Claude Fable 5 is Anthropic's first public Mythos-class model. It tops SWE-Bench Pro at 80.3%.

Anthropic just put its most capable model in everyone’s hands. Claude Fable 5, the first release from what the company calls its Mythos class, went live on June 9 across paid plans and every major cloud. It posts the highest benchmark scores Anthropic has ever shipped publicly, and it costs twice what Opus 4.8 does.

That price is the tell. Anthropic isn’t pitching Fable 5 as your everyday assistant. It’s the model you reach for when a task is hard enough to justify the bill, and the company spent months arguing that the raw version was too risky to sell to anyone at all.

What “Mythos class” actually means

For most of this year Anthropic has been sitting on a more powerful tier of models. It calls them the Mythos class, “a tier of Claude models that sit above our Opus class in capability,” and it kept the strongest one out of public hands on the argument that the model’s offensive cyber skills were dangerous in the wrong hands. The only outsiders who touched it were hand-picked defenders running it under contract.

You’ve already seen what that model can do. Mozilla pointed an early Mythos build at Firefox and shipped 271 fixes for bugs it found. Anthropic’s restricted defense program logged more than 10,000 high or critical vulnerabilities in a single month. Powerful, and exactly why the company was nervous about a wide release.

Fable 5 is the compromise. Same Mythos-class brain, wrapped in safeguards Anthropic describes as “a Mythos-class model that we’ve made safe for general use.” Think of it as the model with a governor bolted on. Its unrestricted twin, Mythos 5, shipped the same day to vetted cyberdefenders and never reaches a normal account.

The benchmarks, and the Stripe receipt

Numbers first. On SWE-Bench Pro, Fable 5 hits 80.3%, against 69.2% for Claude Opus 4.8 and 58.6% for GPT-5.5, per the figures Anthropic published. On FrontierCode, a harder production-coding test, the gap blows open: 29.3% for Fable 5 versus 13.4% for Opus 4.8 and a brutal 5.7% for GPT-5.5. Google’s Gemini 3.1 Pro trails on the coding suite. Anthropic also claims a structural edge on long jobs: “it can run for days, and the longer the task, the larger its lead over our other models.”

The receipt that’ll get passed around engineering Slacks is Stripe’s. The payments company ran Fable 5 at a 50-million-line Ruby migration and said it compressed “five months of engineering work into days”, finishing in a single day what it had budgeted a full team over two months for. Cursor called Fable 5 “the state of the art model on CursorBench.” On Hebbia’s finance benchmark, which tests analyst-grade reasoning over documents and tables, it took the top score of any model tested.

Coding isn’t the only place Anthropic claims a jump. The company points to “exceptional performance in software engineering, knowledge work, scientific research, and vision,” and the design leans into long, agentic jobs rather than one-shot answers. That framing matters for how you’d actually spend the money: a model that holds a plan together across a multi-hour task is worth more to an engineering team than one that wins a single-turn benchmark by a few points. Whether it sustains that over a real workday, and not just a curated demo, is the open question.

Read all of it with the usual caveat. They’re vendor benchmarks and partner testimonials, run by the company selling the model. The independent numbers will land over the next couple of weeks, and that’s when we’ll know whether the SWE-Bench Pro lead holds up outside Anthropic’s own harness.

The safeguards that downgrade you to Opus 4.8

Here’s the part that makes Fable 5 unusual. When its classifiers detect a query in a high-risk area, it doesn’t answer with the Mythos-class model. It hands the request to Opus 4.8, the older and less capable model, and tells you it did.

Three categories trip the wire: offensive cybersecurity, dual-use biology and chemistry, and distillation, meaning attempts to copy the model’s capabilities into another model. Anthropic says “more than 95% of sessions involve no fallback at all,” so for ordinary coding and knowledge work you get the full model. The company also says it stress-tested the guardrails, reporting it ran “an external bug bounty that produced no universal jailbreaks in over 1,000 hours of testing”, with outside red teams also coming up empty.

Then real users got it, and the launch thread turned ugly fast. People reported getting bumped to Opus 4.8 for prompts that aren’t remotely dangerous: a Costco shopping list for pulled-pork sandwiches, a sheep RNA-sequencing analysis, even a question about the model’s own limitations. One developer said a request about “legitimate low-level software engineering” tripped the biology-or-cyber filter. The nickname stuck within the hour: “Feeble 5.” Anthropic says it will “keep refining the safeguards to reduce false positives,” which is a tacit admission the early classifier is trigger-happy. No universal jailbreak found is not the same as a usable model, and right now the filter is the loudest complaint.

A free trial with a June 22 deadline

The pricing is where the goodwill really cracks. Fable 5 lists at $10 per million input tokens and $50 per million output, roughly double Opus 4.8’s $5/$25. Through June 22 it’s bundled into Pro, Max, Team, and seat-based Enterprise plans at no extra cost. After that, the company is blunt: “on June 23, we’ll remove Fable 5 from those plans. Using it after that will require usage credits.”

So the best model Anthropic sells is a free trial that converts to pay-as-you-go in under two weeks. The company says it “aims to restore Fable 5 as a standard part of subscription plans” once capacity allows, but offers no date. Subscribers reading that as “your $200 Max plan won’t include the flagship” are not wrong today. It compounds a second complaint in the thread: that Fable burns through usage limits fast, which makes a future metered model read less like a perk and more like a meter running. One more wrinkle worth knowing before you paste anything sensitive: prompts and outputs from Mythos-class models “are retained for 30 days for trust and safety purposes,” on every platform, regardless of your usual data settings.

What this means for you

If you write code for a living, Fable 5 is the new ceiling, and the FrontierCode spread suggests the lead over Opus 4.8 is real on genuinely hard work. But treat the free window as exactly that. Run your hardest task against it before June 22, a migration or a refactor that’s eaten days, and measure whether the output justifies pay-per-use rates after the clock runs out. For everyday work, Opus 4.8 or a cheaper tier stays your default.

Two things to watch. First, whether independent benchmarks confirm the lead or shave it, as they did with Opus 4.8’s launch numbers. Second, how fast Anthropic tunes the classifier. If you do security work or life-sciences research, test now: you may be paying flagship prices for Opus 4.8 answers every time the filter misreads your prompt, and a “Feeble 5” that won’t touch your actual job is worth nothing at $50 a million tokens.