What is Claude Mythos? And how to see it in action with Claude Fable 5
Instead the company set up Project Glasswing, a limited program that gave roughly 50 partner organizations early access so they could fix bugs before bad actors found them. A near-identical version with safety limits, called Claude Fable 5, went to paying subscribers.
Key Takeaways
- Mythos 5 and Fable 5 are actually the same model; Fable 5 just has increased security guardrails to prevent bad actors from using it to develop cyberattacks or new bioweapons.
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- The idea was that they'd be able to deploy Mythos for defensive cybersecurity purposes before an open Chinese model developed the same capabilities and threw the whole web wide open.
Two months later, we now have a guardrailed version of Mythos-called Fable-that's fully available to the public.
- As a tech journalist, I've been covering AI and security (separately and together) for well over a decade.
My read is that while there's a degree of marketing to Anthropic's claims, Mythos and Fable are the most powerful models ever released.
- At the time, Anthropic claimed it was too dangerous for wide release because it had already uncovered "thousands of high-severity vulnerabilities, including some in every major operating system and web browser."
That's why Mythos Preview was released to Project Glasswing partners.
- Incidentally, in launching Mythos 5 and Fable 5 (the first numbered versions of the two models), Anthropic has made a complete mess of its model numbering convention.
Stats & Key Facts
- #Anthropic announced Claude Mythos on April 7, 2026, then declined to release it to the public because the model found thousands of serious software security flaws on its own.
- #Instead the company set up Project Glasswing, a limited program that gave roughly 50 partner organizations early access so they could fix bugs before bad actors found them.
- #Antropic estimates that the guardrails kick in in around 5 percent of queries, and in those cases, it automatically falls back to Opus 4.8.
Mythos 5 and Fable 5 are actually the same model; Fable 5 just has increased security guardrails to prevent bad actors from using it to develop cyberattacks or new bioweapons. See how Zapier helps you manage, secure, and scale automation across your organization. Three phases to move from disconnected AI pilots to orchestrated systems that scale.
On April 7, 2026, Claude Mythos Preview was officially announced, but it was apparently too dangerous to release. According to Anthropic, Claude Mythos represented a unique cybersecurity threat (they claimed that "the fallout-for economies, public safety, and national security-could be severe.") Instead of releasing Mythos to the general public, they spun up Project Glasswing, a cybersecurity initiative that also involved some big-name companies.
The idea was that they'd be able to deploy Mythos for defensive cybersecurity purposes before an open Chinese model developed the same capabilities and threw the whole web wide open. Two months later, we now have a guardrailed version of Mythos-called Fable-that's fully available to the public. Claude Mythos 5 and Claude Fable 5 are Anthropic's most capable models.
For more details please read the original article at Zapier AI Blog.