shouldn't _idmap: Map<string, Schema> = new Map(); be WeakMap too? otherwise presense of id in meta results in strong pointer of Schema to be created which means it will not be removed from the _map ever even if there are no strong pointers to Schema from outside of this class thus creating mem leak potential.

You're correct. Unfortunately registries need to be iterable for z.toJSONSchema() as implemented (this is why CI is failing), and the fact that it's using WeakRef for the schema map is a bit of a vestige of an older implementation. I think I'll be switching to a plain Map and adding a .clear() method. It's not ideal, but honestly I think a true WeakMap implementation would also cause a lot of mysterious issues for people who don't understand how they work. Regardless, merging this fix 👍 Thank you.