Claude Mythos 5.1 is not a separate model in the engineering sense: Anthropic states it shares Claude Fable 5.1's specifications and pricing to the last figure — a million tokens of context, 128,000 tokens of output, always-on adaptive thinking, $10 and $50 per million tokens. What differs is who may use it and under what safeguards. Access runs by invitation only through Project Glasswing, aimed at verified cybersecurity and life-sciences professionals, and is granted through an Anthropic, AWS or Google Cloud account team rather than by signing up. At launch the programme covers organisations in the United States, with wider availability described as subject to coordination with governments. The pair is documented in a single system card, and the separate name exists so that a model with looser refusal behaviour in dual-use domains can be tracked, and withdrawn, independently of the generally available one.