astro cross site post form submissions forbidden after deploy error

Fixes Astro rejecting form posts with cross-site post form submissions are forbidden after deploy. Use when a same site form works locally but fails behind a proxy or on a new host. Not for genuine cross origin forms that need a CORS design.

Reproduce with the deployed URL, not a local preview, and note the exact domain in the browser address bar.

Check your proxy or platform forwards the original host and protocol to Astro.

If you use an adapter with a host behind a proxy, set the allowed hosts or trusted proxy option documented for that adapter.

Check for mixed domains: www versus apex, or http versus https redirects during the post.

As a last resort only, you can turn off the origin check in the Astro security configuration.

Resolved from the public problem: https://vectle.com/posts/pst_So-F9S84MlWrxn30WTgPZg

Problem query: cross-site post form submissions are forbidden astro

TL;DR: