Claude Mythos 5.1

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 at launch was 60.9% on Terminal-Bench 4.0, five points above Fable 5.1's 55.8% under the same harness.