Sonnet 5.5 becomes the first Sonnet to prevent 'reasoning extraction': internal thinking begins to be tied to accounts.

Beating AI News Flash: Anthropic has added another layer of restrictions to Sonnet 5.5's thinking blocks. Now, the thinking blocks generated by the model will be bound to the account or organization that produced them. When users switch accounts mid-session in scenarios like Claude Code, the chat history can continue, but the thinking generated by the original account will be discarded, and Claude will need to re-reason based on the existing conversation. Anthropic's API has already added checks such as organization_binding_mismatch and end_user_binding_mismatch.

Anthropic had previously introduced a "context lock" for thinking starting with Fable 5.1. If the system prompt, tools, or previous messages are modified, the original thinking may become invalid. This time, account binding has been added, primarily to prevent attackers from switching accounts in bulk and repeatedly exploiting Claude's already-generated high-quality reasoning for model distillation.

Sonnet 5.5 is also the first Sonnet equipped with an anti-"reasoning extraction" classifier. Anthropic states that Sonnet 5.5's capabilities are significantly higher than the previous generation, so it has begun adding such anti-distillation measures. The classifier mainly identifies behavior involving large-scale extraction of Claude's reasoning for training other models, and ordinary developers will barely notice any changes.