Claude Fable 5 blocks biology and chemistry queries

Anthropic released Claude Fable 5 on 9 June 2026, the first Mythos-class model with built-in classifiers that block queries in biology, chemistry, cybersecurity, and chemical distillation. Blocked sessions fall back to Claude Opus 4.8. Routine queries like "what are mitochondria" triggered the block, raising questions about self-regulation versus internal constraints.