I recommend checking out this guide:

https://vercel.com/kb/guide/cloudflare-with-vercel

vercel.com

Should I use Cloudflare in front of Vercel? | Vercel Knowledge Base

Information on using Cloudflare together with Vercel.

you suggested to disabled proxy in cloudflare ?

Yes, as the guide suggests:

Using reverse proxies like Cloudflare will limit Vercel's traffic visibility for security measures, introduce latency that degrades performance, and create cache management issues that may affect reliability. We do not recommend using a reverse proxy in front of Vercel.

Thanks for the clarification. I understand Vercel’s recommendation to disable the proxy, but I have a specific reason to keep Cloudflare enabled for my 18 domains.

The main issue I’m facing is that my websites are crashing frequently with Error 526 (Invalid SSL) and sometimes Error 524 (Timeout) when the proxy is on.

Could you please clarify:

1. Since Cloudflare is a ‘Verified Proxy Lite’ (https://vercel.com/docs/security/reverse-proxy)provider, shouldn’t Vercel automatically handle the SSL/TLS handshake even with the proxy enabled?

vercel.com

Reverse Proxy Servers and Vercel

Learn why reverse proxy servers are not recommended with Vercel's firewall.

2. How can I ensure Vercel correctly renews the ACME challenge for SSL certificates while Cloudflare Proxy is active to prevent these 526 errors?

3. Are there any specific firewall or rate-limiting settings on Vercel’s end that might be triggering these crashes when traffic comes through Cloudflare’s IPs?

I really need to stabilize these 18 domains without losing Cloudflare’s features. Any advanced configuration tips would be appreciated.

I'd love to learn more!

From the docs you referenced:

We do not recommend placing a reverse proxy server in front of your Vercel project as it affects Vercel's firewall in the following ways:

• Vercel's CDN loses visibility into the traffic, which reduces the effectiveness of the firewall in identifying suspicious activity.
• Real end-user IP addresses cannot be accurately identified.
• If the reverse proxy undergoes a malicious attack, this traffic can be forwarded to the Vercel project and cause usage spikes.
• If the reverse proxy is compromised, Vercel's firewall cannot automatically purge the cache.