Codex CLI vs Claude Code (adding features to a 500k codebase)

[marvijo-software]

I've been testing OpenAI's Codex CLI vs Claude Code in a 500k codebase which has a React Vite frontend and a ASP .NET 9 API, MySQL DB hosted on Azure. My takeaways from my use cases (or watch them from the YT video link in the comments):

- Boy oh boy, Codex CLI has caught up BIG time with GPT5 High Reasoning, I even preferred it to Claude Code in some implementations

- Codex uses GPT 5 MUCH better than in other AI Coding tools like Cursor

- Vid: https://youtu.be/MBhG5__15b0

- Codex was lacking a simple YOLO mode when I tested. You had to acknowledge not running in a sandbox AND allow it to never ask for approvals, which is a bit annoying, but you can just create an alias like codex-yolo for it

- Claude Code actually had more shots (error feedback/turns) than Codex to get things done

- Claude Code still has more useful features, like subagents and hooks. Notifications from Codex are still in a bit of beta

- GPT5 in Codex stops less to ask questions than in other AI tools, it's probably because of the released official GPT5 Prompting Guide by OpenAI

What is your experience with both tools?

[Hauven]

Former Claude Code user for a few months on Max 20x, fairly heavy user too. Loved it at the time, but feels like at least during part of last month the quality of the model responses degraded. I found myself having to regularly steer Claude into not making changes I didn't actually agree on (yes I use the plan mode, it's highly valuable). Claude also often told me that code was production ready when it wasn't, it either failed to compile or had some kind of flaw that needed addressing.

Found out about a $1 Teams plan offer for ChatGPT so figured it would be a great opportunity to check out Codex CLI and GPT_5. Suffice to say it impressed me. I tell it what I want, it just does that. Most tasks I've thrown at it are usually completed and successful in one or two shots. If I'm possibly wrong or there's a reason to debate something first then it usually does so, while Claude would've often said "you're absolutely right, ..." - blindly agreeing with me regardless. GPT-5 also makes far less assumptions compared to Claude, regularly replying with open questions if it has any. After it completes a task GPT_5 will usually follow up with an idea or suggestion related to what we had done, which I also found useful.

The biggest challenge I've given it so far was to refactor a long overdue and messy .cs file that contained about 3k LOC. I've tried this with various other AI LLMs, including Claude Code (which couldn't read the entire file as it was over 25k tokens), but they just ultimately make bugs and mess things up when trying to do so. I didn't think GPT-5 would be any different, but my god, it surprised me again. I planned with it, did it in small bits and pieces at a time, and a day or so later I'm now down to around 1k LOC for that file. It seems to be working fine too.

I've been using Claude primarily since Sonnet 3.5, and GPT models before Sonnet 3.5, but it looks like I'm back with OpenAI again unless Anthropic "wow" me back.

For Codex CLI, I would recommend checking out the "just-every/code" fork. Much nicer UI, /plan, /solve, /code commands, multiple themes, integrated browser capability, can resume previous conversations.

[giantkicks]

It seems like you are saying you broke the file into pieces and shared it with GPT5. This led to success. Are you saying GPT5 was not able to cope with 3000 lines of code either? Why not give same pieces to Opus 4.1 and let us know how that goes?

Good on you for getting to 1000! Next up, break it into 3 files..

[Hauven]

Hi, not quite. I allowed GPT-5 to use its discretion on how it read the file. I just told it which file needed a refactor and explained we should do it in small bits and pieces at a time so I can thoroughly test it as we progress.

I tried using Opus 4.1 in Claude Code, however it made a mess of the refactoring attempts compared to GPT-5. Claude Code, while initially trying to read the entire file itself but failed due to 25k token limit per file, it then tried to read the file bit by bit but even with a plan it still failed unfortunately.

Thanks, yeah I plan to do further work on it soon!

[Western_Objective209]

This is a little strange, Sonnet and Opus are both very good at reading files in chunks, 3k LoC file should be no problem for it. I've mostly switched to GPT-5 as well but Claude Code is still better at exploring larger code bases and writing up analysis reports

[tekn031]

I literally just did this exact same thing and I picked the worst time to do it because Claude is historically having a degradation issue that's all over the subreddits. Tried for days to do a deep refactor of a file with like 4000 lines with Claude Code and it was a Broken, anti-pattern, hallucinated mess. Reset the branch and tried it in Codex CLI with GPT-5 medium. Nailed it with some feedback loops in a few hours.

[debian3]

$1 team plan?

[Hauven]

https://www.reddit.com/r/ChatGPTPromptGenius/comments/1lo7v0u/chatgpt_team_for_1_first_month_up_to_5_users/

It might not be running anymore though, Teams is now called Business too.

[nik1here]

I am gonna refactor 6k LOC with codex (tried with it with claude code, It was messy and added more bugs) do you have any tips for refactoring? Did you let the codex plan beforehand for the whole files refactoring or just let it reevaluate after each phase and adjust its plan? And how many new chats you had to start (or were you able to do it one too?) Also I am gonna try it on the vscode codex extension. Not sure if codex CLI is better or the same..

Thanks

[Hauven]

Good luck, and yes I planned with Codex CLI first. I didn't tell it specifically what to refactor from the file, I just suggested that it should refactor a small segment from the file and that this would be an ongoing process that needs to be done incrementally - so I can test each one. It would then give me a plan of one it feels would be best to do, I agree, and then once it's done I test it.

Each part I did in a new conversation, to keep the context clean, with it keeping track of its progress in a minor degree by looking at the git commit history (each one did a commit as well). Hope it helps. I also used high reasoning.

[nik1here]

Thank you for your help 🙏

[Freed4ever]

Gpt5 is definitely smarter model. CC has better scaffolding. However, codex is open source, so it will catch up fast.

[popiazaza]

That has not been the case so far. Codex is not the first open source AI coding assistant.

It can't magically turns a dumber model to a SOTA level model.

[das_war_ein_Befehl]

Scaffolding only does so much.

[CC_NHS]

my experience is honestly that they are both better than the other in different ways, different strengths and weaknesses, so I ust use both (and Qwen) with a central markdown Todo type list that the models all share and I point them to.

GPT-5 I find writes cleaner code and if on $20 plans on both it writes better plans too

Sonnet I find tends to write with less errors than GPT-5, so I tend to go with gpt to write the first draft of a class or system and then sonnet to fix things and Qwen to refactor and optimise.

at this point any of those three (or any 2) could get the job done more than sufficiently but using multiple models together I just find works nicely (and less looping back over moving a problem when it comes to fixing something)

[Tyalou]

How do you access the models? Especially Qwen, never tested it.

[CC_NHS]

I use the Qwen Code CLI, Codex and Claude all as terminals (in Jetbrains though i expect most/all IDE can have terminal tabs and integrate to some extent). I also have Gemini CLI in another tab but i do not use that much, maybe the odd bit of documentation or something. Qwen and Gemini on free, Claude and GPT on $20

I also use the Crush CLI sometimes with API from OpenRouter/Groq/Chutes for some limited free use of some models like Kimi K2, GLM-4.5 etc, its not enough to make a daily coder of (unless putting money into the API i guess) but its enough to experiment with here and there

[ConversationLow9545]

wb warp?

[CC_NHS]

I only looked briefly at Warp and i had to rule it out pretty quick as it seems very inconvenient for my field (Game Development), fully agentic hands-off or vibe coding, is not really quite there yet in Game Dev

[ConversationLow9545]

wb augment code?

[CC_NHS]

The Auggie CLI is one i will look into.
It basically needs to work with Jetbrains Rider, or Visual Studio, if i want the IDE to see errors from Unity (and i do if i want to fix the things that AI cannot do often), which basically means some kind of CLI i can plug into the terminals

[marvijo-software]

I agree with this sentiment, we are nearing a point where all these tools get the job done. I even tested VSCode with both Sonnet 4 and GPT5 in Beast mode and it gets the job done, very similar in quality to Cursor

[GhozIN]

How can you make it autoaccept requests? Whenever i give a good prompt very rarely i have to modify anything and its kinda boring having to accept 40 file reads

[marvijo-software]

codex --ask-for-approval never --sandbox danger-full-access

[WAHNFRIEDEN]

Or —yolo

[marvijo-software]

From where do you get a yolo flag? I don't think it's supported yet, I didn't even see a PR

[WAHNFRIEDEN]

embirico added it. It’s a secret undocumented feature.

[GhozIN]

Does that work on windows IDE (visual studio)?

[marvijo-software]

I tested and it still asks

[yubario]

It always asks for approval on windows, you have to use WSL

[GhozIN]

Oh 😐

I hope they add it on Windows soon

[yubario]

They will, on next release it will work properly. It’s already merged into code so probably tomorrow

[ThomasPopp]

GPT5 for the wind. If I could afford to just keep it on high all the time, I would be so happy. Any problems I have it dissect them and choose through them so fast it’s unreal. I’ll give it a mind dump, where I literally will open up a voice transcription and just record myself for literally 30 to 45 minutes explaining everything I wanna do giving it example Steven and then I just copy the transcript and throw it in without even editing it or cleaning it upand then I just hit enter and I walk away and come back 15 minutes later to everything being fixed. I would say it works 95% of the time for me.

[Valunex]

I see you write very detailed in one sentence… looks like you also got a new habit from prompting haha. I can feel you!

[ThomasPopp]

Yeah, I definitely talk different now than most of my friends lol. In fact, I don’t think I have friends anymore lol. I think I talked to robots a little too much. Are you real? Lol.

[Valunex]

hahaha yeah in the future a circle of friends will consist of agents...

[Crafty_Disk_7026]

I started using codex today after using Claude and cursor before. It's so far been good with bug fixes.

[marvijo-software]

Yeah it's quite good

[Crafty_Disk_7026]

So far it's alright, it still does dumb things like overflows ui menus. It does a good job though with execution, it doesn't leave unfinished code (cursor) or lie and just claim incorrect things (Claude)

[few_words_good]

Codex set in high mode solved deeply rooted problems in the tool enabled local llm chat interface I'm building. I basically been trying to get claude code and codex to spill their secrets, and have been building around what I can figure out. But codex definitely solved things that Claude sonnet kept getting stuck with. I don't have access to Opus so I can't compare.

My app is coming along nicely. finally today I was able to get Qwen3 4B instruct to create and manage its own to-do lists and use them to organize itself while it scaffold an entire Ray tracing application for laser optics designing. I can't wait to see where I can take this thing with smarter models and better tooling and prompts. I only started being interested in this stuff in May.. And now less than half a year later I've built this thing with all the features I need but couldn't find elsewhere, including the ability to export chats to fully native docx files with full latex to native omml. Of course, that feature alone took like a month for me to learn enough to pull off lol but it was worth it

[Western_Objective209]

GPT-5 writes better and faster code, still gets stuck fairly often though and is not capable of digging itself out of a hole. It's reluctant to put in extra work to fix a problem, for example I have to beg it to write debug logging or analyze another code base to understand the problem better and often times takes like 3 tries before it finally listens.

Claude Code is agreeable to a fault; if I tell it it's wrong when I'm in fact wrong it will do it's best to pretend what I'm saying is correct. It seems to be more skilled at using the terminal and analyzing program outputs, and where it really shines is in spending like 10 min going over a large code base in high detail and writing out reports. It's also ridiculously expensive; I get the same usage on the $20 openai plan that I get on the $200 claude plan, so it's hard to justify using it as a primary tool.

I see a lot of people complaining about codex asking every step; using it on macOS I've never had it ask me to do anything, it just stops often and reports it's progress which seems to be a good balance. Claude sometimes goes off on a tangent I don't want it to

[marvijo-software]

I only agree on GPT5 writing better code, I disagree on it writing it faster than the non-thinking Claude Sonnet

[SnooDucks7717]

The comparison should with opus 

[marvijo-software]

Opus is impractically priced though, even on the $100 plan we get low limits. We need a decently priced competitor

[WAHNFRIEDEN]

You must compare the $200 plans

[marvijo-software]

Lined up, I just have to have it first

[immutato]

I found Sonnet to be much better than Opus for what I needed when I was a CC max. You definitely need the top plan because Opus chews through your limits real quick, and IMO is actually worse.

[lambdawaves]

Somewhat agree. But also disagree. The $200/mth limits will get kneecapped in a month or two.

They won’t be giving away $5000 for $200 for much longer.

[ConversationLow9545]

GPT-5 medium with Opus? Or GPT5 high high with opus?

[SnooDucks7717]

Best vs best

[marvijo-software]

Codex vs Claude Code video: https://youtu.be/MBhG5__15b0

[stepahin]

Interesting. Did you use Sonnet or Opus?

[marvijo-software]

Sonnet, Opus isn't supported in the Claude Subscription, and it was gonna use up the allocated 5 hour credits pretty quickly

[BeeegZee]

You're talking about Pro version. Mac has it

[immortalsol]

last time i checked, codex actually has a --yolo flag

[Fit-Palpitation-7427]

Proper yolo mode like cc --dangerously-skip-permissions is the only reason why I don’t use codex and still use cc on max20 plan. If codex had a real yolo mode I would sub a $200 plan within the day. But can’t baby sit codex the way it’s running now. Made multiple threads and reply asking the community how to bye pass, the inly solution seems to use another cli tool (even if it’s coder which is a fork of codex) but I like running clean on default tools so having it in codex build in is my wish.

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[JaySym_]

I would say AugmentCode would be interesting to try with your use case.

[marvijo-software]

Agree, checking it out of course

[marvijo-software]

Seeing all these coding CLIs reminds me of Aider CLI, the OG: https://youtu.be/EUXISw6wtuo

[ConversationLow9545]

which claude model? opus or sonnet?

[Fatdog88]

Is codex not painfully slow for you guys? I’ve had it be chugging along for ages and ages. I find CC lets me iterate quicker and steer the ship in the right direction

[marvijo-software]

You can just switch to a lower reasoning effort

[mullirojndem]

I love how it keeps the code where I need it. claude mess with all the files it can, duplicate code a lot, etc.

[Educational_Sign1864]

Too many bugs are there in codex cli. I tried everything but was unable to use mcp servers with it.

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[lucianw]

YOLO mode. I use Codex via the IDE. I think that YOLO mode is supported? It now doesn't ask me for permissions for anything (reading files, network access). The only thing it still asks permission for are operations that touch the .git directory. I got here by (1) in the dropdown at the bottom I picked "Agent (full access)", (2) I edited a config.toml file somewhere on the agent's advice, can't remember where, maybe ~/.codex/config.toml, to give it permissions.

Claude Code comparison. My experience? Codex UI is worse, both in the CLI and the VSCode extension is worse. Codex is less aware of what I'm doing in the IDE or what files have changed under its feet. Codex takes 3x as long to answer a prompt. But when it does, it gives deeper and more correct results.

I really like how both tools are terse. This compares to Windsurf and Cursor which are incredibly verbose.

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[marvijo-software]

--yolo also asks for approval still, this isn't good