Function Calling / Tool Use

Function calling (also called tool use) is a capability where the model emits structured JSON requesting that specific tools be called with specific arguments, rather than just generating prose. The runtime executes the tool, returns the result, and feeds it back into the next model turn.

This is how AI agents do things in the world: an agent decides "I need to read a file" → emits {"tool": "read_file", "args": {"path": "..."}} → runtime reads the file → result goes back into the conversation → model continues with that knowledge.

Modern open-weight models with strong function calling: Qwen 2.5/3, Llama 3.1+, Mistral Nemo, Hermes 3 (an explicitly agent-tuned Llama fine-tune). The MCP (Model Context Protocol) standard is consolidating tool definitions across providers — a tool defined for Claude Code can be reused by other MCP-aware agents.