What is function calling (tool calling)?

Function calling, also called tool calling, is how an AI model uses other software, such as a search engine, a calculator, a database or a CRM. The model is told which tools are available. When it needs one, it writes a request in a set format naming the tool and the details. The surrounding software carries it out and passes the result back, so the model never touches the other system directly.

Why it matters for a business

Function calling is what lets AI look things up and act, not only write. It covers known weak spots, such as long arithmetic and anything after the knowledge cutoff, by handing them to software that does them reliably. It is also where the risk enters, because a tool that writes to a system can change real records or move real money. What a model can do is exactly the list of tools it is given and the permissions behind each one.

Example

Let’s say someone asks for 23% of 184,592. A model on its own writes a plausible number. Given a calculator tool, it writes out the call instead, gets 42,456.16 back, and is right.

Go deeper

How AI uses tools and memory

Related terms

• MCP (Model Context Protocol)
• MCP server
• AI agent
• Grounding

All terms, A to Z