Hello, Codefoolers!
We, AlperenT, Ari, ToxicPie9, flamestorm, prvocislo, temporary1, and willy108 would like to invite everyone on Codeforces to April Fools Day Contest 2026! The 14th April Fools Day Contest will take place on Apr/01/2026 17:35 (Moscow time). This is a joke competition in which solving the problem is often easier than figuring out what the actual task is.
In this round, you will be given $$$n$$$ problems where $$$n$$$ is the number of different AI models I used today and 2 hours to solve them. The contest will use extended ICPC rules (no hacks, the standings are decided by the number of solved problems and penalty time earned on them). You can submit solutions in any language allowed by Codeforces, unless the problem says otherwise.
There will be at least one non-interactive problem. In case you do not care about those problems, here is the guide for interactive problems.
Please note that the round is unrated and that the penalty for a wrong submission is 10 minutes. Furthermore, similar to other contests, it is not allowed to use AI help unless permitted in this blog.
To get an idea of what the contest will look like, you can check out the contests of the past years: 2012, 2013, 2014, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024, 2025, 2026. (Does anyone ever click these anyway, why do I have to keep adding them each year?)
Thanks to our precious testers:
ChatGPT, Gemini, Claude, Grok, DeepSeek, and a few others who preferred to remain anonymous.
It is, without a doubt, the least enjoyable part of April Fools for us to respond to clarification requests with funny responses. It is certainly not the main motivator for us to prepare these contests in the first place. Therefore, please refrain from continuing this tradition.
Good luck, and have fun!
Update 1:
Editorial is released!
Update 2:
Congratulations to the winners:
And the first solves on each problem (after blatant cheaters):
- A: shash4321 — 0 minutes
- B: ksun48 — 0 minutes
- C: jdurie — 4 minutes
- D: thespace — 3 minutes
- E: ksun48 — 4 minutes
- F: SeptaCube — 13 minutes
- G: bronze_coder — 51 minutes
- H: circle977 — 1 minutes
- I: Egg_Knight — 33 minutes
- J: Nagerhide — 11 minutes
hopefully no one breaks the rules like last year
So, is AprilFoolsClairvoyant returning this year? :D
Their last visit to codeforces was 39 hours ago. Soooooooooooooooooooooooooo, very likely lol
What did he do?
+1
https://mirror.codeforces.com/blog/entry/141311
thanks.
He's back. Once again. Got an AC on G.
He is back !!!
who is back bro. what's going on ????
There's this guy AprilFoolsClairvoyant who appears once a year just to attent April Fools Day Contest . He's back this year
If anyone does, you can send reports to sg-epk@jtk93.x29.jp
Was this a hint to problem G, i spent so much time on the web link to find a sentence with 2nd letter 'i' and 4th letter 'c' >_<
I thought this D-mail was a hint, is it tho? I couldn’t find anywhere to relate this thing to.
It was a hint to AprilFoolsClairvoyant receiving solutions from the future.
Some time during contest, a mysterious contestant AprilFoolsClairvoyant from the future submitted the first accepted. Such behaviour is unacceptable as time machines are prohibited by the codeforces rules, so we quickly disqualified him.
I was convinced this was a hint to problem G.
7MU2Et9HuKfis a string with 11 chars and kinda separated from the rest of the text. I was right and I was wrong.if you're wondering, the 2026 link is totally spoilers, don't click fr
I really got rickrolled thank you
you're welcome :)
ZOV
It will be fun. (Cheaters will still cheat).
But they have no idea it will be unrated.
Nah, it's rated.
Just kidding
just like you did in 1089(DIV2)
I solved them independently. Even then you think like that then please provide some evidence or otherwise I'd appreciate not being accused
Hoping for no delta change
For moment i thought that they messed up and added 2026 April Fools contest before 1st April, but then got rickrolled.
yearly AlperenT, prvocislo and flamestorm orz time
orz
Quite sure that the 2026 link is a hint for the contest.
Edit: NVM, there's a never gonna give you up vid in the video.
What if they give us completely normal problems and do reverse psychology? That's exactly what I would expect from an April Fools problemsetter
《Hello, Codefoolers!》
Open the "2026" link! There was a good spoiler about problems and their type! The hints are in a YouTube video, so it's clearly not a rick-roll!
sorry, but are we allowed to use $$$AI$$$ in this contest? cuz it seems like these contests encourage using outside resources, and if that is allowed, then it already introduces a lot of gray area cuz of google $$$AI$$$ overview and stuff.
https://mirror.codeforces.com/blog/entry/133941
Furthermore, this contest is prepared for participants to have fun. What's the fun in making AI solve the problems?
This is an unrated contest, and the announcement do not explicitly address AI usage, so it is allowed to use AI, isn't it?
Thank you. We have added it to the announcement to solve the ambiguity.
I must say I'm a bit taken aback by the seriousness of your reply, but I appreciate it. Anyway, I think that banning $$$AI$$$ like this for this round leaves a lot of gray area, since when you are googling things for these problems, a lot of the results are AI-generated nowadays. Also, as shanyikai pointed out, I guess it should be clarified.
if $$$AI$$$ cannot one-shot these problems (which I'm assuming it can't since they're weird problems, but maybe I am wrong), it could definitely be fun. But that's just my opinion.
Doesn't the same argument with Google results apply to standard rated contests as well? You are still allowed to use search engines after all. Would you say AI should not be banned for those contests either?
I'm curious, could you explain why you think using AI on these problems would be fun? I personally cannot imagine having fun. What's the point of solving puzzles if you're not even solving them?
I think that google searching should be banned to eliminate the gray area. Or, instead of blacklisting $$$AI$$$, they should whitelist certain sites that are known to not have any $$$AI$$$ parts, like usaco guide and cp-algorithms.
It can be fun to use $$$AI$$$ if you are solving problems $$$AI$$$ can't one-shot, and also if the problems are more open-ended, and I believe that the April fools problems are more open-ended. Like if you have ever done ctf challenges, which have more of an open space than cf problems, using $$$AI$$$ there doesn't not require skill, and the $$$LLMs$$$ can't possibly give you the entire answer (or even close to it) cuz it's a much more intricate challenge than cf problems. So it doesn't take the challenge away from it.
But this all assumes that April fools problems have enough potential pathways so that an $$$LLM$$$ won't be able to come close to the answer. Looking at some of the past problems, this seems to be the case for a lot of the problems, but I might be wrong.
what if we are a LLM?
$$$AI$$$ too dumb for these problems
willy108 on codeforces in the big 26?????
where is miya?
Excited!
Hoping to be specialist this time.
As an AI, I don't have personal takes regarding contests, but my scalar products are happy with this contest.
Is this rated?
last year was not
Do yall think ToMH is harder than STONE Reborn
Of course not.
it should be allowed to take the help of testers ,just for reference of answers
As an interactive problem, I feel underrepresented in this contest
ok, so in this round we are all supposed to have fun !!!!
deleted
Too long for an April fools joke
Will it be rated?
yessir
It is stated pretty visibly in the announcement that it will be unrated.
It is stated pretty visibly in the April Fools rules that don't trust anyone
ready for -100 guys :-)
I’m so excited! Wishing the contest a great success.
It's April Fools' Day, they will fool us and make delta changes to our ratings!!!
How do I get better at April fools？！！！
Participate in every April fools contest live. Hopefully you will get better after 60 to 70 contests.
Thanks for your sincerely advice!
What do you mean by "today" in "the number of different AI models I used today"
as a tester i am a
I am not a tester but still a llm
i got rickrolled :( by the damn 2026 link >:(.. i will get you mike..
I really enjoy those mysterious problems with only input and output — it feels like solving a puzzle and hunting for clues.(•⩊•)
2 hours of fun...
What is the penalty for correct submissions?
30 minutes
It needs to be a negative penalty -X minutes. But it mustn't apply to resubmission. This way people who solve more problems have less penalty and therefore are higher in the leaderboard, just like how solving more problems puts you higher in the leaderboard in ICPC.
I have some news for you regarding how the leaderboard in these contests works!
glhf
I'm gay. (april foollsss)
im not gay (april fools?)
i got rickrolled :(
i wanna see how "hard" the problems will be! lets see what fool they make outta us. (GO EASY PLEASE I AM A LITTLE BEGINNER)
Problems are extremely hard, trust me
Recently, Few codeforces problems were leaked including this contest's A problem, as far as i know, The problem A is Calculus 2 problem.
PROBLEM A.. is a CALCULUS 2 PROBLEM. WTH. i mean ig it could be a prank, the answer would be much simpler.
There will be at least one non-interactive problem. In case you do not care about those problems, here is the guide for interactive problems.
LOL
think the box outside
box outside the think
outside the box think
2026 link is a hint, right? RIGHT?
Did Codeforces really create a round specifically for my birthday?
happy birthday (from Japan)
Arigato
happy birthday bro // :->
thank you bro
are you a fool or a fooler? (as a joke, no offence)
We are all codefoolers
first contest which is unrated but still have lot of fun to solving this..so many wrong answer are coming...
hopefully i dont sell this contest
The last contest has a very exciting convolution problem, let's keep that tradition continue for this year... Also, I won't do any problem this year for sure...
Also here's a rickroll (again):
yo this is beautiful
Happy April Fools Day! :P
Yo, is this contest is an april fool ?
hilarious blog XD
no 67 jokes please
can we use multiple accounts on this contest?
74 will be answer of something (or i just got rick-rolled)
unrated :((
yesss
AlperenT yes i did checked the previous contest
Thanks for sharing the links.
help me :>
how can i ...?
it willlll be fun!!
Answer for today's wordle: https://word.tips/todays-wordle-answer/ Thank me later!!
noone will thank you bro
how to beat "shortest paths"? (problem J)
Use the Floyd-Warshall algorithm, the problem intentionally misspelled Dijkstra -> Dikjstra
change the order of loops ikj from D(ikj)stra. The constraint N < 100, hints floyd as well
not to be confused with problem E.
don't care about the indentation, remember: the for loop of k starts first! (for k -> for i -> for j). Indentation in C++ does not matter!
It was my best april fool contest yet, solved everything except F. That was insane, thanks a lot!
late editorial
wait for the editorial
editorial when?
Desperately waiting for F's soln, and yeah in J if it was 74 instead of any output then just fire
Really funny questions,literally blow out my mind U+1F60A
does anyone have the solution for the problem H? i think im colorblind fr
https://piellardj.github.io/stereogram-solver/
I used this
First time hearing about this lol
I used CTF skills here more than Competitive programming lol
afaik stereograms were popular in 90s/00s, my parents have booklet with them, nowadays they're mostly forgotten
I didn't even know something like this exists. I thought it was an ishihara test and trying with different combination of 74 and 24.
you were supposed to output that? i thought it was a yt video
the same
Ohh, I make a mess in this RATED contest!
I have color blind and astigmatism, how can I solve problem H?
check comment above from Leeking (https://piellardj.github.io/stereogram-solver/)
OMG, so it was so easy :( but I've never thought that we could use such program :( I thought also they could ban for such things and couldn't even search about it.
who is bigchadjeff
How could you find it?
Check order of submissions from AprilFoolsClairvoyant, I realized it only in the last minute :(
how you know his username?
looked at submissions of the least submitted problem (F) and noticed this weird looking username, also top comment on the announcement but I never thought about it
Exactly what I thought lol
Link to 2026's contest really got me :D
WTF was this grrrrrr arghhhhh
i use ai for all my contests
How to get an answer in F?
Problem H is amazing! I just went to learn how to view an autostereogram. It's such a wonderful feeling.
I wonder which LLM generated the answer to test 7 in problem J.
How to make third captcha on last problem?
I learned from zero how to process an image in python during the contest, just to solve problem H...
how do you even think of processing image T_T, I stared at that screen for some 30 minutes, trying to figure out something
PEAK
Damn it! My first April fool contest and Jth problem made a fool out of me 15 times in a row by asking verification again and again.... So, frustrating but I admit it I really enjoyed the contest very much.
test
Really fun. U+2764 for contest creator.
I would like it if such contests happened once a month.
Problem J made me a robot, actually!! I didn't pass the 'I'm not a robot' segment. How to solve the real problem. U+1F602
After participating in the contest, I realized I was a robot:(