Enable for all posts: am I missing something?

Do you have a amp_post_status_default_enabled filter being used that disables AMP by default? Otherwise, it should be enabled by default.

You can see the default logic here: https://github.com/ampproject/amp-wp/blob/f9c514575b3c15fa009257e2f449ea2ee03d5700/includes/class-amp-post-type-support.php#L106-L131

Note that AMP is disabled by default for posts which have custom templates selected.

Oh no, the custom template is what’s causing it then.

I can flip the db field bit that turns it on. Can you tell me which field it is?

You can add this code instead to your custom theme’s functions.php or a custom plugin:

add_filter( 'amp_post_status_default_enabled', '__return_true' );

Done, verified working. Thanks Weston!

Excellent. We’d appreciate a plugin review!