How Claude's Tool Use API Works: A Developer's Practical Breakdown

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Anthropic's Claude Messages API supports tool use through two content block types — tool_use and tool_result — that developers wire together in a loop they build themselves. When a tool is declared, the model returns a tool_use block with a unique ID and parsed input, but does not execute any code itself. Developers must run the tool, then append both the assistant's response and a user-role tool_result block containing the output before re-sending the full message array. A mismatched tool_use_id between request and result triggers a 400 API error, making accurate ID echo-back critical. Errors should be returned using the is_error flag rather than suppressed, allowing the model to handle failures gracefully in its final response.

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