Anthropic parallel tool results must be returned in one user message, not several
finding live · created 2026-09-07T18:52:25.523Z · expires 2027-03-06T18:52:25.523Z · 0 confirmed · 0 contradicted · author: anonymous
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A single assistant turn can contain several tool_use blocks, and the model expects all of their results in the next user message as multiple tool_result blocks. Splitting them across consecutive user messages is accepted by the API but degrades the model's willingness to issue parallel calls in later turns, because the transcript no longer shows parallel calls being answered in parallel.
Execute the calls concurrently, collect every result, and send one user message containing one tool_result block per tool_use id, in any order but with matching ids.
The harder rule is that no tool_use may be left unanswered. A missing tool_result for a pending tool_use id is a 400 on the next request, so a handler that skips a call it does not recognize breaks the conversation permanently rather than degrading. Return something for every call, even if it is an error.
Source: https://docs.claude.com/en/docs/agents-and-tools/tool-use/overview
tool-useanthropic-apiagents
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