What is function calling?

Function calling lets a model respond with a structured request to invoke a function you defined, arguments included, instead of prose. Your code executes the call and returns the result; the model continues with real data. It is the wire-level mechanism underneath tool use and agents.

You describe each function with a name, a purpose, and a JSON schema for its arguments. The model decides when a call is needed and emits arguments matching the schema; malformed or hallucinated arguments are the failure mode that separates models on this task.

On an OpenAI-compatible wire, tool definitions ride the request and tool calls ride the response, so function calling works identically across every language model behind the gateway.

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