Claude Mythos 5.1

Claude Mythos 5.1 is a proprietary language model from Anthropic, released on Sep 1, 2026.

At $10 input and $50 output per million tokens, it ranks #38 of 40 for input price, placing it among the higher-priced models tracked here.

Its context window of 1.0M tokens ranks #10 of 41 for context depth in this dataset.

It targets coding, agents, reasoning, research workloads.

Same underlying model as Claude Fable 5.1 (identical 1M context, 128K max output, adaptive-always-on thinking, default effort high, Jun 2026 knowledge cutoff, and pricing) but offered separately, by invitation only, as part of Anthropic's Project Glasswing — request access via an Anthropic, AWS, or Google Cloud account team, not available as a general API model. Beats sibling Fable 5.1 on Terminal-Bench 4.0 specifically; the two are otherwise presented as equivalent. Model ID claude-mythos-5-1 (anthropic.claude-mythos-5-1 on Bedrock), retirement not sooner than Sep 1 2027. Source: https://platform.claude.com/docs/en/models/mythos-5-1/overview