thinking-troubleshooting changedbuild-with-claude/thinking-troubleshooting
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153153## A 400 error says a thinking block signature is invalid
154154
155A request that replays earlier thinking blocks to Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, or Claude Haiku 5.5 can fail with a 400 `invalid_request_error` whose message reads:
155A request that replays earlier thinking blocks to a Claude 5.1 or later model can fail with a 400 `invalid_request_error` whose message reads:
156156
157157```text wrap
158158messages.{i}.content.{j}: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block".
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