The ability of an AI model to call external functions or tools during a conversation. Instead of just generating text, the model can decide to search the web, run code, query a database, or call an API — then incorporate the results into its response. The model outputs a structured "tool call" that the host application executes.
Why it matters
Tool use is what makes AI models actually useful beyond conversation. It's the mechanism behind code interpreters, web-browsing AI, and every AI agent. Without it, models are limited to what's in their training data.