It would be great to check constraints on a new or edited statement //before// it is saved, so that possible errors can be corrected before the incorrect statement is added.
(I think this feature has been planned, at least as a “nice to have for the future”, for some time – I remember it came up when the API was added – but as far as I can tell we didn’t have a task for it before.)
On a more technical level, this means adding support for passing JSON snippets (for snaks, statements, of even full entities?) instead of entity or claim IDs into `wbcheckconstraints` (or a new API module?), which could also be used externally, e. g. before a large data import. (For instance, the [HarvestTemplates tool](https://tools.wmflabs.org/pltools/harvesttemplates/) already includes a partial [implementation of constraint checks](https://github.com/Pascalco/harvesttemplates/blob/86b38d29262b1705c96439247ae536d6e35ffc42/main.js#L400-L553), and aborts imports which fail some constraints.)