PRODUCTION SCENARIO
With extended thinking enabled, Claude returns an assistant turn holding a thinking block with a signature followed by a tool_use block. The client has run the tool and is sending the tool_result back.
How should the assistant turn be included in that request?
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Answer: Echo the assistant turn back exactly as returned, thinking block included
Inside a tool-use turn Claude is still constructing one response, so the assistant message goes back exactly as it arrived. Dropping the thinking block breaks the reasoning that produced the tool call and is only allowed outside tool use. The signature is what the API verifies, so omitting it fails the check even with the text intact. Rebuilding the turn from parsed blocks is the cause the docs name most often for the 400 that says thinking blocks cannot be modified.
Official Source: https://platform.claude.com/docs/en/build-with-claude/thinking ↗