Claude Fable 5.1 and Claude Mythos 5.1

2026-09-01 · Publisher : Anthropic · Domain : Artificial Intelligence

Anthropic launches Claude Fable 5.1 and Claude Mythos 5.1, presented as its most advanced models in coding and knowledge work, less costly than Fable 5 and with more precise safeguards.

What changes

  • Estimated cost 25% lower than Fable 5 for typical workloads, up to about 45% on highly agentic work (cache read at $0.25).
  • More precise safeguards: vulnerability detection in source code is now allowed, biology interventions on benign requests reduced by 85%, 60% fewer cyber false positives.
  • Mythos 5.1: access via the Cyber Verification Program and the Life Sciences Verification Program (US organizations for now).
  • Enterprise Frontier Safeguards: zero retention possible with customer-controlled cloud infrastructure.

### Specifications - Context window: 1 M tokens. Maximum output: 128,000 tokens. Adaptive thinking always on. - Knowledge cutoff: June 2026 (models page). - Availability: Fable 5.1 on all platforms; identifiers `claude-fable-5-1` and `claude-mythos-5-1`. ### Pricing | Item | Price (USD per million tokens) | | --- | --- | | Input | 10 | | Output | 50 | | Cache write 5 min / 1 h | 12.50 / 20 | | Cache read | 0.25 | | Batch (input / output) | 5 / 25 | ### Benchmarks Scores published by the vendor (read via a summary of the page; to be cross-checked against the official sheet). | Evaluation | Fable 5.1 | Fable 5 | Opus 5 | | --- | --- | --- | --- | | Terminal-Bench 4.0 | 55.8% | 42.0% | 52.3% | | Terminal-Bench-Science 0.1 | 52.6% | 24.7% | 29.0% | | GDPval-AA v2 | 1853 | 1723 | 1824 | | OSWorld 2.0 (partial) | 77.9% | 72.9% | 75.4% | | Humanity's Last Exam (no tools) | 60.9% | 57.8% | 56.6% | | Humanity's Last Exam (with tools) | 65.0% | 63.8% | 63.6% | | AutomationBench | 31.4% | 17.1% | 26.9% | | CursorBench 3.2.0 | 73.4% | 70.5% | 70.0% | Mythos 5.1: 60.9% on Terminal-Bench 4.0. ### Key takeaways Fable 5.1 lowers the effective cost of the top tier, especially for agents with a high cache rate, and relaxes some safeguards without changing the base price.

Links

Tags : LLM, Anthropic, Claude, Fable, Mythos, Agents