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Why is running another CDN on top of Vercel not recommended?

Understanding the problem

The Vercel CDN was designed to carefully perform new releases of your application without downtime. Therefore, once a new production deployment is ready, we will purge our cache globally and replace the current deployment being served at your production domain with the new version.

If you use a CDN on top of Vercel, you may experience issues since our infrastructure cannot control those providers. A new release on Vercel can cause other CDNs to serve stale content referencing files that no longer exist. You can see an example of the issue below:

Overview of possible issues sites using another CDN on top of Vercel may introduce to users.

A common symptom of the issue described above can be an increase in errors reported by your users after a production deployment is created. The stale HTML is returned by the 3rd party CDN but other assets were purged from it, returning a 404. For that reason, we recommend users to either configure the 3rd party CDN with a very short TTL in the cache or disable it completely.

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