Anthropic's preserved thinking feature in Claude Fable 5.1 and Claude Opus 5.5 stops API accounts created on or after August 31, 2026 from editing Claude's prior context

Anthropic describes new defenses against illicit distillation of Claude, including preserved thinking, a two-stage classifier, and account bans, which cut extracted reasoning tokens 10-100x. It argues distillation threatens national security and urges all developers and platforms to act.