# Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1

Claude Fable 5.1 is available now, while Claude Mythos 5.1 for cyberdefenders and life scientists is available through trusted access programs.

- Fable 5.1 scores 52.6% on Terminal-Bench-Science 0.1 and 55.8% on Terminal-Bench 4.0.
- Cache reads cost 75% less than Fable 5, cutting practical costs by 25% for typical workloads and up to 45% for agentic tasks.
- Cybersecurity safeguards flag benign requests 60% less often, and biology fallback rates dropped by about 85%.
- Enterprise Frontier Safeguards with zero data retention will roll out in phases starting this fall.

## Why it matters

Developers and researchers get more capable reasoning and coding models with reduced caching costs and fewer false-positive safety refusals.

## Sources

- [Anthropic: Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1](https://x.com/claudeai/status/2094848572143407483)

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Summarized by dstilled on 2026-09-01. https://dstilled.ai/story/6d8601b5-d53d-4829-befc-44d7f84cdf33