“The era of large model distillation is over? Fable 5.1’s rewritten API completely eliminates the possibility of distillation.”
On September 2, Anthropic launched Fable 5.1, which strictly verifies the consistency of the “thinking blocks” in API calls, completely blocking the path for illegal distillation of large models. This mechanism requires that the thinking blocks returned by clients must be exactly consistent with the system prompts, tools, and historical messages that generated them; any alteration will result in a service rejection. Additionally, a “non-strict mode” is available, allowing context modifications while automatically deleting thinking blocks to maintain dialogue continuity. This update aims to curb the use of fake accounts and automated scripts to大规模 steal model reasoning logic, preventing the risk of AI becoming uncontrollable due to the “disconnection between capabilities and security”. The new rules apply only to new API accounts created after August 31, 2026; existing old accounts and C-end users are not affected. Legal developers can migrate by maintaining context consistency or enabling the non-strict mode, which will also significantly reduce call costs and shorten response times by improving prompt cache hit rates.