Function Calling

Function calling lets a model output a structured, schema-conformant request to invoke a named function with arguments, instead of only free text. The application runs the function and returns the result for the model to reason over. It is the low-level mechanism that turns a text generator into something that can act, and it underlies both tool calls and the Model Context Protocol.

Because a function call can read data or trigger a real action, it is the highest-leverage and highest-risk thing a model does. Each call is a natural place to authorize against policy, scope to the caller's purpose, and record - so a model can only invoke what it is entitled to, with the arguments and the outcome captured as evidence.