Only MuxForms public form API URLs on muxforms.com (submit or schema GET paths).

What is a good Origin value?

The exact URL users see: https://www.yoursite.com with scheme and host, no path or trailing slash.

204 with no Allow-Origin?

Your origin is not on the form allowlist. Add it in the dashboard and run the test again.

See /guides/fixing-cors-errors-form-submit for fetch, Server Actions, and embed patterns.

The MuxForms form CORS tester sends an OPTIONS preflight to your submit URL with a chosen Origin header and shows Access-Control-Allow-Origin and related headers so you can fix browser blocked requests.

Tool

Form CORS tester

Paste endpoint URL and production origin. See preflight status before you debug in DevTools.

Endpoint URLTest Origin (your website)

Server-side probe sends OPTIONS with your Origin header. Browser row shows what this page's origin sees (useful for comparing). Only MuxForms /api/v1/forms/… URLs are allowed.

Fix CORS on form submit · Submit API docs

Common questions

What URL can I test?: Only MuxForms public form API URLs on muxforms.com (submit or schema GET paths).
What is a good Origin value?: The exact URL users see: https://www.yoursite.com with scheme and host, no path or trailing slash.
204 with no Allow-Origin?: Your origin is not on the form allowlist. Add it in the dashboard and run the test again.
Deeper guide?: See /guides/fixing-cors-errors-form-submit for fetch, Server Actions, and embed patterns.

Preflight green, inbox happy.

Allowlist once per form; embed and fetch both respect it.

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