Anthropic launches Claude Fable 5.1: 2x jump on Terminal-Bench-Science, 25% cheaper
heathercmiller · x · 2026-09-02
- Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 — the same model with different safeguard levels. Fable 5.1 is generally available; Mythos 5.1 is limited to trusted access programs for cybersecurity and life sciences work.
- On the Stanford-led Terminal-Bench-Science benchmark (70 tasks in v0.1 evaluating AI agents on scientific research workflows), performance more than doubled: 24.7% on Fable 5 → 52.6% on Fable 5.1. The benchmark became the #1 featured benchmark just 5 days after launch; the team plans a harder v0.2.
- Pricing: typical token-billed workloads cost 25% less than Fable 5, driven by cheaper cache reads; highly agentic workloads save up to 45%.
- Data retention: the new Enterprise Frontier Safeguards (EFS) give enterprise customers zero-data-retention-level privacy with data stored in customer-controlled infrastructure, rolling out in phases this fall; until then eligible customers can use Fable 5.1 with zero data retention.
- Safeguards: false positives drop significantly — 60% fewer false positives in cybersecurity.
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