Thank you for your interest in helping improve CS50's problem sets and checks! Please read this document in its entirety before contributing.
Note: This repository and all other CS50 repositories are not the place to submit your completed problem sets or to request help with assignments. If you're stuck on a problem set, please consult one of our many communities.
This repository contains the check50 checks, submit50 configuration, and problem specifications for CS50's online courses. There is no single main branch: each course offering has its own branch, named <year>/<course>, and most branches stop changing once that offering ends. Pull requests should target the current branch for the course:
| Course | Current branch |
|---|---|
| CS50x | 2026/x |
| CS50's Introduction to Programming with Python (CS50P) | 2022/python |
| CS50's Introduction to Databases with SQL | 2024/sql |
| CS50's Introduction to Programming with R | 2025/r |
| CS50's Introduction to Programming with Scratch | 2024/scratch |
Branches named <year>/fall, <year>/spring, and <year>/summer belong to on-campus offerings and are not open to outside contribution.
The check50 or submit50 slug for a problem tells you exactly where it lives. For example, in
check50 cs50/problems/2026/x/mario/less
2026/x is the branch and mario/less is the folder within it. Whenever you open an issue or pull request, start from the slug. It is the fastest way for staff to identify the right branch.
Bugs in the check50 or submit50 tools themselves (as opposed to a particular problem's checks) belong in cs50/check50 or cs50/submit50 respectively. The projects for CS50's Introduction to Artificial Intelligence with Python (CS50 AI) live in ai50/projects, not here.
CS50 uses GitHub to host code, track issues, discuss feature requests, and accept pull requests. Please do not email staff members to propose changes.
- Open an issue to report a bug or suggest an improvement (see below). You do not need to fork anything to do this.
- Optionally, once the issue has been discussed, open a pull request with your proposed change (see below).
Click New issue and choose Bug report or Feature request. GitHub will present a form; please complete every field. The first fields ask which course you are taking and for the problem's slug, so that staff can tell which branch your report concerns even if you are unsure yourself.
When describing a bug, include the exact command you ran and its full output, as long as doing so does not violate CS50's Academic Honesty policy.
- Find the right branch. Take the branch name from the problem's slug (e.g.
2026/xfromcs50/problems/2026/x/mario/less) and confirm it is the current branch for that course in the table above. Changes are normally accepted only on current branches; branches for past offerings are kept stable so that students' results remain reproducible. If you believe a past branch needs a fix, open an issue first. - Fork this repository and create your branch from that course branch, not from the default branch, unless the default branch is the one you are changing.
- Make your change. Keep it focused on a single problem or issue.
- Open the pull request against the same course branch you started from. GitHub pre-selects the default branch as the base, so double-check the base dropdown before you submit.
- In the description, link the issue your pull request addresses (
Fixes #123) and state the slug(s) affected. If the same fix applies to another course's branch as well, say so; staff will cherry-pick rather than asking you to open several pull requests.
A member of the CS50 staff will review your issue or pull request and follow up as needed. If a pull request was opened against the wrong branch, staff will usually retarget it, but telling us the slug up front saves everyone time.