Tool Use with Claude: Client Tools, the Tool Loop, Strict Mode and Server Tools
For tool use in Claude you define each tool with a name, a description and a strict JSON Schema, run the tools while stop_reason is tool_use and send all tool_results back in a single user message; authorization belongs inside the tool. tool_choice any returning 400 on Opus 5.5, parallel calls, web search and tool search, the SDK's tool runner and testing the loop with a fake HttpMessageHandler.
15 Nov 2026
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18 min
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5
