This is a better introduction to it: https://mariozechner.at/posts/2025-11-30-pi-coding-agent/

I have nothing against Codex's cli or OpenCode. I just found pi is sufficient for me and it's easy to understand what's going on under the hood.

By ChatGPT I mean the subscription plan, not the web UI. I only use gpt-5.3-codex with pi.