well that's fucking annoying:

blossom.primal.net returns 401 with preview: 'invalid x tag': the server is rejecting the Nostr auth event we’re forwarding (it decodes the event and finds an x tag it doesn’t like). That’s coming from blossom.primal.net itself, not the proxy.

Nostrcheck.me returns 500 with Error: Unexpected end of form: their backend never receives the full multipart payload (likely because it insists on a fixed content length and rejects chunked uploads).