For now, issues and patches can be filed at https://github.com/internetarchive/fatcat.
To start, the back-end (fatcatd, in rust), web interface (fatcat-web, in python), bots, and this guide are all versioned in the same git repository.
rust/HACKING documents for some common tasks and
gotchas when working with the rust backend.
When considering making a non-trivial contribution, it can save review time and duplicated work to post an issue with your intentions and plan. New code and features will need to include unit tests before being merged, though we can help with writing them.