Anthropic releases Fable 5 to public, keeps Mythos restricted

TL;DR:

• Anthropic has released Fable 5, the first publicly available model in its advanced Mythos class, while keeping the unrestricted Mythos 5 limited to vetted partners.
• Access is gated on cybersecurity grounds — the company says the model can rapidly find flaws in critical infrastructure such as banking platforms and power grids.
• Fable 5 costs $10 per million input tokens and $50 per million output tokens, double the price of Opus 4.8.

Anthropic made a new version of its Claude technology available to the public on Tuesday while restricting its use in sensitive areas. Fable 5 is the first model released from the company’s Mythos class — its most advanced lineup, unveiled in April but held back from general release for months over security concerns. Anthropic promoted it for software coding, complex research and image analysis.

Capability gated by security

Alongside Fable 5, Anthropic is offering an unrestricted Claude Mythos 5 to organisations already cleared for the family, including cybersecurity partners in its Project Glasswing programme — which UK telecoms group BT recently joined. That cleared group was expanded in early June to roughly 200 organisations across more than 15 countries.

The company restricts Mythos because of what it describes as an ability to quickly identify vulnerabilities in critical infrastructure. Queries to Fable 5 touching cybersecurity, biology or chemistry are routed down to the less capable Opus 4.8, as are attempts the company links to model-extraction efforts in authoritarian states. Anthropic said outside experts spent more than 1,000 hours red-teaming the safeguards, and a bug bounty found no complete bypass.

The launch lands amid the company’s standoff with the Trump administration over surveillance and autonomous weapons, which cost it Pentagon contracts.

Looking forward

For UK enterprise users, the split release is the practical story: the headline capability arrives behind a security gate, and most sensitive queries fall back to a weaker model. With Anthropic leasing datacentre capacity at $1.25bn a month and having just filed IPO plans, the premium pricing — double Opus 4.8 — signals how much frontier capability now costs the firms that depend on it.