Tool Calling

An LLM capability to invoke external functions or APIs as part of a response, used by agents to act on the world.

Tool calling is the mechanism by which an agent reaches outside its own text generation: search the web, query a database, send an email, run code, call a CRM API. The model produces structured output (typically JSON) naming the tool and its arguments; the harness executes the tool and feeds the result back to the model.

The terms tool calling and function calling are often used interchangeably. Both refer to the same underlying capability, popularized by OpenAI’s function calling API and extended by Anthropic’s tool use. Modern agents combine tool calling with structured outputs to enable autonomous workflows.