If you use Codex or Claude Code in a terminal, you’ve seen the shift: the tool doesn’t just answer questions about your code. It reads files, edits them, runs commands, and reacts to the output. That’s the practical meaning of “agent”, and it changes what you supply and what you check.

This post is for developers who use these tools but haven’t studied how agents work. We’ll walk through one small change in an unfamiliar repository and mark where the engineer sets the boundary and verifies the outcome.