Function calling
Function calling is the mechanism by which a language model returns a structured, schema-conforming request to invoke a named function, instead of returning free text.
It is the foundation of agent behaviour and of any reliable structured output. Validation still belongs on the application side: a model can emit a call that satisfies the schema and is still semantically wrong, so arguments should be checked before anything irreversible happens.
Related terms
- Tool use
Tool use is a language model’s ability to call defined external functions — querying a database, sending a request, performing a calculation — by emitting a structured call that an application executes and returns results for.
- MCP (Model Context Protocol)
The Model Context Protocol is an open standard that defines how AI applications connect to external tools and data sources through a server interface, so one integration works across any MCP-compatible client.
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