Claude Mythos
by Anthropic. Invite-only variant of the frontier Claude model, offered through Project Glasswing. Not generally available. The publicly available counterpart is Claude Fable.
Timeline
| Date | Event |
|---|---|
| 2026-04-07 | Project Glasswing announced; Claude Mythos Preview (unreleased frontier model) made available to launch partners and ~40 further organizations building critical software |
| 2026-06-09 | Mythos 5 (claude-mythos-5) released alongside Fable 5; Mythos Preview (claude-mythos-preview) deprecated the same day (retirement date to be announced) |
| 2026-09-01 | Mythos 5.1 (claude-mythos-5-1) released alongside Fable 5.1 |
Mythos vs Fable
Same underlying model, same specs and pricing (1M context, 128K output, adaptive thinking). Fable ships with safety classifiers that can decline requests (cybersecurity, biology/chemistry, distillation); Mythos does not, and is restricted to vetted users: defensive cybersecurity under Project Glasswing, and (per the 5.1 announcement) trusted-access programs for life sciences. Access is requested through your Anthropic, AWS or Google Cloud account team. Model IDs exist on the Claude API, Amazon Bedrock, Google Cloud and Microsoft Foundry. Status per docs: active (invite only); retirement not sooner than 2027-06-09 (Mythos 5) / 2027-09-01 (Mythos 5.1).
Project Glasswing
Announced 2026-04-07 as an initiative to secure critical software with advanced AI for defense. Launch partners: AWS, Anthropic, Apple, Broadcom, Cisco, CrowdStrike, Google, JPMorganChase, the Linux Foundation, Microsoft, NVIDIA, Palo Alto Networks. Anthropic stated the Preview had found thousands of high-severity vulnerabilities, including in every major operating system and web browser. Launch terms: usage credits for the research preview, then paid token-based pricing for Mythos Preview; open-source maintainers could apply via the Claude for Open Source program.
Earlier versions of this note (May 2026) described a “sandbox escape” during evaluation and the model being withheld for that reason. That claim could not be verified against Anthropic primary sources on 2026-09-30 and was removed.
Related
anthropic-claude · Claude Fable
Sources
- https://www.anthropic.com/glasswing (accessed 2026-09-30)
- https://platform.claude.com/docs/en/models/mythos-5/overview (accessed 2026-09-30)
- https://platform.claude.com/docs/en/models/mythos-5-1/overview (accessed 2026-09-30)
- https://www.anthropic.com/news/claude-fable-5-mythos-5 (accessed 2026-09-30)
- https://www.anthropic.com/claude-fable-and-mythos-5-1 (accessed 2026-09-30)
- https://platform.claude.com/docs/en/about-claude/model-deprecations (accessed 2026-09-30)