Claude Mythos 5.1

Claude Mythos 5.1 is a limited-access Anthropic model released on September 1, 2026, as claude-mythos-5-1.

Lab: Anthropic
Release date: September 1, 2026
Status: Limited access
API model ID: claude-mythos-5-1
Context window: 1,000,000 tokens
Max output: 128,000 tokens

Anthropic released Claude Mythos 5.1 on September 1, 2026, the date in the Claude Platform release notes, as claude-mythos-5-1, with a 1M-token context window by default and 128K max output tokens. Access is limited: Anthropic makes it available to vetted cyber defenders through its Cyber Verification Program and to life sciences professionals through its Life Sciences Verification Program, currently for a set of US organizations.

The release notes list it for Project Glasswing participants. It also powers Anthropic’s Claude Security vulnerability-scanning product.

Availability: Vetted organizations only, through the Cyber Verification Program and the Life Sciences Verification Program (currently a set of US organizations); Project Glasswing participants; Claude Security.
Official source: anthropic.com
Last checked: October 5, 2026

Questions

Who can access Claude Mythos 5.1?

Vetted cyber defenders through the Cyber Verification Program and life sciences professionals through the Life Sciences Verification Program, currently a set of US organizations, plus Project Glasswing participants.

What is the context window of Claude Mythos 5.1?

It has a 1M-token context window by default and 128K max output tokens.

When was Claude Mythos 5.1 released?

Anthropic released it on September 1, 2026, the date in the Claude Platform release notes.

What does Claude Mythos 5.1 power?

It powers Anthropic’s Claude Security vulnerability-scanning product.

Source

Release dates, model IDs and limits come from each lab’s own announcement, changelog, documentation or deprecations page, linked on every model with the date it was last checked. How we pick and check sources: editorial standards.