Claude Mythos

Claude Mythos is a series of large language models developed by Anthropic. It is the most complicated set of models in the Claude product line. The first model in the series was Claude Mythos Preview. Anthropic did not release the model to the public, citing the model's ability to find software vulnerabilities. Starting in April 2026 under the name Project Glasswing, some companies were given access to Claude Mythos to scan for security vulnerabilities in critical software. The public had mixed reactions to the announcement of Claude Mythos Preview.

In June 2026, Anthropic publicly released Claude Fable 5, a "Mythos-class" model made available for general use with a set of safeguards, alongside Claude Mythos 5, a restricted-access version of the same underlying model with those safeguards lifted in some areas. According to Anthropic, the two models are identical apart from their safeguards; when Fable 5's classifiers flag a request relating to cybersecurity, biology and chemistry, or model distillation, the response is instead handled by the less capable Claude Opus. Mythos 5.1 and Fable 5.1 were released in September 2026.

According to industry estimates reported by the Financial Times, Mythos has approximately 8 trillion parameters, while Fable 5 has approximately 5 trillion parameters.