What it is
Tool use, often called function calling, lets a language model request that an external function run on its behalf. The model cannot search the web or query a database itself, so it emits a structured request and the surrounding system executes it.
The loop
The interaction follows a clear cycle.
- The developer describes available tools, each with a name and an input schema
- When the model decides a tool is needed, it outputs a structured call with arguments
- The application runs the tool and feeds the result back into the conversation
- The model reads the result and continues, possibly calling more tools
This separation keeps the model focused on deciding what to do, while real code does the doing.
Why it matters
Function calling turns a text predictor into an actor.
- It grounds answers in fresh, real data instead of memorized text
- It lets the model take actions like sending an email or booking a slot
- The schema makes the model output machine readable, so it plugs into normal code
Key idea
Tool use lets a model emit structured function calls that the surrounding system runs, grounding answers and enabling real actions.