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HTTP/2 and HTTP/3

HTTP/2

HTTP/2 is an improved protocol for web traffic. It takes fewer resources, uses those more effectively and gracefully falls back on clients where it is not supported.

HTTP/3

HTTP/3 is the newest version of HTTP. It runs over QUIC, a UDP-based transport, instead of TCP, which makes connection setup faster and copes better with lossy networks. Browsers learn that your site offers HTTP/3 from an alt-svc response header and switch to it on later requests.

HTTP/3 is enabled by default on every site. Browsers that can’t use it, or can’t reach it over UDP, fall back to HTTP/2 automatically, so most sites never need to change this setting. It is separate from the HTTP/2 setting: turning one off does not affect the other.

When to turn it off

Consider turning HTTP/3 off if visitors on iPhones and iPads (Safari and other iOS browsers) report pages that hang or connections that drop, while the same pages load fine on desktop browsers. Some mobile networks and devices handle QUIC poorly, and turning HTTP/3 off moves those visitors back to HTTP/2.

Turning HTTP/3 off

  1. Open the Cache Rules > Configuration page of your dashboard.
  2. In the HTTP/3 card, choose Disabled and click Update.

The change reaches every edge server within 10 to 30 minutes. You can turn it back on the same way. If you use an AI agent connected to your account, it can make the same change with the set_cache_configuration tool by setting http3 to disabled.

Checking the change

Once the change is live, your site answers with alt-svc: clear, which tells browsers to stop using HTTP/3:

curl -sI https://yourdomain.com | grep -i alt-svc

While HTTP/3 is on, the same command shows alt-svc: h3=":443"; ma=86400.

Browsers remember the old alt-svc header for up to 24 hours, so some visitors may keep trying HTTP/3 for up to a day after you turn it off. A private or incognito window, or restarting the browser, picks up the change right away, which is the quickest way to confirm it fixed the problem.