Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, the same model with different restrictions. Fable is open to everyone. Mythos requires verification for cybersecurity and life sciences work. With open protein-design tools, almost 50% of designs worked across 12 targets, versus 10-15% normally. Binding affinity was 10 times higher than leading Adaptyv Bio contest entries for 3 targets.

The model also built a new elevation map for a third of Venus from Magellan radar images. Resolution improved to 2 to 3 km. Terminal-Bench-Science reached 52.6%, up from 24.7%. Cached reads are 75% cheaper, cutting typical task costs by about 25% and agentic costs by up to 45%.