codex is written in rust fwiw

smol has implementations in Go, Python, Clojure, PHP

https://github.com/smol-env/smol

out of the box an agent only needs to be able to do http requests and call tools (which might again be just http requests or shelling out)

there is no inherent reason for why an agent has to be in JavaScript or Typescript

but they are popular languages and come with runtimes and libraries for http requests, steaming, TUI (terminal ui) and so on which can help