Why do I get the message "Serve images in next-gen formats"?

Both GTmetrix and Google PageSpeed Insights will display the warning "Serve images in next-gen formats" if your images are not delivered in WebP or AVIF format.

If that is on purpose and you don't want to serve WebP or AVIF images, that's fine, simply ignore the warning. But if you do want to serve WebP or AVIF images, continue reading.

ShortPixel Image Optimizer (SPIO)

If you use ShortPixel Image Optimizer, first make sure to enable the proper options:

If you already had everything configured to deliver WebP or AVIF files with the plugin, have a read at the following articles:

If you are using the method "Using the <PICTURE> tag syntax", then you should take a look at The "Using the <PICTURE> tag syntax" option doesn't work; WebP images won't show up.
On the other hand, if you are using "Without altering the page code (via .htaccess)", have a read at Delivering WebP images via .htaccess.

ShortPixel Adaptive Images (SPAI)

If you use ShortPixel Adaptive Images, you simply need to activate the option "Next-gen Images Support" on its settings. For more information, check this step-by-step: Step-by-step guide to install and use ShortPixel Adaptive Images (SPAI).

Bear in mind that if you have both SPIO and SPAI, you need to properly configure them: I already used SPIO for image optimization, what settings should I use with SPAI?

In case you had already everything properly configured, and your WebP/AVIF images are still not present, check the following article: ShortPixel Adaptive Images doesn't serve WebP images, why is that?

Autoptimize

When using Autoptimize, you simply need to activate the option "Optimize Images". That's enough to serve your images in WebP format, which should get rid of the message "Serve images in next-gen formats".

If you happen to use ShortPixel Image Optimizer with Autoptimize, make sure to read this one too: I want to start using Autoptimize with ShortPixel Image Optimizer, what should I do?