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Leeking's blog

API is Broken
By Leeking, history, 6 months ago, In English

Hi Codeforces team and MikeMirzayanov,

I know maintaining Codeforces is a huge amount of work, especially with the number of submissions and API requests every day, so thanks for all the effort.

I wanted to ask about a small issue with the API. I use it to hide problem tags while solving, so I can try problems completely on my own. Recently, without the API working properly, I can’t reveal the tags with View tags, and I also can’t see the problem rating unless I enable Show tags for unsolved problems on the Problemset page.

Also, could Codeforces maybe support this natively? Ideally, there would be an option to show only the problem rating by default, while keeping the tags hidden behind a button like View tags.

I think this would be useful for people who want to choose problems by rating but avoid spoilers from tags. Thanks again for everything you do for the platform.

Hello

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6 months ago, hide # |
 
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can relate, also it would be better off if showing only the rating of the problem without the tags was built in into codeforces

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6 months ago, hide # |
 
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Tags can really spoil some problems for me , especially when I need to practice randomly and enhance my ability to recognize the problem tags, adding the option to hide it would be great.

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6 months ago, hide # |
 
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Because they do not want their API LEEKING.

:) Just kiding

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6 months ago, hide # |
 
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I can confirm that I have also been getting this error message for a while (at least the past few weeks, maybe even the past month or more). My guess is that this has something to do with the extension's code itself; perhaps the extension is using a certain feature in the API that has changed recently.

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6 months ago, hide # |
← Rev. 11  
Vote: I like it +6 Vote: I do not like it

The API for the tags is not broken, your extension is broken

It is normal, my extension is working fine, here's the API for 2227H:

You can still see tags. Here's the API page: https://codeforces.me/api/problemset.problems

Please stop with the API is broken blogs

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    6 months ago, hide # ^ |
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    Most likely extension using contest.standings which is not broken but blocked somewhy

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      6 months ago, hide # ^ |
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      yea, that could be the case, contest.standings needs authentication like API key and signature to access but extensions don't have them, but does it have problems?

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    6 months ago, hide # ^ |
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    Ok fine, in the post I didn't explain the problem because MikeMirzayanov can easily identify it, but I'll explain it to you.

    DevTool

    Spoiler

    You can clearly see (in the Console tab, at the bottom of the page) the message: Error: Uncaught Error: CF API: HTTP error 400: {"status":"FAILED","comment":"contestId: You have to be authenticated to use this method"}

    The Problem & Why It Exists

    Codeforces recently tightened its API security, specifically locking down the contest.standings endpoint. In the past, extensions could fetch this data in the background anonymously or by piggybacking on your active browser login cookie. These older extensions just send naked, anonymous requests, the server instantly drops the hammer with that 400 error (check spoiler above).

    How to Fix It & What I Changed

    Since the extensions lack the complex math to generate an apiSig, I dug into the source code and bypassed the blocked endpoint entirely (another topic for another day). I changed the extension's requestURL to point to https://codeforces.me/api/problemset.problems. Since this is a public endpoint, Codeforces still allows anonymous background fetching here.

    Spoiler

    However, making this swap required a critical logic fix. When fetching contest.standings, the code only had to search a list of ~10 problems, so it just checked if the problem index matched (e.g., "Is this problem A?"). Now that we are fetching the global problemset, checking only the index would grab the very first problem "A" in Codeforces history!

    To fix this, I updated the loop logic to match both the Contest ID and the Problem Index: from simple if(problemsList[i].index == index) to if(problemsList[i].contestId == id && problemsList[i].index == index), which is roughly a 1000x increase in data being processed. It's noticeably slower, but manageable.

    TLDR;

    While this new approach completely bypasses the security block and makes the extension work perfectly again, it is significantly heavier. The old method downloaded a tiny, lightning-fast JSON payload containing data for a single contest. The new method forces the browser to download a massive payload containing the entire history of Codeforces problems (nearly 10,000 problems) on every page load, which is roughly a 1000x increase in data being processed. It's noticeably slower, but manageable.

    I hope MikeMirzayanov or KAN replies to this blog and lets us know what the plan is. If he isn't trying to fix this from the server side or revert the API restrictions, I may just fork these extensions, implement some local caching so it doesn't lag the browser, and release the fixes for the community myself. It will take a ton of my time, but it's still worth it.

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