I have a fairly large web application with ~200k LoC.
Gave the same prompt to Sonnet 4.5 (Claude Code) and GPT-5-Codex (Codex CLI).
"implement a fuzzy search for conversations and reports either when selecting "Go to Conversation" or "Go to Report" and typing the title or when the user types in the title in the main input field, and none of the standard elements match, a search starts with a 2s delay"
Sonnet 4.5 went really fast at ~3min. But what it built was broken and superficial. The code did not even manage to reuse already existing auth and started re-building auth server-side instead of looking how other API endpoints do it. Even re-prompting and telling it how it went wrong did not help much. No tests were written (despite the project rules requiring it).
GPT-5-Codex needed MUCH longer ~20min. Changes made were much more profound, but it implemented proper error handling, lots of edge cases and wrote tests without me prompting it to do so (project rules already require it). API calls ran smoothly. The entire feature worked perfectly.
My conclusion is clear: GPT-5-Codex is the clear winner, not even close.
I will take the 20mins every single time, knowing the work that has been done feels like work done by a senior dev.
The 3mins surprised me a lot and I was hoping to see great results in such a short period of time. But of course, a quick & dirty, buggy implementation with no tests is not what I wanted.