# Claude Fable 5.1: Benchmark Gains and 25-45% Lower Cost

> A 75% cut to cache-read pricing drives 25-45% lower workload cost, alongside benchmark gains over Fable 5 and new anti-distillation protections.

Published: 2026-09-03
URL: https://daniliants.com/insights/claude-fable-5-1-benchmark-gains-and-lower-cost/
Tags: anthropic, model-release, llm-benchmarks, pricing

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## Summary

A screen-share walkthrough of Anthropic's announcement post for Claude Fable 5.1, with a parallel more-permissive Claude Mythos 5.1 tier, covering benchmark gains over Fable 5 and rival GPT-5.6 Sol, a 25-45% cost cut via cheaper cache reads, a new customer-controlled-infrastructure privacy option, and safety changes including anti-distillation protections. The source is a creator scrolling and narrating the actual post rather than adding independent commentary.

## Key Insight

**What Fable and Mythos are**

Per the post, Claude Fable 5.1 and Claude Mythos 5.1 are the same underlying model. Mythos is a trusted-access variant with more permissive safeguards for vetted individuals and organizations; Fable is the standard-safeguard public release.

**Benchmarks (Fable 5.1 / Fable 5 / Opus 5 / GPT-5.6 Sol)**

- Terminal-Bench-Science 0.1: 52.6% / 24.7% / 29.0% / 22.4%
- Terminal-Bench 4.0 (agentic coding): 55.8% / 42.0% / 52.3% / 37.3%
- GDPval-AA v2 (knowledge work, Elo-style): 1853 / 1723 / 1824 / 1711
- OSWorld 2.0 (computer use, partial): 77.9% / 72.9% / 75.9% / not reported
- Humanity's Last Exam (no tools): 60.9% / 57.8% / 56.6% / not reported
- AutomationBench: 31.4% / 17.1% / 26.9% / 19.6%
- CursorBench 3.2.0: 73.4% / 70.5% / 70.0% / 67.2%

Fable 5.1 leads Opus 5, Anthropic's own higher tier, on every listed benchmark, and leads GPT-5.6 Sol everywhere it's reported.

**Pricing**

Cache reads now cost $0.25 per million tokens, a 75% cut on that line item. Net effect is roughly 25% cheaper for typical workloads and up to 45% cheaper for complex coding and highly agentic tasks, because agentic workloads lean more heavily on cache reads. The cost index shows Fable 5 at 100 versus Fable 5.1 at 75 for typical workloads and 55 for highly agentic ones, measured over four weeks of actual August 2026 usage.

**Data retention and privacy**

A new Enterprise Frontier Safeguards system stores enterprise customer data in infrastructure the customer controls rather than Anthropic's, giving zero-data-retention-equivalent privacy while keeping adversarial-use protections. It rolls out in phases starting later in fall 2026; until then, eligible customers can already run Fable 5.1 with standard zero data retention.

**Safeguards changes**

- Cybersecurity false-positive rate down roughly 60% versus Fable 5.
- Fable 5.1 can now be used to discover software vulnerabilities for defensive security work but is still blocked from developing exploits. Dual-use tasks still route to Opus models.
- New anti-distillation protection: for API accounts created from the 5.1 launch date onward, it's no longer possible to manually edit Claude's prior turns in a multi-turn conversation while preserving the transcript of extended thinking. This closes a publicly known technique for distilling Claude's reasoning via edited conversation history. Existing accounts are unaffected for now.
- A new biology access program built with the US government will enable Mythos 5.1's advanced biology capabilities for vetted scientists.

**Availability**

Live on AWS, Google Cloud, and Microsoft Azure alongside the Claude API directly, with the API model identifier `claude-fable-5-1`.