Cannot Find Hook that is changing the_content()

It’s unusual for the_content to be rewriting image classes like this; it sounds like it could be a plugin (or theme function) which is doing this. Have you disabled all plugins just to check?

Having said that, most of the functions in core that hook into the_content are located in wp-includes/default-filters.php (apart from the oembed functions).

You can remove any of them by adding the following to your theme’s functions.php, eg. to remove the wpautop function:

remove_filter("the_content", "wpautop");

However none of these should be messing with these classes. The wp_make_content_images_responsive function will be messing with image tags, but it should only be checking the classes, not changing them, which is why it sounds like these changes could be happening in a plugin.

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