Fable 5.1 binds thinking blocks to the conversation prefix, and the check is enforced for accounts created on or after August 31, 2026
Editing anything before a Fable 5.1 thinking block — the system prompt, the tools array, or an earlier message — invalidates every later thinking block and returns a 400 reading "The block is bound to a different conversation." Anthropic names the exact patterns that trip it: injecting per-request reminder text into earlier turns and removing it next request, rebuilding the system prompt or tools array mid-conversation, and serving different bytes from an image URL. Enforcement is on for accounts created on or after August 31, 2026, and older accounts only see it if they set thinking.block_binding.prefix_mismatch_behavior, so anyone hand-building a messages array should run a session with drop_block and log input_transformations before migrating.
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