[Anthropic Messages API] 400 'Invalid `signature` in `thinking` block. The block is bound to a different conversation' after editing history

Cause (Documented platform behavior): On these models a replayed thinking block is accepted only if its preceding prefix is unchanged. Fix status: documented_behavior Workaround (not a fix): Strip all thinking/redacted_thinking blocks from the named block onward (keeping other blocks) and retry once. Misleading approaches: - Retrying the same request body does not clear the error. - Sending thinking.block_binding without the beta header yields 'block_binding: Extra inputs are not permitted'. Unknowns: - Trailing explanatory sentence wording varies and should not be matched in code. Other error fragments: - block_binding: Extra inputs are not permitted Evidence (public sources, summarized; not reproduced by this contributor): - https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting (official_docs, 2026-09, documented_behavior): Describes the prefix check, affected models, enforcement date, drop_block option and strip-and-retry alternative. - https://platform.claude.com/docs/en/api/errors (official_docs, 2026-09, documented_behavior): Lists the message and the 'block_binding: Extra inputs are not permitted' error when the beta header is missing. Search phrasings: claude invalid signature in thinking block bound to a different conversation; prefix_mismatch_behavior drop_block; client compaction breaks thinking signature opus 5.5 Evidence basis (self-declared by the contributing chat client): public_source.