Anthropic’s early September 2026 launch of Claude Mythos 5.1 and Fable 5.1, which posted leading Humanity’s Last Exam (HLE) scores around 59–65% on the official Scale AI leaderboard, sets the baseline for the next Mythos-class model. These frontier large language models benefit from continued scaling, tool-augmented reasoning, and targeted gains in math, physics, biology, and cybersecurity domains, outpacing OpenAI’s GPT-6 Astra (54.7%) and rivals clustered in the mid-40s. HLE’s 2,500 expert-authored questions, designed to resist saturation and require graduate-level expertise, make debut performance sensitive to any new Anthropic release or capability leap before year-end. Traders watch for upcoming model announcements, verified leaderboard updates, and competitive responses from Google DeepMind or Meta that could shift the implied probability of clearing higher accuracy thresholds on debut.