Introducing Claude Fable 5.1 and Claude Mythos 5.1

We’re introducing Claude Fable 5.1 and Claude Mythos 5.1. They’re the world’s most advanced models for coding and knowledge work—and their research capabilities offer an early glimpse of how AI models will contribute to scientific progress.Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is available only through our trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences.Alongside its increased capabilities, Fable 5.1 takes important steps towards addressing the feedback we’ve received from customers on price, data retention, and safeguards.Price. Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token. This is because we’re reducing our pricing on cache reads (where the model reads inputs that have already been processed and stored). For highly agentic work, the savings will often be much larger—up to approximately 45%.Data retention. Our new system of Enterprise Frontier Safeguards (EFS) gives customers complete privacy (the same as a zero data retention policy) while still being state-of-the-art at preventing adversarial use. EFS works by storing data in cloud infrastructure controlled entirely by the customer, not Anthropic. It will be made available to enterprise customers in phases, beginning later this fall. Until EFS is available, eligible customers will be able to use Fable 5.1 with zero data retention.Safeguards. We’ve improved our safeguards to reduce false positives (where the system flags benign content). In cybersecurity, our newest safeguards block 60% fewer false positives than before. In part, this is because Fable 5.1 can now be used to discover software vulnerabilities—though not develop exploits for them. In biology, we’ve established an access program, developed in partnership with the US government, to enable access to Claude Mythos 5.1’s advanced biology capabilities. We expect to open enrollment for scientists soon.A new performance frontierClaude Fable 5.1 sets a new standard on coding, knowledge work, and long-running problem-solving tasks. The charts below show that Fable 5.1 is capable of much higher performance than its predecessor, Fable 5. And when set to Low or Medium effort, Fable 5.1 achieves similar or better results than Fable 5 at a much lower cost. (Note that Fable 5.1 defaults to High effort in Claude Code, and to Medium in Claude Cowork and on Claude.ai.)Terminal-Bench-Science 0.1Accuracy vs CostFable 5.1Fable 50102030405060Score (%)101520304050Mean cost per task (USD, log scale)lowmedhighxhighmaxlowmedhighxhighmaxTerminal-Bench-Science 0.1 scores by cost (log scale), at each effort level.Fable 5.1 avoids shortcuts that result in poorer-quality work, and it’s smart enough to fix the root causes of software issues. For example, in testing by the investment firm Millennium, Fable 5.1 found the cause of a rare crash on their internal systems that none of their engineers (or any other model) had been able to explain after several years of trying.Here, you can see how Fable 5.1 compares across various benchmarks:Fable 5.1Fable 5Opus 5GPT-5.6 SolAgentic scientific researchTerminal-Bench-Science 0.152.6%24.7%29.0%22.4%Agentic codingTerminal-Bench 4.055.8%60.9% (Mythos 5.1)42.0%52.3%37.3%Knowledge workGDPval-AA v21853172318241711Computer useOSWorld 2.077.9%partial72.9%partial75.4%partial—partial41.7%strict36.1%strict39.6%strict—strictMultidisciplinary reasoningHumanity's Last Exam60.9%no tools57.8%no tools56.6%no tools—no tools65.0%with tools63.8%with tools63.6%with tools—with toolsBusiness workflowsAutomationBench31.4%17.1%26.9%19.6%Agentic codingCursorBench 3.2.073.4%70.5%70.0%67.2% Fable 5.1 was evaluated with its production safeguards enabled. On tasks where these safeguards intervened, Fable 5.1 and Fable 5 scored a zero on OSWorld 2.0, and Fable 5 scored a zero on AutomationBench. In all other interventions from our safeguards, cybersecurity tasks were completed by Claude Opus 4.8, and biology tasks were completed by Claude Opus 5. This likely reduces the performance of Fable 5.1 and Fable 5 on these benchmarks. Terminal-Bench-Science 0.1: The standard error is ±3.5–4.5 pts per model. The public leaderboard (3 trials/task, Claude Code harness) reports Claude Opus 5 at 30.0% and Claude Fable 5 at 21.4%; our setup reproduces them at 29.0% and 24.7%, respectively, both within noise.