An architect is implementing tool use with the Claude API and receives the following in the assistant response: a content block with type 'tool_use' containing an id, name, and input object. The next step requires constructing the tool_result message correctly. Which structure is correct for returning tool results to Claude?
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Short Explanation
Here's the deal — you submit the tool result as a new user message whose content array includes a tool_result block. That block must carry the matching tool_use_id and the result in its content field, with is_error optional.
Full Explanation
The correct structure is the new user message with role 'user' and content as an array containing a tool_result content block whose tool_use_id matches the original tool_use id and whose content field holds the result. The Anthropic API requires tool results to be submitted as a user-turn message where content is an array containing a tool_result content block. The tool_result block must include tool_use_id, matching the id from the tool_use block, content, which may be a result string or content array, and optionally is_error. The role of this message must be 'user'. The option that adds a new message with role 'tool' is wrong because 'tool' is not a valid role in the Anthropic messages API — only 'user' and 'assistant' are valid roles. The option that adds a new assistant message with the tool result as a string is wrong because tool results must come from the user role, not the assistant role, and must use the structured tool_result content block type rather than a plain string. The option that appends the tool result directly to the original assistant message's content array is wrong because messages are immutable after creation — you cannot append to an existing message; you must create a new user message with the tool result.