Function calling / tool use
Function calling, or tool use, is a language model’s ability to request that a defined external operation be run (look up a record, check a calendar, create a booking), receive the result, and continue the conversation using it.
Also called: tool use, tool calling, actions
A model on its own can only produce text. Function calling gives it a set of named tools, each with a description and the inputs it takes. When the conversation needs one, "is Thursday at two free?", the model emits a structured request instead of a guess, the surrounding system runs the request against the calendar, and the answer comes back into the conversation for the model to relay.
This is the mechanism behind real-time integration. A post-call integration sends data out after the conversation; function calling lets the agent read and write systems while the caller is still on the line, which is what turns "I’ll pass that on" into "you’re booked". Under the hood the tools are usually APIs or webhooks.
It also bounds what the agent can do. An agent can only take the actions it has been given tools for, which makes the tool list a clear statement of what it can do and a natural place for guardrails.
Related
aiReceptionist does this: Workflow builder (self-serve)