If you are hunting for an open-source Anki-algorithm alternative for handwriting, it helps to separate two things that often get tangled: the scheduling algorithm, and the practice it schedules.
The algorithm is the easy, open part. The practice is where almost everything actually goes wrong.
The scheduler is a solved, open commodity
Anki’s spacing was long powered by SM-2, a well-documented algorithm, and now uses FSRS, the Free Spaced Repetition Scheduler, which is open source and available as a library. In other words, the math that decides “show this card again in 9 days” is not proprietary magic. It is published, studied, and reusable.
So if your goal is access to a good open scheduling algorithm, you already have it. That is not the bottleneck for remembering how to write Chinese characters.
The bottleneck is what gets scheduled
Here is the part that matters. A scheduler only strengthens the skill you actually rehearse. If your scheduled reviews show you a character and ask “did you know it?”, you are rehearsing recognition. Writing from memory is recall, a different and harder skill, and no scheduler will build it unless your reviews make you produce the character by hand. We unpack this in the case for a dedicated Hanzi writing app and, on the memory side, in the forgetting curve for Hanzi.
This is why “I use Anki with FSRS and still cannot write characters” is so common. The algorithm is fine. The reps are the wrong kind.
Where Hanzi Write Practice stands, honestly
Hanzi Write Practice is not open source. If auditable, self-hostable code is a hard requirement for you, this is not that tool, and that is a legitimate need to take elsewhere.
What it does is apply proven spaced repetition to the right reps: each review hides the character and asks you to draw it from memory on a practice grid, then checks your stroke order, pinyin, and meaning. The characters you miss come back on schedule and collect in a focused difficult pile. The scheduler is ordinary; the discipline of from-memory writing is the point. For building it into a routine, see Chinese character writing practice that sticks.
If you love open tooling, keep your open scheduler for everything else. For the specific job of not forgetting how to write Hanzi, what you need is recall practice, by hand, every day.
Join early access and put your reps where they count.