Anthropic releases Claude Fable and Mythos 5.1

Anthropic introduced Claude Fable 5.1 and Mythos 5.1, with improvements in coding, professional work and scientific research. Both use the same underlying model. Fable is generally available, while Mythos is restricted to trusted access programmes for work including cybersecurity and life sciences.

Lower cache-read prices are expected to reduce typical token-billed costs by about 25 percent compared with Fable 5. Anthropic estimates savings of up to approximately 45 percent on some highly agentic workloads. The company also reported fewer false positives in its cybersecurity safeguards and said Fable can now help discover software vulnerabilities, although exploit development remains restricted.

A forthcoming enterprise safeguards system will store monitoring data in infrastructure controlled by customers, with a phased rollout planned for later in the autumn.

Access conditions are becoming part of the model distinction: the same core capabilities can reach different users under substantially different controls.