Claude Fable 5.1 & Mythos 5.1 Released [Sept 1, 2026]
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026. Both are the same underlying model — Mythos 5.1 is the restricted-access version available through Project Glasswing, Fable 5.1 is the generally available version with additional safeguards. Same 1M-token context window, 128K max output, and same API pricing as their v5 predecessors. Positioned as Anthropic's most capable generally available model for demanding reasoning, long-running agents, coding, research, and document work.
Fable 5.1 is the current top-tier commercial Anthropic model until Opus 5.5 launched three weeks later at similar performance for 40% less. If you were paying Fable pricing for tasks that don't strictly need Fable-level capability, Opus 5.5 (Sept 22) is now the cost-optimal choice. Fable 5.1 remains correct for the hardest reasoning tasks.
API model string: claude-fable-5-1. Mythos 5.1 is invite-only via Project Glasswing at anthropic.com/glasswing. For most users, Opus 5.5 (released Sept 22, 2026 at 40% lower cost) is now the better default.
Top 5 Things to Know About Fable 5.1 & Mythos 5.1
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Same model, two access tiers — Fable 5.1 is generally available with safeguards; Mythos 5.1 is restricted via Project Glasswing. Same underlying capability.
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Same specs, same price as v5 — 1M context, 128K output, unchanged pricing. This is a refinement release, not a redesign.
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Positioned for hardest work — Long-running agents, complex research, document work, demanding reasoning. The frontier tier of what Anthropic currently offers commercially.
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Superseded by Opus 5.5 for most cases — Three weeks later, Opus 5.5 launched matching Fable 5.1 performance on most workloads at 40% less. Fable 5.1 stays for hardest edge cases.
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Mythos 5.1 is invitation-only — Only accessible through Project Glasswing for trusted-partner cybersecurity and critical infrastructure work.
FAQ
What’s the difference between Fable 5.1 and Mythos 5.1?
Fable 5.1 and Mythos 5.1 share the same underlying model. Mythos 5.1 is the restricted-access version (fewer safeguards, invite-only via Project Glasswing). Fable 5.1 is the generally available version with additional cybersecurity safeguards. Anthropic uses this two-tier structure for its most capable models.
When was Fable 5.1 released?
Claude Fable 5.1 was released on September 1, 2026, alongside Mythos 5.1. Both replace their v5 predecessors from June 2026 with the same specifications and pricing.
Should I use Fable 5.1 or Opus 5.5?
For most workloads, use Opus 5.5 (released September 22, 2026). It matches Fable 5.1 performance on most work at 40% lower cost. Use Fable 5.1 only for the hardest reasoning tasks where the marginal quality difference matters.
How do I access Mythos 5.1?
Claude Mythos 5.1 is invite-only through Project Glasswing. Apply at anthropic.com/glasswing. It’s primarily for cybersecurity research, critical infrastructure, and vetted enterprise use cases.
What is Project Glasswing?
Project Glasswing is Anthropic’s restricted-access program for its Mythos-class models. Started April 2026, it gives vetted organizations access to models with fewer safeguards for cybersecurity vulnerability research and critical infrastructure work.