cliAnthropic's CLI for serious AI-assisted coding, and the tool that taught me how to talk to an AI like a real collaborator. It lives in your terminal, reads your whole project, and holds context across sessions through a CLAUDE.md file you write yourself. The standout features are deep file awareness, persistent project memory, and multi-file edits that actually land where you expect them. A few months ago this was my unhesitating first recommendation, and it's still where I'd send anyone who wants to learn the craft properly. I'd reach for it when you care about understanding what the model is doing, not just shipping the output. The skills you build here transfer to every other tool on this list.