end completion of complex legal tasks under an all-pass standard, Fable 5 scored 13.3%. That sounds low until you see the field: Opus 4.8 sits at 10.4%, GPT-5.5 at 2.1%, and Gemini 3.1 Pro at 0.0%. On Harvey’s BigLaw Bench, Fable 5 hit 93.4%, the highest score any Anthropic model has posted. Harvey’s lawyer evaluators called out drafting and markup analysis specifically: catching off-market provisions, term sheet deviations, and inconsistencies in counterparty redlines, and tracing defined terms accurately across large document sets.
For complex legal work, there is currently no contest. One model leads, and it leads by a lot.
What six hours of testing told me
Benchmarks are one thing. Here’s what I actually saw today.
The writing is better than Opus 4.8, at least for my use cases, and Opus 4.8 was already the best writing model I’d used. It’s also fast. Noticeably, almost surprisingly fast, which is not what I expected from a model this large.
The coding has been rock solid. Every initial test I ran produced one-shot outputs well beyond what Opus 4.8 gave me on the same prompts. And inside the Excel add-in, the difference is almost comical. It’s far faster than Opus or Sonnet, it asks smarter clarifying questions before it starts working, and the output comes back polished rather than 80% done.
Early reviews match what I’m seeing. Andrej Karpathy called it a genuine step change rather than an incremental release, and the customer quotes Anthropic published, from Cursor, GitHub, Cognition, and others, all describe the same pattern: long, hard tasks that previous models couldn’t finish, now finished.
The part your general counsel needs to read
Now the tradeoff, and for my legal audience this is not a footnote.
Anthropic is requiring 30-day data retention for all traffic on Mythos-class models. All of it. First-party and third-party surfaces, regardless of how you access the model. They’ve stated clearly that the data won’t be used to train future models and won’t be used for anything beyond safety purposes, that human access is logged, and that deletion happens after 30 days in almost all cases. The stated reason is defending against novel jailbreaks and attacks that play out across many requests.
That’s a reasonable security posture. It’s also a meaningful departure from the zero-retention arrangements many firms negotiated, and it applies even if your enterprise agreement says otherwise for older models. Harvey flagged the same thing in its announcement and made Fable 5 opt-in for exactly this reason.
So before anyone at your firm runs client matters through Fable 5, your information governance people need to answer one question: can this engagement tolerate 30-day retention by the model provider? For a lot of work, the answer is yes. For some matters, it won’t be. Know which is which before Monday.
What to do this week (Monday is too far away!)
Three things. First, try it now: Fable 5 is included in Pro, Max, Team, and seat-based Enterprise plans through June 22, after which it moves to usage credits until capacity catches up. That’s a “free” window to test the best model on the market against your real work. Second, run your own bake-off. Take three tasks you did last week with your current model and rerun them in Fable 5. Compare. Third, get the retention question in front of whoever owns your AI governance policy before broad rollout, not after.
OpenAI and Google will answer. ChatGPT 5.6 and Gemini 3.5 Pro are coming, and this leapfrog game never stays settled for long.
But today, right now, the most capable AI model in the world is sitting in your Claude account. Part of your subscription, for the next two weeks.
I’d go find out what it can do.
Here’s where this leaves you. The most capable AI model ever released is sitting in your Claude account right now, part of your subscription until June 22, and it leads the legal field by a margin that won’t be ignored by the firm across the street. The only thing standing between you and a real answer is two weeks of testing and one retention question your governance team can settle in a single meeting. If you want help structuring that bake-off, or thinking through which matters can tolerate the new retention terms, reach out: steve@intelligencebyintent.com. The firms that win with AI aren’t the ones that move first. They’re the ones that move first with their eyes open.