Anthropic dropped two models with identical weights: Mythos 5 for trusted partners (Project Glasswing), Fable 5 for everyone else, with safeguards layered on top. The numbers are generational. 80.3% SWE-bench Pro vs GPT-5.5's 58.6. 59% HLE without tools. Stripe reportedly migrated 50 million lines of code in 24 hours. Karpathy called it a "major-version-bump step change" (23.7k likes).