# Claude Fable 5.1

Anthropic, released 2026-09-01

**Verdict: conditional.** Anthropic's new frontier flagship is the most capable model publicly available, and it largely fixes the safeguard over-triggering that made Fable 5 frustrating for routine coding. The verdict stays conditional on cost: independent measurement shows more spend per task than Fable 5 despite the cheaper cache reads, because the model thinks longer. Reach for it on demanding reasoning and long-horizon agentic work. For everyday coding, Claude Opus 5.5 at $4/$20 per 1M is Anthropic's recommended starting model and the better default. Last verified 2026-09-30.

## Pricing

- Input: $10 / 1M tokens
- Output: $50 / 1M tokens
- Context: 1M context, 128K output; cache $0.25/1M

## Strengths

- Demanding reasoning and long-horizon agentic work — Anthropic's own guidance is to reach for Fable 5.1 on its most demanding tasks
- Agentic scientific research: 52.6% on Terminal-Bench-Science 0.1 against Opus 5's 29.0% and Fable 5's 24.7%
- Context-heavy agent loops — cache reads dropped 75% to $0.25 per 1M, which Anthropic puts at up to ~45% cheaper on highly agentic workloads
- Long-document and whole-repo work: 1M-token context with 128K max output
- Security-adjacent coding that Fable 5 refused — cyber safeguards now permit vulnerability discovery, cutting Claude Code interventions ~60% per session
- Cost-sensitive everyday coding — Claude Opus 5.5 at $4/$20 per 1M is Anthropic's recommended starting model, and Anthropic says it performs at the level of Fable 5.1 on most work at a much lower rate
- Latency-sensitive paths — adaptive thinking is always on and Anthropic rates its comparative latency as the slowest in the lineup
- Budgeting by rate card alone — Artificial Analysis measures ~20% higher cost per task than Fable 5 despite the cache-read cut, because effort drives token count
- Fact-recall work where a confident wrong answer is expensive — at max effort it attempts 93.4% of AA-Omniscience questions, trading more hallucinations for its higher accuracy at no net Omniscience gain

## Benchmarks

- Artificial Analysis Intelligence Index: 66 (https://artificialanalysis.ai/articles/claude-fable-5-1)
- Terminal-Bench-Science 0.1: 52.6% (https://www.anthropic.com/claude-fable-and-mythos-5-1)
- Terminal-Bench 4.0: 55.8% (https://www.anthropic.com/claude-fable-and-mythos-5-1)
- CursorBench 3.2.0: 73.4% (https://www.anthropic.com/claude-fable-and-mythos-5-1)
- Humanity's Last Exam (no tools): 60.9% (https://www.anthropic.com/claude-fable-and-mythos-5-1)
- Humanity's Last Exam (with tools): 65.0% (https://www.anthropic.com/claude-fable-and-mythos-5-1)
- AutomationBench: 31.4% (https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/)
- OSWorld 2.0 (strict): 41.7% (https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/)
- GDPval-AA v2: 1853 (https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/)
- AA-Omniscience accuracy (max effort): 67.2% (https://artificialanalysis.ai/articles/claude-fable-5-1)

## Sources

- [Anthropic — Claude models overview](https://platform.claude.com/docs/en/models/overview)
- [Anthropic pricing page](https://platform.claude.com/docs/en/about-claude/pricing)
- [AiCybr — Fable 5.1 / Mythos 5.1 pricing, benchmarks, access](https://aicybr.com/blog/claude-fable-5-1-mythos-5-1-pricing-benchmarks)
- [Anthropic: Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1)
- [Anthropic - Introducing Claude Opus 5.5](https://www.anthropic.com/claude-opus-5-5)
- [Artificial Analysis: Claude Fable 5.1 tops the Intelligence Index](https://artificialanalysis.ai/articles/claude-fable-5-1)
- [MarkTechPost — Anthropic releases Claude Fable 5.1 and Mythos 5.1](https://www.marktechpost.com/2026/09/01/anthropic-releases-claude-fable-5-1-and-claude-mythos-5-1-52-6-on-terminal-bench-science-and-75-cheaper-cache-reads/)

Canonical: https://neomanex.com/models/claude-fable-5-1