Claude Mythos 5.1
Released
Claude Mythos 5.1 is an AI model released by Anthropic on Tuesday, Sep 1 2026, the same day as Claude Fable 5.1. It has a 1M token context window. Benchmark results (shown below) cover Terminal-Bench 4.0.
Benchmarks
About
Claude Mythos 5.1, released September 1, 2026 alongside Claude Fable 5.1, continued the pairing Anthropic set up with the Claude 5 generation: the same frontier model as that day's Fable release, offered by invitation only to Project Glasswing participants without the dual-use safety measures that let Fable ship to everyone. It succeeded Claude Mythos 5 and shared Fable 5.1's specifications and pricing — a 1M-token context window, 128K output tokens, always-on adaptive thinking, $10 per million input tokens and $50 per million output, with cache reads cut to $0.25 per million.
Access went through Anthropic, AWS or Google Cloud account teams rather than the console, and the model was listed on the Claude API, Amazon Bedrock, Google Cloud and Microsoft Foundry at launch. Like Mythos 5 before it, it required 30-day data retention and was not offered under zero data retention without express authorisation. The one documented behavioural difference from Fable 5.1 was that Mythos 5.1 did not enforce the check that rejects a conversation whose earlier turns were edited after a thinking block was produced. The one figure Anthropic published for it was 60.9% on Terminal-Bench 4.0, five points above Fable 5.1's 55.8% under the same harness.
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Frequently asked questions
Claude Mythos 5.1 was released by Anthropic on Tuesday, Sep 1 2026.
Claude Mythos 5.1 was built by Anthropic. AI safety company building the Claude family of models. Founded in 2021 by former OpenAI researchers.
Claude Mythos 5.1 reports 1 tracked benchmark score — Terminal-Bench 4.0: 60.9%. Scores are the figures published at release by Anthropic. It holds the best score among all models tracked here on Terminal-Bench 4.0.
Claude Mythos 5.1 has a context window of 1M. That is the maximum amount of input plus output the model can hold in a single request.
No. Claude Mythos 5.1 is a proprietary model. The weights are not published — it is available only through the provider's own API, apps, or partner platforms.
Anthropic's previous tracked release was Claude Fable 5.1 on Sep 1 2026. It is the most recent Anthropic model tracked on AI Release Tracker.