Codex: Three reasons. I've used all extensively, for multiple months.

Main one is that it's ~3 times slower. This is the real dealbreaker, not quality. I can guarantee that if tomorrow we woke up and gpt-5.2-codex became the same speed as 4.5-opus without a change in quality, a huge number of people - not HNers but everyone price sensitive - would switch to Codex because it's so much cheaper per usage.

The second one is that it's a little worse at using tools, though 5.2-codex is pretty good at it.

The third is that its knowledge cutoff is further in the past than both Opus 4.5 and Gemini 3 that it's noticeable and annoying when you're working with more recent libraries. This is irrelevant if you're not using those.

For Gemini 3 Pro, it's the same first two reasons as Codex, though the tool calling gap is even much bigger.

Mistral is of course so far removed in quality that it's apples to oranges.

The Claude models are still the best at what they do, right now GLM is just barely scratching sonnet 4.5 quality, mistral isnt really usable for real codebases and gemini is kind of in a weird spot where it's sometimes better then Claude at small targeted changes but randomly goes off the rails. Haven't tried codex recently but the last time I did the model thought for 27 minutes straight and then gave me about the same (incorrect) output that opus would have in 20 seconds. Anthropics models are their only moat as demonstrated by their cutting off of tools other then Claude code on their coding plans.

I tried codex, using my same sandbox setup with it. Normally I work with sonnet in code, but it was stuck on a problem for hours, and I thought hmm, let me try codex. Codex just started monkey patching stuff and broke everything within like 3-4 prompts. I said f-this, went back to my last commit, and tried Opus this time in code, which fixed the problem within 2 prompts.

So yeah, codex kinda sucks to me. Maybe I'll try mistral.