async vs. sync depends exactly on what you are doing in what context. If this is in a network service, you need async. For a command line utility, sync is the appropriate paradigm in most simple cases, but just knee-jerk saying "async is better" is not correct. My snippet is based on the OP snippet for context.

Stop to think about "normal app" as like desktop app. Android isn't a desktop platform, there is no such this. A "normal" mobile app let the system control the lifecycle, not the dev. The system expect that, the users expect that. All you need to do is change your mindset and learn how to build on it. Don't try to clone a desktop app on mobile. Everything is completely different including UI/UX.

As I said above, the deciding factor is what is meant by syntax. If the request was sent with a Content Type of application/json, then yes, the request is syntactically valid because it's valid JSON syntax, but not semantically valid, since it doesn't match what's expected. (assuming a strict definition of what makes the request in question semantically valid or not). If, on the other hand, the request was sent with a more specific custom Content Type like application/vnd.mycorp.mydatatype+json that, perhaps, specifies exactly what fields are expected, then I would say that the request could easily be syntactically invalid, hence the 400 response.

From a client side/application (as a whole) standpoint this is an internal endpoint, for the router of SvelteKit this would be an external resource.

It seems inelegant to me to split this into two different modules, one to include, the other to extend.

include adds instance methods, extend adds class methods. This is how it works. I don't see inconsistency, only unmet expectations :)

which might or might not be useful depending on what is the content of the commit.