Anthropic Puts a 'Context Lock' on Claude's Thinking Chain: Change One Prompt, and the Entire Encrypted Reasoning Trace Is Voided
Starting August 31, 2026, Anthropic added ‘context binding’ to Claude Fable 5.1’s encrypted thinking chain - thinking blocks must be sent back exactly as they were produced, paired with the same system prompt, tools, and conversation history, or they’re voided outright. The move closes a distillation attack that used smaller models as decryption hostages to steal flagship-model reasoning, but it also collides with common dev techniques like sliding windows, history compression, and dynamic prompts. Includes a decision table for keeping vs. discarding thinking blocks.