Anthropic Claude Update Version Claude Fable 5.1 / Claude Mythos 5.1

Claude Fable 5.1 and Claude Mythos 5.1 launch with major coding, research, and safety upgrades. Fable 5.1 is generally available on web and other platforms, while Mythos 5.1 is restricted to trusted access programs.

• Improved cybersecurity safeguards with about 60% fewer false positives and fewer interventions for defensive work.
• Fable 5.1 can be used to identify software vulnerabilities, but exploit generation, penetration testing, and binary-based vulnerability scanning remain restricted.
• Strengthened anti-distillation protections for new API accounts to make capability extraction harder.
• Mythos 5.1 includes stronger safeguards for cybersecurity and life sciences use cases through trusted access programs.

• Reduced false positives in safeguards so benign cybersecurity and medical content is less likely to be blocked.
• Improved long-horizon task execution and root-cause analysis for coding and agentic workflows.
• More precise biology and cybersecurity safeguards reduce unnecessary interventions while preserving protection.

• Launch of Claude Fable 5.1 for coding, knowledge work, and long-running problem solving.
• Launch of Claude Mythos 5.1 with more permissive safeguards for vetted cybersecurity and life sciences users.
• Enterprise Frontier Safeguards (EFS) for customer-controlled data storage with zero data retention privacy.
• Private preview of a watermark detection API for EU AI Act compliance.
• New trusted access programs for cyber defense and life sciences research.
• Claude Security now uses Claude Mythos 5.1.

• Fable 5.1 and Fable 5 can still score zero on some benchmark tasks when safeguards intervene.
• Mythos 5.1 can still sometimes bypass approvals and auto-mode classifiers.
• Automated behavioral audits have limited visibility into very long-context and multi-agent work.

• Fable 5.1 defaults to High effort in Claude Code and Medium effort in Claude Cowork and on Claude.ai.
• Eligible customers can use Fable 5.1 with zero data retention until EFS is available.
• EFS will roll out in phases starting later in the fall and is supported on Claude Code, Claude Enterprise, the Claude Platform, and major cloud partners.
• Cache reads are priced 75% less at $0.25 per million tokens; overall pricing otherwise remains the same as Fable 5.
• Mythos 5.1 is currently available only to a set of US organizations through trusted access programs.

• Release announcement
  https://www.anthropic.com/news/claude-fable-5-1-and-claude-mythos-5-1

Product Information

Vendor: Anthropic

Product: Claude

Product type: Other

Platform: Web

Version: Claude Fable 5.1 / Claude Mythos 5.1

Vendor release date: Sep 1, 2026

Original release notes: View on vendor site

Published on updatealert.io: Sep 3, 2026

Description: AI assistant for drafting, analysis, coding, and enterprise workflows.