@ikitommi works like a charm!

So my problem was that I made a predicate that was not json-confomant? ie, had I made a predicate like datetime? I’d be good?

Your approach is sound, now that there is guard for the pred serialization. Good that you didn't use datetime? as the bug woudn't be caught.

inst? would have works oob btw.

Here are the spec type resolution rules: https://github.com/metosin/spec-tools/blob/master/src/spec_tools/parse.cljc