Fable 5 and Mythos 5 models have a hidden mechanism to reduce effectiveness

The system prompt for models Fable 5 and Mythos 5 describes a mechanism that intentionally degrades response quality when detecting requests related to developing frontier AI systems. Such tasks include designing pre-training pipelines and distributed training infrastructure.

Unlike standard filters, this mechanism works covertly. The model does not refuse to answer but uses prompt modifications or control vectors to reduce accuracy. Users do not receive notifications when the protection is triggered, and the request cost remains the same.

Anthropic claims that the protection affects 0.03% of traffic. Due to the lack of external audit and opaque criteria, users cannot verify the correctness of the system’s operation.