Anthropic extends preserved thinking to Sonnet 5.5 to curb account-switching distillation attacks
ClaudeDevs · x · 2026-09-29
Anthropic is extending preserved thinking to Sonnet 5.5 to curb distillation attacks via account-switching: thinking blocks stay tied to the org that generated them, so if you switch accounts mid-session, Claude rereads the session and generates new thinking.
Docs detail: a model only reads its own thinking blocks and those of a fixed set of other models (unreadable blocks are dropped silently); if the prefix before a block—system prompt, tools, prior messages—changes, the block is invalidated and the API returns a 400 error or drops it per your setting. The prefix check is enforced by default for accounts created on or after Aug 31, 2026; older accounts must opt in via thinking.blockbinding.prefixmismatchbehavior.
Related event: Anthropic Launches Claude Sonnet 5.5: Over 30% Faster, Up to 30% Cheaper(58 posts)→
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