Anthropic Launches Claude Fable 5.1 and Claude Mythos 5.1 With Lower Prices and Tighter Safeguards

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, described as its most advanced models for coding and knowledge work. The two share an underlying model but differ in safeguards: Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programs covering cybersecurity and life sciences. Cache reads now cost 75% less, at $0.25 per million tokens, cutting typical workload costs by roughly 25% and agentic task costs by up to about 45%; input and output pricing stays at $10 and $50 per million tokens. A new Enterprise Frontier Safeguards system keeps customer data on customer-controlled cloud infrastructure, rolling out in phases from this fall, with zero data retention available until then. Cybersecurity safeguards now produce about 60% fewer false positives, and Fable 5.1 may be used to find software vulnerabilities but not to build exploits. In testing by the investment firm Millennium, the model traced a rare crash that had gone unexplained for four to five years. Mythos 5.1 designed protein binders with hit rates near 50% across 12 targets, and Fable 5.1 trained a neural network to map a third of Venus at two-to-three-kilometer resolution using NASA Magellan radar data, released under a Creative Commons license ahead of NASA VERITAS and ESA EnVision. Anthropic signed the EU AI Act Code of Practice on Transparency in July 2026 and added watermarking to outputs of models released after August 2, 2026, with a detection API in private preview.