Jetpack Boost Image CDN: HTTP/3 and disabling automatic image resizing

Hi there,

Thanks for the detailed breakdown, including the response headers and DevTools protocol info, that context makes both questions much easier to answer.

On Question 1 (HTTP/3):

Yes, HTTP/3 is supported across our infrastructure, including our CDN. We enabled it for all of Automattic’s services in late 2023, so the Image CDN endpoints (i0/i1/i2.wp.com) are served from HTTP/3-capable edge servers. You can read the announcement here:

https://wordpress.com/blog/2024/01/31/http3/

Whether an individual request actually negotiates HTTP/3 comes down to what the edge advertises to the browser at request time. In your case the response carries “Alt-Svc: clear”, which tells the browser to discard any advertised alternative protocol, so it has nothing to upgrade to on that request and stays on HTTP/2 (h2). That is why DevTools reports h2. This reflects what that endpoint is currently advertising rather than a limitation of your Apache/HTTP/2 origin, and it is not something you can force from your side.

On Question 2 (the fit= transformation):

There isn’t a UI toggle to keep the Image CDN enabled while turning off the automatic resizing, the CDN resizes and optimizes by design. Changing that behavior is possible, but it lives in developer filters rather than a setting. A couple of supported hooks are relevant here:

• jetpack_photon_pre_args lets you modify the arguments appended to CDN image URLs before the request is made, so you can adjust or remove transformation parameters. Reference: https://jetpack.com/resources/jetpack-hooks-control-jetpack-cdn/
• The Photon module exposesjetpack_photon_image_downsize_string and jetpack_photon_image_downsize_array, which filter the arguments added when an image goes through the CDN. Reference: https://developer.jetpack.com/module/photon/

One thing worth knowing: the fit=1920,1789 value is derived from the dimensions in your image markup (the srcset/sizes attributes WordPress generates), not an arbitrary downscale of your original file. So in many cases adjusting the markup dimensions changes what the CDN requests.

There is also a jetpack_photon_noresize_mode filter (https://developer.jetpack.com/hooks/jetpack_photon_noresize_mode/). It tells Photon to skip WordPress’s own uploaded-image resizing and use the CDN’s resize capabilities instead, so it isn’t a clean “never resize anything” switch. I would test its effect on a staging copy before relying on it.

Since this involves custom filter code specific to your setup, the exact snippet would be development work beyond what the plugin does out of the box, but the hooks above are the supported way to get there.

Happy to help interpret the behavior if you try one and see something unexpected.