Function Calling
An older name for tool use — getting an LLM to invoke defined functions.
Function calling is essentially synonymous with tool use, with slightly different connotations. "Function calling" was the term used early on (especially in OpenAI's API) to describe getting the model to emit a structured request to call a developer-defined function. "Tool use" became the broader term as the practice expanded beyond simple function invocation to include browser actions, computer use, and code execution.
In 2026, both terms appear in the wild and refer to roughly the same thing: structured, schema-validated invocation of defined capabilities by an LLM. MCP has standardized the protocol; function calling is the technical mechanism.
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