I am one of the part-time staff members at AtCoder responsible for investigating cheating involving AI. Let me explain what happened during my free time today.
What I did in my free time today
I was browsing Codeforces blogs, as I often do for fun.
I found a post asking AtCoder to unban a certain user. The banned user was also in the comments, strongly insisting that they had not cheated. The comment section was full of people criticizing AtCoder. I immediately reviewed the case again. The user had been banned yesterday for cheating that took place three months earlier. After carefully rechecking the evidence, I also concluded that they had clearly used AI. (I will not explain the details because they relate to our methods for detecting AI use. The same applies to the cases below.)
The post also named another person who was said to have been falsely banned. I did not feel like spending even more time on this, but I carefully reviewed that case as well. That user had blatantly cheated one month earlier.
Next, I found another post in which someone claimed that AtCoder had banned them by mistake. I reviewed the case, and once again, the user had cheated.
A little later, another "wrongly banned user" was added to the first post. I reviewed that case as well. I doubt I need to tell you the result.
Finally, I found a comment on an AtCoder-related blog saying, "AtCoder banned me. I didn't cheat. Please unban me." I reviewed the case, and of course, that user had cheated too.
(Total time spent: a full two hours.)
Are you kidding me? Do all these people cheat and then somehow forget that they did it?! And the commenters kept making fun of AtCoder's banning system, without even knowing that we seriously review every claim of a false ban whenever we come across one!
Until now, I had taken claims of false bans on Codeforces seriously. However, after seeing so many claims from liars or people who had somehow forgotten their own cheating in a single day, I changed my mind. From now on, when I see a Codeforces post about a false ban, I will handle it as follows:
- If the author had a max rating of LGM as of the end of 2025, I will treat it as a serious appeal that could affect their entire competitive programming career, and I will carefully review the case.
- Otherwise, I will ignore it.
Unfortunately, this seems to be the most reasonable way to avoid being misled by cheaters' lies.
Please do not let cheaters convince you that AtCoder often bans innocent users by mistake. Also, if you find a blog post about a false ban by AtCoder, feel free to reply with a link to this article. Unfortunately, most people who write such posts are liars, people who have forgotten their own cheating, or fools who were tricked by cheaters into writing the post.
Some tips
- If you believe AtCoder banned you by mistake and want the ban removed, the right thing to do is contact AtCoder through its official support channel, rather than write a post on Codeforces. Also, AtCoder’s official ban policy is described in the following article: https://codeforces.me/blog/entry/154651
- This should be obvious, but just to be clear: even if you have entered 100 contests and competed honestly in 99 of them, cheating in even one makes you a cheater. And of course, AtCoder may ban you for cheating in a past contest.
