Codex defaults to medium, but I later switched to high.
During my time using Codex, there was one thing that always felt a bit awkward: the default thinking level is medium, but when chatting online about GPT-5.4, everyone’s tone is very strong. When it comes down to actually using it, what exactly is the difference between medium, high, and xhigh? The official documentation hasn’t provided a particularly straightforward chart.
My current conclusion is quite clear: for daily coding, I prefer to start directly at high. Medium isn’t unusable; it’s fine for quick tasks, minor tweaks, or exploring directions. But when dealing with multi-file modifications, ambiguous requirements, and needing to judge while looking at code, medium easily wastes computational power in the wrong places. I actually don’t use xhigh often; I save it for really difficult tasks where I get stuck.