# Claude Mythos 5

Claude Mythos 5 is an AI model released by Anthropic on Jun 9 2026. It has a 1M context window.

## Facts

| Field | Value |
| --- | --- |
| Model | Claude Mythos 5 |
| Developer | Anthropic |
| Release date | Tuesday, Jun 9 2026 |
| Licensing | Proprietary |
| Context window | 1M |

## About Claude Mythos 5

Claude Mythos 5, released June 9, 2026 alongside Claude Fable 5, is the unrestricted half of a pair: the same Claude 5 frontier model offered to vetted organisations without the dual-use safety measures that let Fable 5 ship to everyone. Where Fable 5's classifiers flag a request touching cybersecurity, biology and chemistry, or model distillation and hand it to a less capable model, Mythos 5 answers it. It succeeded Claude Mythos Preview, which Anthropic disclosed on April 7, 2026 and put in the hands of roughly forty organisations under Project Glasswing — a group that had grown to about 150 across more than fifteen countries by June 2.

Anthropic published no separate benchmark table for the model. In testing by the UK AI Security Institute the Mythos line ranked first for cyber capability, ahead of Claude Opus 4.6 and a tie between GPT-5.4 and GPT-5.3-Codex, and Anthropic reported vulnerabilities found across every major operating system and browser, including 271 in Firefox that Mozilla confirmed. Access moved faster than capability: on June 12, 2026 the US government barred both Mythos 5 and Fable 5 to non-US nationals on national-security grounds, a restriction the Department of Commerce lifted on June 30. On August 21, 2026 Anthropic opened Mythos 5 to Claude Enterprise customers as scans inside Claude Security, with integrations into partner security products to follow — the same deliver-the-output-not-the-model posture OpenAI took with Daybreak and Google with the CodeMender pilot.

## Questions and answers

### When was Claude Mythos 5 released?

Claude Mythos 5 was released by Anthropic on Tuesday, Jun 9 2026.

### Who made Claude Mythos 5?

Claude Mythos 5 was built by Anthropic. AI safety company building the Claude family of models. Founded in 2021 by former OpenAI researchers.

### What is the context window of Claude Mythos 5?

Claude Mythos 5 has a context window of 1M. That is the maximum amount of input plus output the model can hold in a single request.

### Is Claude Mythos 5 open source?

No. Claude Mythos 5 is a proprietary model. The weights are not published — it is available only through the provider's own API, apps, or partner platforms.

### What came before and after Claude Mythos 5?

Anthropic's previous tracked release was Claude Fable 5 on Jun 9 2026. It was followed by Claude Sonnet 5 on Jun 30 2026.


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Canonical page: https://aireleasetracker.com/model/anthropic/claude-mythos-5
Full dataset: https://aireleasetracker.com/llms-full.txt · JSON: https://aireleasetracker.com/models.json
Source: AI Release Tracker (https://aireleasetracker.com). Benchmark scores are the figures published by the releasing lab at launch.