Anthropic splits Claude 5.1 into public Fable and gated Mythos

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 in an announcement, and the line that matters most is easy to skim past: "Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards." Fable ships broadly across AWS, Google Cloud and Azure. Mythos is walled behind a trusted access program open only to vetted cyberdefenders and life scientists at US organizations.

Headline prices stay at $10 per million input tokens and $50 per million output. What moved is the cache read: down 75%, to $0.25 per million. Anthropic says that translates to workloads roughly 25% cheaper than Fable 5, and "up to approximately 45%" on highly agentic tasks, the workloads where a large cached prompt is replayed turn after turn.

Cognition's co-founder is quoted saying the model "matched or edged out Fable 5 in our testing at a lower cost per task." Anthropic reports 73.4% on CursorBench 3.2.0, 55.8% on Terminal-Bench 4.0, a partial 77.9% on OSWorld 2.0, and 60.9% on Humanity's Last Exam without tools. On a new Terminal-Bench-Science variant the score jumps to 52.6% from Fable 5's 24.7%.

Two safety changes are worth reading closely. Cybersecurity false positives drop 60%, and Fable 5.1 now "permits vulnerability discovery (not exploit development)," a shift that lands in a week when Anthropic is also fielding the Aurora ransomware disclosure that a crew ran Cursor and Claude Sonnet against more than 20 organisations, and a separate sandbox escape response that pulled 150 engineers off other work. The second is quieter: an anti-distillation mechanism that blocks manual context editing in multi-turn conversations, aimed at competitors trying to lift capabilities from Anthropic's models.

Enterprise Frontier Safeguards, which the company says give customers "complete privacy" while still being state-of-the-art, are set to roll out in fall 2026.