# Claude Mythos 5.1

Claude Mythos 5.1 is an Anthropic language model (limited access) released Sep 1, 2026; Anthropic's newest language model in our dataset is Claude Haiku 5.5. Proprietary, API access only. Anthropic's access-limited Claude Mythos successor for Project Glasswing participants, launched with Claude Fable 5.1 (release-notes date Sep 1, 2026). Verified against vendor sources on Oct 26, 2026.

Last verified: 2026-10-26 · Evidence: Reported · Canonical: https://theagenticweekly.com/models/claude-mythos-5-1 · JSON: https://theagenticweekly.com/api/models/claude-mythos-5-1

## Key facts

| Fact | Value | Source |
|---|---|---|
| Vendor | Anthropic |  |
| Released | Sep 1, 2026 | https://platform.claude.com/docs/en/release-notes/overview |
| API id | `claude-mythos-5-1` | https://platform.claude.com/docs/en/release-notes/overview |
| Context window | 1,000,000 tokens (1M) | https://platform.claude.com/docs/en/release-notes/overview |
| Max output | 128,000 tokens | https://platform.claude.com/docs/en/release-notes/overview |
| Licence | Proprietary (API) | https://platform.claude.com/docs/en/about-claude/models/overview |
| Pricing | $10 in / $50 out per 1M tokens | https://platform.claude.com/docs/en/release-notes/overview |

## What we tested

Nothing yet. We have not run Claude Mythos 5.1 ourselves, so every fact here is Reported from the sources below.

## Not verified, left out

- Anthropic's newsroom announcement for Fable 5.1 and Mythos is dated Sep 15, 2026 on our Fable 5.1 page; this page uses the API release-notes date.

## Sources

- [Claude Platform release notes](https://platform.claude.com/docs/en/release-notes/overview)
- [Anthropic models overview](https://platform.claude.com/docs/en/about-claude/models/overview)

Data licence: CC BY 4.0. Data: Agentic Weekly (https://theagenticweekly.com), CC BY 4.0.