A common technique for distillation attacks involves extracting the model’s chain of thought. To make these attacks more difficult, we are introducing a change to the Messages API: it will no longer be possible to edit Claude's context prior to thinking blocks in a multi-turn conversation.

We've done this in the least disruptive way we can: it applies only to new accounts using Fable 5.1 (distillation often makes use of large numbers of new, fake accounts), although we plan to roll out this change to all users for future model releases.
We hope to find better solutions to distillation attacks in the long run.

https://t.co/km7CtFVNyV