Update: Anthropic has walked back this policy after outrage from developers . The company now says Fable 5's safeguards for frontier LLM development will be visible to users instead of silently degrading the model.

I didn't expect to read this in a model card.

Fable 5 model card :

we’ve implemented new interventions that limit Claude’s effectiveness for requests targeting frontier LLM development (for example, on building pretraining pipelines, distributed training infrastructure, or ML accelerator design). Using Claude to develop competing models already violates our Terms of Service, but enforcing this restriction through our safeguards avoids accelerating the actors most willing to violate these terms. Unlike our interventions for cybersecurity, biology and chemistry, and distillation attempts, these safeguards will not be visible to the user. Fable 5 will not fall back to a different model.…