Anthropic has launched Claude Fable 5.1 and Claude Mythos 5.1, calling them its most advanced models for coding and knowledge work. The two are the same underlying model with different safeguard levels: Fable 5.1 is generally available today, while Mythos 5.1 is restricted to trusted access programs for cyberdefenders and life scientists. The announcement rocketed to the top of Hacker News within hours.
The release takes aim at the biggest complaints about Fable 5. Fable 5.1 costs about 25 percent less for typical workloads, and up to 45 percent less for highly agentic work, driven by cheaper cache-read pricing. Anthropic also introduced Enterprise Frontier Safeguards, giving customers zero data retention by storing data in infrastructure the customer controls, rolling out in phases from this fall. On benchmarks, Fable 5.1 scores 55.8 percent on Terminal-Bench 4.0 and 65 percent on Humanity's Last Exam with tools, while improved cyber safeguards block 60 percent fewer false positives.
Early partners report real-world wins. Millennium says Fable 5.1 found the root cause of a rare crash its engineers could not explain for years, and Cognition is moving Opus 5 traffic to Fable 5.1 on launch day. The launch arrives as AI agent security incidents draw scrutiny, making Anthropic's dual-model approach a bet that capability and guardrails can scale together.