Claude Fable 5.1

Overview

Claude Fable 5.1 is Anthropic's newest generally-available frontier model, announced September 1, 2026, alongside a restricted-access twin, Claude Mythos 5.1 (the same underlying model with fewer safeguards, available only to trusted-access customers). Both run on a 1,000,000-token context window with 128,000 max output tokens. The headline change is cache-read pricing, cut 75% versus Fable 5, which Anthropic says lowers typical workload cost roughly 25% and highly agentic workload cost up to roughly 45%. Mythos 5.1 is positioned as Anthropic's most capable model for cybersecurity defense and life-sciences research (threat intelligence, vulnerability discovery, red teaming, drug discovery, and biodefense screening), scoring 52.6% on Terminal-Bench-Science and meeting or exceeding the cyber capabilities of the prior Claude Mythos 5. Mythos 5.1 shares Fable 5.1's $10/$50-per-million-token API pricing, and is reached only through Anthropic's trusted-access pathways: the Cyber Verification Program for defensive security work, and the Life Sciences Verification Program (opened September 17, 2026) for vetted biology research.

This is a sourced directory entry. Features and capabilities are reported in the linked sources; this entry does not document independent Aymorphic testing.

Key features

• 1,000,000-token context window, 128,000 max output tokens
• Cache reads at $0.25/million tokens, down 75% from Fable 5
• Batch processing at half price for asynchronous workloads
• US-only inference option at 1.1x standard pricing
• Mythos 5.1: same model, restricted trusted-access twin tuned for cybersecurity and biology, 52.6% on Terminal-Bench-Science
• Mythos 5.1 access gated via the Cyber Verification Program and the Life Sciences Verification Program

Use cases

• Coding and knowledge work
• Long-running agents and multistep research
• Document-heavy professional work
• High-volume agentic workloads sensitive to cache-read cost

Pricing

$10/million input tokens, $50/million output; cache read $0.25/million (75% cut from Fable 5); batch processing $5/$25 per million; US-only inference at 1.1x standard rate.

Pricing reflects the source record and may change. Confirm current terms with the provider.

Dates and sources

Released

1 Sept 2026

Record updated

23 Sept 2026

Record last checked

23 Sept 2026

• anthropic.com/claude-fable-and-mythos-5-1
• 9to5mac.com/2026/09/01/anthropic-upgrades-claude-with-new-fable-5-1-model-details-here/
• venturebeat.com/technology/anthropics-claude-fable-5-1-and-mythos-5-1-arrive-with-a-75-cost-reduction-for-fable-cache-reads
• 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/
• claude.com/blog/bringing-claude-mythos-5-to-more-defenders
• anthropic.com/news/life-sciences-verification-program