any idea why a request would return a CORS error when I sent a lot of IDs to a query, but works fine with just a couple?

Are you running a select which uses a GET request? Not sure about causing CORS error, but you might be exceeding URL length limit. I have found CORS errors can come back for a variety of reasons having nothing to do with actual CORS error (not just on Supabase).

yes, it is using a get request, and probably this is the reason as I am seeing the URL gets huge. Is it possible to send the request as post instead???

Not that I know of. Use an RPC function as that uses POST or split up are probably only two options. Someone else ran into this a few days ago and went with RPC.

Do you happen to know URL limit lenght? I could split up if I knew the limit... I have no experience with RPC functions, but I'll see if I can check them out... thanks a lot btw!

I'm not sure it is absolutely set in stone, but about 2000