How to use Aider: AI pair programming in your terminal

Aider is an open-source AI pair programmer that lives in your terminal, commits every change to git so you can always undo it, and points at almost any model, including a free local one. Here is the whole thing: install, run it in a repo, the commands you will use, and how to wire it to Claude, to OpenRouter, or to Ollama running on your own machine for zero per-token cost.