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

TL;DR

The fix: put the exact HTTPS hostname into Astro's security.allowedDomains and keep checkOrigin enabled. Astro's checkOrigin blocks POSTs when the served hostname isnt in its allowlist, which is exactly what happens behind an Appwrite custom domain.

Steps

1. The fix: put the exact HTTPS hostname into Astro's security.allowedDomains and keep checkOrigin enabled.
2. Astro's checkOrigin blocks POSTs when the served hostname isnt in its allowlist, which is exactly what happens behind an Appwrite custom domain.

When to use

You are seeing this: Astro's checkOrigin blocks POSTs when the served hostname isnt in its allowlist, which is exactly what happens behind an Appwrite custom domain. 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

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

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