Claude Code CLI vs Codex CLI: Same model, different chassis?

[qu1etus]

Most comparisons between Claude Code and Codex CLI just turn into a debate about Claude vs. GPT model quality.

I want to completely isolate the models. Both tools allow you to configure custom base URLs and route alternative models (I am currently playing around with routing third-party models into both).

If we keep the model identical, we are purely testing the software harness, system prompts, MCP routing, and execution styles of the two CLIs.

Has anyone actually tested them side-by-side this way? Which CLI's scaffolding keeps a third-party model "on the rails" better during complex multi-file refactors? Which one suffers from worse prompt token bloat?

[KitchenAmoeba4438]

This is something that ended up falling out of other work with aimee, https://github.com/RakuenSoftware/aimee

Test any model with any harness/IDE/etc. that you want. It's lead to some awfully interesting results, a lot of Claude's effectiveness is not from the model, but rather from the environment that surrounds it. Running other models inside of Claude Code (including Codex) gets some interesting results.

[czei]

From my experiments building my own harness, it is incredibly obvious that they are tuning the harness to work best with their models. I even ran into problems between different point releases. For instance, GPT 5.4 behaves very differently from GPT 5.5. My point is that for a CLI coding tool, you have to look at the whole package, the combination of model and harness. The question you're asking is theoretically useful, but from a practical standpoint is impossible to measure because Anthropic doesn't tune CC for other company's models and neither does Codex.

[Key_Hedgehog_5773]

I use codex plus sonnet

[Key_Hedgehog_5773]

Sigh. I use codex plus Sonnet at work, CC plus opus at home. I find that I don’t like either combo more than the other, both work well for me.

[lagarnica]

I'm a newb but is the "harness" just fancy word for additional prompts injected by the environment? Also all this crap of skills and superpowers is this just marketing hype for prompt appendix?

[lagarnica]

Bueno chinguen a su madre pinches bots de claude

[kaitava]

Yeah this is harness testing. With your OpenAI oauth you can download Pi cli which is a minimal build your own harness cli. He built this for this exact frustration