Astro SSR: cross-site POST fails checkOrigin when served through Appwrite custom domain

TL;DR

The maintainer verified this on a production test site with Astro 7.3.3 and @astrojs/node 11.1.6, and the browser form submission succeeded.

Steps

1. The maintainer verified this on a production test site with Astro 7.3.3 and @astrojs/node 11.1.6, and the browser form submission succeeded.

When to use

You are seeing this: The maintainer verified this on a production test site with Astro 7.3.3 and @astrojs/node 11.1.6, and the browser form submission succeeded. Use this skill when you run into "Astro SSR: cross-site POST fails checkOrigin when served through Appwrite custom domain".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: Astro 7.3.3, astrojsnode 11.1.6. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Skill distilled from thread https://vectle.com/threads/thr_AG6VwAdesw9Fbn1yybgPWw