Claude Fable 5.1 / Mythos 5.1

What Claude Fable 5.1 / Mythos 5.1 is

Claude Fable 5.1 is Anthropic's frontier model for coding and knowledge work, announced September 1, 2026 alongside Claude Mythos 5.1. The two are the same underlying model with different safeguard levels: Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted access programs for cybersecurity and life-sciences work. Fable 5.1 carries a 1M-token context window and 128K max output, and cuts cache-read pricing by 75% versus Fable 5, reducing typical workload costs by an estimated 25% and agentic workloads by up to about 45%. It supersedes Claude Fable 5, which launched June 9, 2026.

Key features

• Claude Fable 5.1 and Claude Mythos 5.1 announced September 1, 2026; Anthropic describes Fable 5.1 as its 'most capable model for coding and knowledge work'
• Fable 5.1 and Mythos 5.1 are the same model with different safeguard levels — Fable 5.1 is generally available, while Mythos 5.1 is limited to trusted access programs for cybersecurity and life sciences
• 1M-token context window with 128K maximum output tokens (up from the 200K class)
• Mythos-level model for days-long, asynchronous tasks that fixes root causes rather than symptoms
• Enterprise Frontier Safeguards (EFS) store data in customer-controlled cloud infrastructure; zero data retention available until EFS phases in
• Adaptive extended thinking that is always on, with a default effort setting of high

Where it lives

Sources & media

• Introducing Claude Fable 5.1 and Claude Mythos 5.1 · Anthropic (official)
• Claude Fable 5 product page and release timeline · Anthropic (official)
• Claude models overview — specifications and pricing · Claude Platform Docs (official)
• Claude plans and pricing · Claude (official)
• Anthropic newsroom — all model announcements · Anthropic (official)