Tool use (function calling)
The model's ability to emit structured calls — run a command, read a file, search — that the harness executes and feeds back.
Tool use is what separates a coding agent from a chat model. The model outputs a call with arguments, the scaffold runs it, and the result becomes the next observation. Coding agents typically expose a small set: read/write file, grep, run shell, run tests. Model Context Protocol (MCP) standardises how external tools are described; 34% of developers building agents use MCP servers per Stack Overflow 2025.