Not suggesting you build it for commercial or open-source reasons... it's purely a learning exercise.
So many people are picking up these agentic tools and can suddenly build anything, but if you make your own mini Claude Code or Codex, you actually appreciate what it's doing under the hood and start using the real ones way better. So absolutely not about competing with the OpenCodes, Cursors, or Devins of the world.. but if you experiment with it, you come out understanding them better, and thus can go build the stuff only you want but better.