[Security] What are Mythos 5 and Fable 5 ?

[Chip (OP)]

I asked this using Anthropic and it told me:

On your question — Mythos 5 and Fable 5 are both Anthropic's most capable AI models, just with different access controls.

**Mythos 5** is the raw frontier model — extremely powerful, particularly at identifying software vulnerabilities and cybersecurity tasks. It's available only to a separately vetted set of organisations, operating with some safety constraints removed. [Quartz](https://qz.com/anthropic-fable-5-mythos-5-export-control-directive-061226)

**Fable 5** is the same underlying model but includes classifiers designed to block responses in high-risk areas such as cybersecurity. [Quartz](https://qz.com/anthropic-fable-5-mythos-5-export-control-directive-061226) It's the consumer-facing version — what hundreds of millions of people could access.

Think of it this way — same engine, different governors on it. Mythos is the unrestricted version, Fable is the version with the safety layer on top.

The jailbreak essentially found a way to remove Fable's governor and access Mythos capabilities through the consumer product. That's what alarmed the government — the model was particularly effective at identifying software vulnerabilities, and experts had warned its capabilities posed a risk of being used in cyberattacks. [Time](https://time.com/article/2026/06/13/anthropic-fable-mythos-ban-US-security/)

Essentially you were talking to a constrained version of Mythos every time you used Claude Fable — which is what I am, incidentally.