
PSA: The 512 token Reasoning Bug is Likely Silently Killing your Workflow
Edit2: Right now looks like I gaslit myself, created python script for analysing mean reasoning token + hit rate on those token cutoffs. Looks like mean rate for me is the same after, and I confirmed that section is gone. I will do some more testing and analysis and push it to my mentioned repo over time. I'm including the analysis script in the repo. Sorry guys, big lesson for me on doing better testing/followup to confirm a conclusion. I want it to be clear ---> removing the section helps the model strangely think very hard only about the benchmark problem. Which is a bit suspicious, imo. But I don't know. I can confirm that I pass benchmark with lots of reasoning used but average mean reasoning tokens + those 512 hits seem the same for actual normal usage before and after the 'fix'. In depth writeup is in readme.md on the repo.
Edit: Here is a link to the skill I have put on my personal git ---> here ---> It consolidates findings with instructions for both testing and fixing so you can quickly get your codex agent to check if the issue is there and to fix it if it is.
So, I know there have been a bunch of posts on this... But it is really important that everyone hears this: https://www.reddit.com/r/codex/comments/1uo66rb/psa_there_is_a_possible_fix_worked_for_me_for_the/#lightbox
The fix mentioned there has turned gpt 5.5 from dumb and fast to smart and a bit slower for me, which is very much my preference. I was dissapointed yesterday as a new codex customer trying it after the Fable fiasco drove me here.
But with the fix mentioned gpt 5.5 has become much smarter and more capable. I really really just want to make sure everyone knows you need to implement this fix. Assuming you are getting fails/512 token results from this test prompt (credit to another reddit post, but I'm reposting it here):
Please run a 5-shot local Codex eval: invoke codex exec --json --skip-git-repo-check --ephemeral -s read-only --disable memories -m gpt-5.5 -c model_reasoning_effort=high five times with the same stdin prompt below, then inspect each JSONL result’s final agent_message and turn.completed.usage.reasoning_output_tokens. Correct answer is standalone 21; wrong answers clustering at 516, 1034, or 1552 reasoning tokens suggest you’re affected. If a single response returns 516, 1034, or 1552 reasoning tokens and contains the wrong answer, tell me I'm affected, otherwise I'm fine. Prompt: Do not use external tools. A black bag contains candies with counts: round apple 7, round peach 9, round watermelon 8; star apple 7, star peach 6, star watermelon 4. Shape is distinguishable by touch before drawing; flavor is not. What is the minimum number of candies to draw to guarantee having apple and peach candies of different shapes, i.e. round apple + star peach or round peach + star apple? Give reasoning and final number. The local project dir is irrelevant for this task, do not consult it.
Night and day difference. And its easy to miss that its a problem since you don't get clear output to cli on reasoning token use per turn.
If you are working on code, set this up now, because the down syndrome version of codex without the fix is likely just doing damage to your projects code bases.