AI coding glossary
Function Calling (Tool Use)
Also known as: tool calling, tool use, tool calls
In one sentence
An LLM capability where the model decides to invoke a structured function or tool with arguments, instead of (or alongside) generating text, the foundation of AI agents.
Full definition
Function calling (also tool calling, tool use) is how an LLM reaches outside its own weights to call external functions: read a file, query a database, hit an API, run a shell command. The model is given function schemas; at inference time it produces structured calls instead of text; the runtime executes them and feeds results back into the next turn. This is the building block of every AI coding agent in 2026, Claude Code, Cursor agent mode, Codex CLI, Windsurf Cascade, Cline, Antigravity 2.0 all use function calling under the hood. MCP (Model Context Protocol) standardizes the tool surface; SKILL.md standardizes the natural-language instructions wrapped around it.