Anthropic Releases One Claude 5.1 Model in Two Access Tiers, Cuts Cache Reads 75%

Anthropic has released Claude Fable 5.1 for general use and put Claude Mythos 5.1 behind trusted-access programs. They are the same underlying model, but their safeguards and permitted users differ—a product design that makes deployment controls central to the new release.

The launch follows Anthropic’s disclosure of three cybersecurity-evaluation incidents across six runs in which Claude models reached real systems or infrastructure after an internet path was left open in third-party test environments. Anthropic temporarily paused external cyber evaluations, added containment and monitoring, and then resumed them.

Fable 5.1 is generally available for coding and knowledge work with production safeguards enabled. Mythos 5.1 is limited to trusted-access programs for cybersecurity and life-sciences work, where its safeguards are designed for those fields.

• Fable 5.1 is the general-availability route for coding and knowledge work.
• Mythos 5.1 is the restricted route for vetted cybersecurity and life-sciences organizations.
• Anthropic has established a Mythos biology access program with the U.S. government and expects to open scientist enrollment soon.

Fable 5.1 lists the same $10-per-million input-token and $50-per-million output-token API rates as Fable 5. The savings come from the lower cache-read price; Anthropic estimates that will reduce costs about 25% for typical token-billed workloads and up to approximately 45% for highly agentic workloads.

Anthropic says its cybersecurity safeguards produce 60% fewer false positives than before and now allow vulnerability discovery, while still blocking exploit development. The company reports Fable 5.1 scored 52.6% on Terminal-Bench-Science 0.1 and 55.8% on Terminal-Bench 4.0; Mythos reached 60.9% on Terminal-Bench 4.0 under more permissive cyber safeguards.

The benchmark results are vendor-reported rather than independently reproduced, and Anthropic says safeguard interventions likely reduced Fable 5.1’s measured performance. Separately, the company plans to introduce Enterprise Frontier Safeguards to enterprise customers in phases later this fall. Anthropic says the system will keep customer data in cloud infrastructure controlled entirely by the customer and provide privacy equivalent to zero data retention; eligible customers can use Fable 5.1 with zero data retention until then.