Claude Fable 5, the first Mythos-class model made public by Anthropic

Anthropic launches Claude Fable 5, a Mythos-class AI model beating GPT-5.5 on SWE-Bench Pro with advanced safety guardrails and a $10/M input token price.

Anthropic is launching Claude Fable 5, a Mythos-class model, meaning a capability tier above the Opus range, but made safe for general use. The firm presents it as its most capable model ever made available to the public, state-of-the-art across nearly all tested benchmarks, with an advantage that widens as tasks become longer and more complex. Stripe reports that during testing, the model compressed months of engineering work into days, executing in one day a migration on a 50-million-line Ruby codebase that would have taken an entire team more than two months. In terms of figures, Fable 5 achieves 80.3% on SWE-Bench Pro, far ahead of GPT-5.5 (58.6%) and Gemini 3.1 Pro (54.2%).

The novelty of this launch lies in its safety mechanism. Mythos-class capabilities in cybersecurity and biology could offer a real advantage to malicious actors: Fable 5 therefore incorporates classifiers that detect queries related to cybersecurity, biology and chemistry, or model distillation. When one of them is triggered, the response is handled by Claude Opus 4.8 and the user is informed. Anthropic acknowledges having set these guardrails in a deliberately conservative manner, even if it means flagging harmless queries, with the switchover occurring in less than 5% of sessions on average.

In parallel, Anthropic offers Claude Mythos 5, the same underlying model but with certain guardrails lifted, reserved for a small group of cyber defenders and infrastructure providers via the Project Glasswing program, in collaboration with the U.S. government. The firm describes it as having the strongest cybersecurity capabilities in the world. A trusted access program dedicated to biology is also planned to accelerate biomedical research, providing access to Fable 5 with the bio guardrails lifted but cyber protections maintained.

Both models are priced at $10 per million input tokens and $50 per million output tokens, which is less than half the price of Claude Mythos Preview. Fable 5 is available now, via the API under the identifier claude-fable-5, and included at no extra cost with Pro, Max, Team, and Enterprise subscriptions until June 22, after which its usage will transition to credits while Anthropic adjusts its capacity.