Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1, saying they are the same underlying model with different levels of safeguards. Fable 5.1 is generally available. Mythos 5.1 is limited to trusted access programs, with safeguards designed for cybersecurity and life sciences work, Anthropic said. The Verge reported the same-day launch and said Fable 5.1 is available on all platforms. Anthropic’s developer documentation lists Fable 5.1 for all customers on the Claude API and partner platforms, and lists Mythos 5.1 for Project Glasswing participants only.
Anthropic’s Fable product page says Fable 5.1 is available to Pro, Max, Team, and Enterprise users and is priced at $10 per million input tokens and $50 per million output tokens. Cache reads now cost $0.25 per million tokens, 75 percent less than Fable 5. Anthropic said that change will cut typical token-billed workloads by an estimated 25 percent and highly agentic work by up to about 45 percent. The Decoder reported that Artificial Analysis, which helped Anthropic with pre-release testing, disputes those savings: cheaper cache reads save about $1.40 per task on agentic workloads, but Fable 5.1 at max effort costs about 20 percent more per task than Fable 5 because it uses roughly 1.7 times as many output tokens.
Anthropic said it improved safeguards to reduce false positives and that its newest cybersecurity safeguards block 60 percent fewer false positives than before. The company said Fable 5.1 can be used to discover software vulnerabilities but not to develop exploits for them. The Verge reported that Anthropic will still redirect some cybersecurity tasks to Opus models, including penetration testing, exploit generation, and binary-based vulnerability scanning.
Anthropic said it established an access program, developed in partnership with the U.S. government, for Mythos 5.1’s advanced biology capabilities, and that it expects to open enrollment for scientists soon. The Decoder reported that Mythos 5.1 is currently limited to U.S. organizations through a Cyber Verification Program for defensive security work and a Life Sciences Verification Program developed with the U.S. government.