When does the Yoast SEO filter hook wpseo_sitemap_urlimages fire?

Question

I am trying to attach a simple function to the Yoast SEO filter hook wpseo_sitemap_urlimages and get the function to run, but I cannot do so. My code is:

function tp_filter_wpseo_sitemap_urlimages($images, $post_id) {
    error_log("test message");
    return $images;
}
add_filter('wpseo_sitemap_urlimages', 'tp_filter_wpseo_sitemap_urlimages');

To get this function to run, I have tried opening the following Yoast XML sitemaps:

• sitemap index (https://example.com/sitemap_index.xml)
• post sitemap (http://example.com/post-sitemap.xml)
• page sitemap (http://example.com/page-sitemap.xml)

My eventual goal with this is to add additional images to the Yoast post and page sitemaps (per these Yoast instructions).

The only support Yoast has offered for this is to direct me to this page.

Answer 1

What I wanted to say in my previous comment:

This hook is only fired when the sitemap is being built. After that, it's cached and always served from cache. That means you'll never see any change on the frontend because it's all cached.

To disable the cache you can use the following code during development:

add_filter( 'wpseo_enable_xml_sitemap_transient_caching', '__return_false' );

After that, the sitemap is never served from cache but always built when viewing the sitemap in the frontend. Then you'll see some changes.