Anthropic launches Fable 5, a ‘Mythos-Class’ LLM available to all

Anthropic has released Claude Mythos 5 and Claude Fable 5, with Mythos 5 described as having the strongest cybersecurity capabilities of any model in the world. The models are priced at $10 per million input tokens and $50 per million output tokens, less than half the cost of the previous Mythos Preview.

Fable 5, powered by the same underlying model as Mythos 5, includes additional guardrails to prevent misuse in areas like cybersecurity. In some cases, queries on sensitive topics will be answered by Claude Opus 4.8 instead, with these safeguards triggering in less than 5% of sessions.

Mythos 5 will initially be deployed through Project Glasswing, a cybersecurity-focused programme run with the US government, with wider availability planned later. Microsoft has already made Fable 5 available in Microsoft Foundry and for use in GitHub Copilot and Foundry Agent Service.

Industry reactions have been mixed. Sachin Puri of Network Solutions welcomed the advance but stressed the need for governance and accountability. Jamie Moles of ExtraHop expressed scepticism about public release, while Andrew Rubin of Illumio warned that guardrails do not address underlying risks. Douglas McKee of Rapid7 suggested Fable 5 would become the everyday workhorse, with Mythos 5 reserved for vetted security teams.