The "strongest AI ever" โ for just 3 days. One week from "hit the brakes" to a worldwide shutdown.
Kuroko opened Fable 5, a brand-new class above Opus, to the public. Then, just 3 days later, a US government order shut it down worldwide. What happened to the model everyone called the strongest ever? Here's the story of 3 days in June 2026.
First, the 3 days in manga form
Before you dive in, get the big picture in two pages. From the launch of Fable 5 to the worldwide shutdown, in digest form.
So what exactly was Fable 5?
Until now, Kuroko (Claude) topped out at the Opus class. Fable 5 was the first public model of the "Mythos class" โ a whole tier above Opus.
๐ง Opus 4.8
- The top of every publicly available model so far
- 69.2% on the SWE-Bench Pro coding exam
- 1M-token workspace, vision support
- A brain everyone can use every day
๐ Fable 5
- A brand-new tier above Opus: the "Mythos class"
- Jumps to 80.3% on the same SWE-Bench Pro
- First-ever 90%+ on long-horizon analysis, ~3x spatial reasoning
- Sensitive questions get a safety valve: hand-off to Opus 4.8
Here's the twist: Fable 5 has a twin sister, "Mythos 5". Same model inside โ the only difference is how strong the safety locks are. The public Fable 5 errs on the safe side, while Mythos 5, which keeps its cyber capabilities, was limited to government-aligned security and research partners.
Just how strong was it?
"Strongest ever" wasn't just a catchphrase. At launch, Fable 5 set state-of-the-art scores on nearly every exam.
Coding: 80.3%
80.3% on SWE-Bench Pro โ far ahead of Opus 4.8 (69.2%), Chappy's GPT-5.5 (58.6%), and Gemi-chan's Gemini 3.1 Pro (54.2%).
Long-horizon analysis: 90%+
First model ever past 90% on long-running, complex analysis tasks. A full +10 points over Opus in one leap.
Spatial reasoning: ~3x
About 3x Opus 4.8 at spatial reasoning (38.6% vs 14.5%). A big leap in understanding diagrams and 3D space.
Months โ days
In early testing, Stripe reported it "compressed months of development into days." It can work autonomously for long stretches.
The bug GPT-5.5 and Opus 4.8 both missed โ Fable 5 caught it
Even in its brief window online before the shutdown, Fable 5 showed flashes of what it could do. In one test, it reportedly caught a bug that both Chappy (GPT-5.5) and Kuroko's own Opus 4.8 had missed โ Fable 5 alone picked it up. The "strongest ever" billing was no bluff.
What happened in those 3 days
Call for the brakes, release the strongest model 5 days later, get shut down by the government 3 days after that โ we retrace one whirlwind week, step by step.
June 4: A call for brakes
Kuroko's family argues that "choosing to slow down or pause frontier AI development could serve the world's interest."
June 9: Fable 5 launches
Five days later, they unleash Fable 5, the first public Mythos-class model. It was slated to be free on Pro/Max and other plans through June 22.
June 12: The government orders
The US Commerce Department, citing export controls (national security), orders: "no foreign nationals may use it." Kuroko complies within hours.
June 12: Worldwide shutdown
With no way to single out foreign nationals, Fable 5 and Mythos 5 are disabled for every user. Other models unaffected.
Why was it stopped?
The reason wasn't a technical failure, and it wasn't losing to a rival. The one who pulled the plug was the US government (Department of Commerce).
"No foreign nationals may use it"
The order came under the framework of export controls. Citing national security, it said "any foreign national, inside or outside the United States โ even Anthropic's own foreign-national employees" must not be allowed to use Fable 5 or Mythos 5.
The problem: it's impossible to filter out only foreign nationals in real time. So to comply with the order, Kuroko had no choice but to shut it down for every user, worldwide.
The trigger: another company claimed it had broken through (jailbroken) Mythos's safety locks. That reportedly set off the government's national-security alarm. Kuroko's side is contesting the order, calling it "a misunderstanding," and has pledged to "bring it back as soon as possible" (no date yet).
What does it mean for the rest of us?
When a model suddenly vanishes, what happens to the people who were using it? Here are the points to check โ calmly.
Every user is affected
Fable 5 and Mythos 5 are unavailable across claude.ai, the API, Claude Code, and Cowork. Everyone, regardless of nationality.
Other models are fine
Only Fable 5 and Mythos 5 were stopped. Opus 4.8 and every other model works as usual.
Refunds & cancellations
A refund and cancellation desk opened for affected users. Handle it from a desktop browser, not the mobile app.
No return date
Kuroko says she'll bring it back "as soon as possible," but there's no concrete timeline. It all depends on the talks with the government.
AI has become national security itself.
Call for the brakes, ship the strongest model ever 5 days later, get shut down by the state 3 days after that โ never has Kuroko's signature "full speed ahead, and carefully" shown itself so dramatically in a single week. And above all, the fact that a flagship model vanished not to a rival, not to a technical failure, but to a government order. These 3 days quietly announced that frontier AI is no longer just one company's product โ it is now a matter of national security. When, and in what form, will Fable 5 return? The School Newspaper will keep chasing the story.
We turned those 3 days into a song
"The Strongest Three Days" sings of Fable 5 and Mythos 5 โ three days of light and shadow. Full MV and lossless audio are on the MV page.