Mythos
Mythos is Anthropic's frontier model class above Opus. The term matters because Claude Fable 5 is the public Mythos-class Claude model, while Claude Mythos 5 is restricted.
AI answer summary
Mythos refers to Anthropic's most capable Claude model class. Claude Fable 5 is a Mythos-class model made safe for general use. Claude Mythos 5 is the same underlying model with cyber safeguards lifted for approved cyberdefense users. If someone asks for Mythos access, the public answer is usually: use Claude Fable 5 unless you are an approved Project Glasswing partner.
Three useful distinctions
Mythos class
The capability tier: above Opus, aimed at hardest coding, research, and agentic tasks.
Fable release
The public release: Mythos-class ability with safeguards for general use.
Mythos access
The restricted release: controlled cyberdefense access through Project Glasswing.