A field note from shipping Sitecore Content SDK (App Router) on Vercel — where a custom-domain access restriction and an internal editing fetch collided into a cryptic 500.

One of those Sitecore issues that looks simple at first, but turns into a few hours of digging.

We had Sitecore Pages working perfectly against the open *.vercel.app URL, but the moment we switched to our protected custom domain, editing started failing with a rather unhelpful 500. The interesting part was that the problem wasn’t really Pages, Vercel, or the domain restriction on its own — it was how the Content SDK makes an internal request back to the app during editing.

Here’s what we found, and what finally fixed it.