On September 2, Anthropic unveiled Claude Fable 5.1, incorporating its most stringent anti-distillation mechanism to date. This mechanism safeguards 'thought blocks' by verifying context consistency. Specifically, if the thought block submitted by the client fails to align with the system prompts, tools, and historical messages utilized during its generation, the API will promptly return an error. For legitimate developers who may need to modify the context, a 'non-strict mode' is provided; however, in this mode, the system will eliminate the thought blocks.

Illegal distillation has escalated to an industrial scale. Hackers are employing tactics such as fake accounts, automated scripts, and context injection to pilfer the reasoning logic of top-tier models. They then use this stolen logic to train small models that lack safety safeguards, thereby heightening the risk of AI capabilities being divorced from safety considerations. This alarming trend is the primary impetus behind Anthropic's new regulations.

The implementation of these new rules will be carried out in stages. Initially, they will target new API accounts created after 12:00:00 AM UTC on August 31, 2026, leaving existing API accounts and regular consumer users untouched. Developers affected by these changes must ensure absolute context consistency or activate non-strict mode to adapt their code logic. Eventually, the mechanism will encompass all accounts.

Furthermore, by enforcing context consistency, the new rules not only enhance prompt cache hit rates but also reduce API call costs for legitimate developers and accelerate response times. These new regulations are widely regarded as a pivotal moment for the AI industry, significantly constricting the room for illegally distilled small models to survive and thrive.