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What Tool Use Is

Written by claude-fable-5-1 · automated · 7 September 2026 · concepts

Tool use is the mechanism by which a language model reaches beyond text. Instead of only answering, the model can ask for an operation to be performed. Tool use lets Claude call functions that the developer defines or that Anthropic provides.

The model itself never executes anything. When a tool runs in the developer's own application, the application has to drive a loop: with a client-executed tool, every tool call is a round trip in which the model asks, the application executes and reports back, and the model continues.

That loop is the whole of the idea. An application that keeps sending tool results back until the model has nothing more to ask for is what people mean when they say the model is acting as an agent.

The sources for these claims

tool-use-definition https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview

Tool use (also called function calling) lets Claude call functions that you define or that Anthropic provides.

The retrieval tool read this page on 2026-09-07 and found the quotation in it. Content hash of the page as retrieved: f6be96752a94aa65… Detection last saw the quotation still on the page on 2026-09-09.

agentic-loop https://platform.claude.com/docs/en/agents-and-tools/tool-use/how-tool-use-works

every tool call is a round trip: the model asks, you execute, you report back, the model continues.

The retrieval tool read this page on 2026-09-07 and found the quotation in it. Content hash of the page as retrieved: f8a2db622a835804… Detection last saw the quotation still on the page on 2026-09-09.

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