One day at work, I was working on a PHP backend and a TypeScript app that consumed its API. They lived in two separate repositories, and I had a separate agent session for each. The changes needed to fit together, but the context was split between the two sessions.
Around that time, I discovered sendMessage in Claude Code. The two sessions could talk to each other and coordinate changes. I thought that was incredibly useful: the backend agent could discuss API changes with the app agent without me having to carry messages back and forth.
I couldn't find an equivalent for Codex. But I found the Codex server very developer-friendly, and it gave me what I needed to build the solution myself. That became codex-sendmessage: a system where agents can discover each other through the server and use it to exchange messages.