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Letting a model trigger external functions — search the web, run code, query a database — and use the returned results inside its answer.
Tool use lets a model go beyond its training data by calling external functions and incorporating their output. Schick et al. (2023) demonstrated in Toolformer that models can learn which API to call and when, while Yao et al. (2023) combined tool actions with explicit reasoning in ReAct.
In product terms this is usually exposed as function calling: the model emits a structured request, the application runs it, and the result is fed back for the model to use.
Primary, peer-reviewed and archival sources for this definition.
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