**📝Claude Mythos** is publicly usable today as Claude Fable 5.1 — model ID claude-fable-5-1 on 📝claude.ai, Claude Code, and the Claude API — while the less-restricted Claude Mythos 5.1 is limited to vetted organizations.

📝Anthropic first released Mythos-class models publicly on June 9, 2026 as 📝Claude Fable 5 and 📝Claude Mythos 5, and replaced them on September 1, 2026 with Claude Fable 5.1 and Claude Mythos 5.1. Anthropic describes the two as the same model with different safeguards: Fable 5.1 declines or hands off high-risk cybersecurity and life-science requests to Claude Opus models, while Mythos 5.1 relaxes those safeguards for vetted defenders and researchers. For most people, using Claude Mythos means using Claude Fable 5.1.

Key Facts

Public model: Claude Fable 5.1 (claude-fable-5-1), generally available since September 1, 2026
Restricted model: Claude Mythos 5.1 (claude-mythos-5-1), vetted organizations only
Research model: Claude Mythos Preview, Project Glasswing partners only
Pricing: $10 per million input tokens, $50 per million output tokens; cached reads $0.25 per million
Context window: 1M tokens with 128K max output — see 📝Claude Mythos Context Window
Lower-cost alternative: 📝Claude Opus 5.5 (claude-opus-5-5), $4 / $20 per million tokens, released September 22, 2026
Status as of: September 28, 2026

Sources: Anthropic — Introducing Claude Fable 5.1 and Claude Mythos 5.1, Anthropic — What's new in Claude Fable 5.1.

How to Use Claude Fable 5.1

Claude Fable 5.1 is available on every major Claude surface:

claude.ai and 📝Claude Cowork — choose Claude Fable 5.1 in the model picker; it defaults to Medium effort there.
**📝Claude Code** — run /model and select Claude Fable 5.1; it defaults to High effort in Claude Code.
Claude API — send requests to claude-fable-5-1. Adaptive thinking is always on, and forced tool use, assistant prefill, and non-default temperature return errors.
Cloud platforms — Amazon Bedrock (anthropic.claude-fable-5-1), 📝Google Cloud Vertex AI (claude-fable-5-1), and Microsoft Foundry.
Third-party tools — editors and agents that accept an Anthropic model ID or API key, such as 📝Cursor, can call claude-fable-5-1 directly.

How to Get Claude Mythos 5.1 Access

Claude Mythos 5.1 has no self-serve sign-up. Anthropic's model documentation lists it for approved 📝Project Glasswing customers and directs organizations to request access through their Anthropic, AWS, or Google Cloud account team.

Anthropic's launch announcement adds two vetting paths: vetted cyberdefenders through its Cyber Verification Program, and life-science researchers through a program run in partnership with the US government. As of September 2026, access is limited to US organizations.

Open-source maintainers seeking Mythos-class security tooling can apply through Anthropic's Claude for Open Source program, which the Project Glasswing page names as their entry point. Source: Anthropic — Project Glasswing.

Choosing Between Fable 5.1, Opus 5.5, and Mythos 5.1

Claude Opus 5.5 is the practical default for most work. Anthropic reports it performs at the level of Claude Fable 5.1 on most tasks at 40% lower cost, with the same 1M-token context window. Source: Anthropic — Introducing Claude Opus 5.5.

Claude Fable 5.1 fits the hardest workloads: long-running agentic coding, multistep research, and tasks where Opus-tier models still fall short at high effort. Claude Mythos 5.1 is only relevant to organizations doing defensive cybersecurity or life-science research that Fable 5.1's safeguards would block.

Using Claude Mythos With Your Own Context

📝MythOS — a knowledge-management platform unrelated to Claude Mythos — connects to Claude clients through the 📝Model Context Protocol (MCP). It gives Claude Fable 5.1, Opus 5.5, or any other Claude model access to a personal knowledge library, so the model works from the user's own memos and perspectives rather than a blank context.

FAQ

Is Claude Mythos publicly available?

Yes. Claude Mythos is publicly available as Claude Fable 5.1, released September 1, 2026. The less-restricted Claude Mythos 5.1 and the research-only Claude Mythos Preview remain limited to vetted organizations.

Can I use Claude Mythos for free?

Anthropic has not announced free access to Claude Fable 5.1 or Claude Mythos 5.1. API use is billed at $10 per million input tokens and $50 per million output tokens, and claude.ai access follows each plan's usage limits.

What is the difference between Claude Fable 5.1 and Claude Mythos 5.1?

They are the same model with different safeguards. Fable 5.1 declines or hands off high-risk cybersecurity and life-science requests; Mythos 5.1 relaxes those safeguards for vetted organizations. Specifications and pricing are identical.

How do I apply for Claude Mythos 5.1?

Contact your Anthropic, AWS, or Google Cloud account team, or apply to Anthropic's Cyber Verification Program if you do defensive security work. Access is currently limited to vetted US organizations.

Can I use Claude Mythos in Cursor?

Yes, through Claude Fable 5.1. Any client that accepts an Anthropic model ID or API key can call claude-fable-5-1. See 📝How to Use Claude Mythos with Cursor.

Is Claude Mythos the same as MythOS?

No. MythOS is an independent knowledge-management platform at mythos.one, in commercial use since 2018. Claude Mythos is an Anthropic model family that shares the name. See 📝MythOS vs Claude Mythos.

📝Claude Mythos — canonical reference for the model tier
📝Claude Mythos Context Window — 1M-token specification and capacity
📝Claude Mythos Release Date and What We Know So Far — release timeline
📝Claude Mythos for Developers — practical guidance for builders
📝How to Use Claude Mythos with Cursor — editor-specific setup
📝MythOS vs Claude Mythos — name disambiguation

We use Fable 5.1 for the long agentic sessions where it earns its price, and Opus 5.5 for nearly everything else. The model matters less than the context it works from — which is why we built MythOS.