Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1 on Sept. 1, 2026, in a post from the official Claude account on X. Fable 5.1 cuts cache-read pricing by 75% from Fable 5, to $0.25 per million tokens, according to the company’s product announcement.
The two releases use the same underlying model with different safeguards. Fable 5.1 is generally available, while Mythos 5.1 is limited to Anthropic’s trusted access programs.
Input and output prices remain unchanged from Fable 5 at $10 and $50 per million tokens. Anthropic estimates that typical token-billed workloads will cost about 25% less, with savings of up to about 45% for complex coding and highly agentic tasks.
Anthropic said Fable 5.1 was available that day across its platforms, including Amazon Web Services, Google Cloud and Microsoft Azure, under the claude-fable-5-1 API ID. Its product page also lists Microsoft Foundry and access through Pro, Max, Team and Enterprise plans.
Cursor made Fable 5.1 available on Sept. 1 and reported a 73.4% score at maximum effort on CursorBench 3.2. Netlify added it the same day to its AI Gateway and Agent Runners. Microsoft Foundry lists the model as hosted on Anthropic infrastructure.
Anthropic reported a 52.6% score for Fable 5.1 on Terminal-Bench-Science 0.1, compared with 24.7% for Fable 5. On Terminal-Bench 4.0, Fable 5.1 scored 55.8%, Fable 5 scored 42.0% and Mythos 5.1 scored 60.9%. The company attributed the Fable-Mythos gap to tasks where Fable’s cyber safeguards intervened.
Fable 5.1’s cyber safeguards block 60% fewer false positives than before. The model may identify software vulnerabilities, but it may not develop exploits, run penetration tests or perform binary-based vulnerability scanning. Its biology safeguards fire 85% less often on benign elementary biology and medical questions than those launched with Fable 5.
Anthropic also detailed Enterprise Frontier Safeguards, which store monitoring data in customer-controlled cloud infrastructure. The company said it built the system with more than 100 customers and with AWS, Google Cloud and Microsoft Azure. Go Big News reported on Aug. 19 that OpenAI was testing abuse monitoring that keeps no customer data.
Anthropic is a public benefit corporation headquartered in San Francisco, California. Founded in 2021 by seven former OpenAI employees, it introduced Claude publicly in March 2023. In May 2026, Anthropic raised a $65 billion Series H at a $965 billion post-money valuation and said its run-rate revenue had crossed $47 billion.
Enterprise Frontier Safeguards will roll out in phases starting later this fall. Eligible customers may use Fable 5.1 with zero data retention until then. Mythos 5.1 is currently limited to a set of U.S. organizations, with the Life Sciences Verification Program launching as an invite-only beta and Mythos access planned for the Cyber Verification Program in the near future.
