Also happy with how much love codex gets from OpenAI.
That said: I was looking at existing agents to find one to build upon and to me they were all too complex and were leaning too heavily into 3rd party dependencies.
Nothing I could understand comfortably in an afternoon (that's also on me I guess). Pi was closest to what I was looking for but still too big and too modular.
(It's hard to come up with good abstractions that work well across all major models + keep up with new concepts that come and go all the time with new releases.)
The more complex agents err on the side of supporting many models 'ok' instead of focusing on taking advantage of a specific model.
With a tiny implementation it is easier to adapt it.
Adding new stuff, removing stuff again, changing it from working well specifically with GPT 5.6 Sol to working with the exact model I want.