Here is the 15 sec coding test I used to instantly filter out most applicants
92 comments
·December 17, 2025hairband_dude
kevin061
This is an excellent point. I did not think of that.
sond
Instead of the zero-width div, you could set up an event listener for the copy event (using addEventListener() method) which calls .clipboardData.setData() on the ClipboardEvent to change it to your modified code.
That should avoid messing things up for people with screen readers while still trapping the copy+pasters.
Aloisius
Or add aria-hidden=true
I've had clipboard events and the clipboard API disabled in my browser to prevent websites from intercepting them for ages. I can't be the only one.
imglorp
How about don't let them copy it at all. Show them the problem on a shared screen and they should speak the answer in 15 or 30 seconds.
If someone is visually impaired, it's short enough you can just read the problem text to them.
gs17
> Show them the problem on a shared screen
I'm pretty sure the intent is to weed people out well before they get to a point where you could share a screen with them. He mentioned a few people "resubmitted the application", so sure this is probably an initial step.
skeledrew
Can't read the problem when the point is to catch those who copy-paste the code.
userbinator
For better or worse, screen readers tend to be less easily tricked by things like that now.
null
thaumasiotes
You got different source than I did. For me it's a span of finite width, but with a font size of 1px.
CharlieDigital
What most interviews get wrong is that there are usually just a few "bullet points" that if you see them, you instantly know that the the candidate at least has the technical chops.
Instead of creating a test that specifically aims for those bullet points, many technical assessments end up with convoluted scaffolding when actually, only those key bullet points really matter.
Like the OP, I can usually tell if a candidate has the technical chops in just a handful of really straightforward questions for a number of technical domains.
Maxatar
If you have a test that can identify a good candidate quickly then you have honestly struck gold and can genuinely use that to start your own company. I mean this with absolute sincerity.
One of the absolute hardest part of my business is really hiring qualified candidates, and it's really demoralizing and time consuming and unbelievably expensive. The best that I've managed to do is the same that pretty much every other business owner says... which is that I can usually (not always) filter out the bad candidates (along with some false negatives), and have some degree of luck in hiring good candidates (with some false positives).
ttoinou
Yeah sourcing developers / collaborating with developers is a huge barrier of entry. More than others factors such as deciding what software to produce for the market
antonymoose
I’d love for you to elaborate :)
But along that thought, I’ve always held that a human conversation is the best filter. I’ll ask you what do you work on recently, what did you learn, what did you mess up, what did you hate about the tool / language / framework.
I strongly believe your ability to articulate your situation corresponds with your ability to do the job.
CharlieDigital
Take your current technical assessment and think about the types of responses or code submissions that really impressed you. What was special about them? What did you see in the response that drew a positive reaction from your team?
Can you re-frame your process or the prompt to only elicit those specific responses?
So instead of a whole exercise of building a React app or a whole backend API, for example, what would really "wow" you if you saw the candidate do it in the submission for a project? Could you re-frame your whole process so you only target those specific responses and elicit specific outputs?
Now that you've taken what was previously a 2 hour coding exercise (for example) and distilled down to 3-4 key questions, you can seek the same outputs in 15-30 minutes instead.
There are several advantages to this:
1) Many times, candidates know the answer but they actually can't figure out what you're looking for when there's a lot of cruft around the question/problem being solved. You can end up losing candidates that know how to solve the problem the way you want, but because of the way the question was posed, the objective is opaque.
2) It saves a lot of time for both sides. Interviewer doesn't have to review a big submission, candidate doesn't have to waste time doing a long project.
3) By condensing the cycle, you can evaluate more candidates and you can hopefully select a top candidate before they get another opportunity. You shorten the cycle time because the candidate doesn't have to find free time to do a project or sit down for long interviews, you don't need to have people review the code submissions, etc.
eska
There are quite a few people who can code but have 0 social skills and can’t talk well or at all. In that sense I have to disagree with you, but I still wouldn’t hire them because they drag teams down.
ttoinou
But how do you know if the remote candidate used AI or his brain?
stavros
For anyone who missed the (poorly-explained) trick, the website uses a CSS trick to insert an equals sign, thus showing different code if read or if copied/pasted. That's how the author knows whether you solved it in your head or pasted it somewhere.
kevin061
The thing I found particularly fascinating, because the article was talking about discarding AI applicants, is that if you take a screenshot and ask ChatGPT, then it works fine (of course, it cannot really see the extra equals sign)
So this is not really foolproof, and also makes me think that feeding screenshots to AI is probably better than copy-pasting
UncleOxidant
Sure, it's not foolproof, but a large percentage of folks would just copy&paste rather than taking the screenshot. Now that may start to change.
There was story like this on NPR recently where a professor used this method to weed out students who were using AI to answer an essay question about a book the class was assigned to read. The book mentioned nothing about Marxism, but the prof inserted unseeable text into the question such that when it was copy&pasted into an AI chat it added an extra instruction to make sure to talk about Marxism in relation to this book (which wasn't at all related to Marxism). When he got answers that talked extensively about the book in Marxist terms he knew that they had used AI.
divbzero
This didn’t work for me because Reader Mode popped up and showed the “hidden” equals sign.
psygn89
Thanks, I was wondering how in the hell that many would get the answer wrong and what is this hidden equal sign he was talking about.
Maybe the question could be flipped on its head to filter further with "50% of applicants get this question wrong -- why?" to where someone more inquisitive like you might inspect it, but that's probably more of a frontend question.
imron
People are focusing on the > vs the >=, but for me the key point is being able to hold logic and variables in your mind over several iterations of a loop.
I’ve used similar tests in interviews before (a function that behaves like atoi and candidates have to figure out what it’s doing) and the good candidates are able to go over the code and hold values in their head across multiple iterations of a loop.
There are many candidates who can’t do this.
delduca
Best test is to listen to 2h meeting without too much details and have to figure out how to ship the feature.
tagyro
4real
better yet, listen to a 1h meeting and compare notes/action points
preinheimer
I think it’s important to test these systems. Let some % of candidates who get this wrong through to the next stage and see what happens. Does failing this test actually correlate with being a bad fit later?
If you want to ineffectivly filter out most candidates just auto-reject everything that doesn’t arrive on a timestamp ending in 1.
gs17
> Let some % of candidates who get this wrong through
Really, the better test would be to not discriminate on it before you know it's useful, but store their answer to compare later.
preinheimer
You're right. I agree.
jgilias
_How_ can you be a good hire for a _software engineering_ position, if you can’t get that one correct though?
gs17
It depends why they didn't get it "correct" (asked ChatGPT bad, used Python REPL not so bad, used screen reader very not bad) and what "correct" even means for this problem.
There's a bizarro version of this guy who rejects people who do it in their head because they weren't told to not use an interpreter and he values them using the tools available to solve a problem. In his mind, the = is definitely part of the code, you should have double checked.
jgilias
Oh. I was reading this on a phone, and didn’t realise there’s hidden equal sign (though it’s mentioned).
That does change it. In that I can see how false negatives may arise. Though, when hiring you generally care a lot more about false positives than negatives.
thaumasiotes
> Let some % of candidates who get this wrong through to the next stage and see what happens.
This isn't a good methodology. To do your validation correctly, you'd want to hire some percentage of candidates who get it wrong and see what happens.
Your way, you're validating whether the test is informative as to passing rate in the next stage of your hiring process, not whether it's informative as to performance on the job.
(Related: the 'stage' model of hiring is a bad idea.)
apothegm
That was REALLY weird to read. In reader mode the comparator in the first conditional was >=. But without reader mode it was just >.
amelius
Apparently they don't want to hire people who don't use reader mode.
But then the question is, how do you reach people who filter out the job ads?
gnabgib
Other way around.. if you use reader mode you generate the wrong answer ("tricked" like an LLM). At least, it's wrong according to the author/a former CTO.
It also excludes users of Lynx, cURL, possibly people using accessibility tools, those with custom/overriding style sheets.
ano-ther
Was that the trick? When copying the text, it is also >=, which is why an online search or AI tools probably give the wrong answer as the article asserts. If you correct the code then at least Claude gives the right answer.
gs17
The trick is that the = has CSS styling with "opacity: 0; font-size: 1px;".
bmacho
In normal mode the question is different than in reader mode, or when copied.
Thus, if you get the wrong answer, you "cheated" (or used reader mode)
Piraty
exactly the reason why you NEVER should copy-paste code from a website into your terminal, even if that has paste protection (https://lwn.net/Articles/749992/)
null
llm_nerd
Which is literally the point of the post. They have the = in >= at an opacity of 0 and a font-size of 1, which means it doesn't appear if styles are used properly. And their point is that candidates that copy/paste such trivial code into an AI/interpreter will get -11 because it just sees the >=.
Though a gap in their process is that, as you mentioned, various reading modes also remove styles and likewise might see it. Though a reader should filter out opacity:0 content.
gs17
> into an AI solver
Not just an "AI solver", a Python interpreter will also do it "wrong". The idea is that it's so simple that anyone qualified should be able to do it in their head, so they get the answer without the equals sign (but IMO a qualified applicant might also know it takes 5 seconds to run it in the repl and it'd be better to be correct than to use the fewest tools, or might be using a screen reader).
skeledrew
I'm one of those hit by the reader mode "bug". I really wouldn't want the mode to give me anything beyond raw text+images, as that can be abused.
koakuma-chan
What did it say? Their website doesn't load.
koakuma-chan
Oh now it works, ok I got it right, I am hired
rumplestilts
[dead]
farazbabar
I like it, a test so bad, it just might work! I think the trick is not the equal sign, trick is to keep it so simple and small that most qualified people will not try to short circuit it.
skeeter2020
My take-away: if you're doing simple coding problems like this for an interview with the "CTO", that's a very bad smell.
ndsipa_pomu
It's not for getting an interview with the CTO, but a very early filter to weed out poor candidates. But yes, if that's the only question then it's not going to discover talent.
Incipient
I don't like to give non value add replies...but this is hilarious in its simplicity. I honestly thought I was losing my marbles - how could anyone NOT get it ri...ohhh! You sneaky you!
metadope
The only correct answer is... both answers (1 and -11).
That is, if you're really interested in pursuing the position.
Not only are you willing to take their tests, but you go beyond what is required, for your own benefit and edification.
That's why, when presented with the URL during the interview, you immediately load it, and right-click View Source into another tab, while simultaneously making small talk with the former CTO interviewer.
Even though you're a backender, you know enough frontend to navigate the interspersed style and html and javascript and so, you solve both puzzles, and weave into the conversation the two answers, while also deciding that this is probably not the gig for you, but who knows, let's see how they answer your questions now...
sointeresting
is this a joke
prisenco
I got the right answer but it was so easy I went in with doubt I had done it right.
Which I understand is my issue to work on, but if I were interviewing, I'd ask candidates to verbalize or write out their thought process to get a sense of who is overthinking or doubting themselves.
gs17
> I went in with doubt I had done it right.
And if in your doubt you decided to run it through the interpreter to get the "real" answer, whoops, you're rejected.
peacebeard
That doubt is valid. Anyone reading this blog post (or in an interview, given the prevalence of trick interview questions) would know there must be some kind of trick. So, after getting the answer without finding a trick, it would be totally reasonable to conclude you must have missed something. In this case, it turns out the trick was something that was INTENDED for you to miss if you solved the problem in your head. At the end of the day, the knowledge that "I may have missed something" is just part of day to day life as an engineer. You have to make your best effort and not get paralyzed by the possibility of failure. If you did it right, had a gut feeling that something was amiss, but submitted the right answer without too much hemming and hawing, I expect that's typical for a qualified engineer.
Peeking at the source, it's just a zero-width div, which is not accomodating of people with disabilities. This might open you up to litigation if you disqualify a blind person on the grounds he gave the wrong answer 'using AI', when he might have just been answering the question his screen reader read out.