Glossary
What Is Function Calling in AI?
Function calling is a feature that lets an AI model decide to run one of your predefined tools, such as looking up an order or booking an appointment, and then use the result to answer the user. The model picks the right tool and fills in the details, but your own code does the actual work.
Most AI chatbots only know what they were trained on. Function calling fixes that gap. You give the model a short list of tools it's allowed to use, each with a name and a few inputs, and the model decides when to call one based on what the person asks.
Here's a plain example. A customer types, "Is the blue jacket in size medium in stock?" Without function calling, the bot can only guess. With it, the model sees you have a tool called check_inventory that needs a product and a size. It fills in "blue jacket" and "medium," your code runs the lookup against your real inventory, and the answer comes back. The bot then replies with the live number instead of a made-up one.
The key thing to understand is that the model doesn't run anything itself. It only outputs a structured request, like "call check_inventory with these values." Your system reads that request, runs the function, and hands the result back to the model so it can write a normal, friendly reply.
This is what turns a chat widget on your website from a talker into a doer. The same idea powers a voice agent that can quote a price, check if a time is open, or start a return while the caller is still on the line.
You stay in control of what the AI can touch. You decide which tools exist, what data they reach, and what they're allowed to change. If a tool isn't on the list, the model can't use it.
Related terms
Frequently asked questions
Does function calling mean the AI runs code on my server by itself?+
No. The model only suggests which tool to call and what inputs to use. Your own code decides whether to run it and does the actual work, so you keep full control over your data and systems.
Why not just let the AI answer from memory instead of calling a function?+
Because the model's training data is fixed and doesn't know your live prices, stock, or schedule. Function calling pulls in real, current information from your systems, so the bot gives accurate answers instead of guessing.
Do I need function calling for a website chatbot or voice agent?+
You need it whenever the bot has to do something specific, like check an order, book a slot, or quote a real price. For answering general questions from your help docs, it isn't required, but it's what lets the agent actually complete tasks.