How do I make 301 redirection from `/%post_id%/` to `/%postname%/`?

I’ve written my own solution, sharing it and hoping it will help someone.

Add this to your functions.php:

<?php

add_action('parse_request', 'redirect_postid_to_postname');
function redirect_postid_to_postname($wp)
{
    // If /%post_id%/
    if (is_numeric($wp->request)) {

        $post_id = $wp->request;
        $slug = get_post_field( 'post_name', $post_id );

        // If the post slug === the post number, prevent redirection loops.
        if ($slug !== $post_id) {
            
            // Adding url parameters manually to $redirect_to
            $parameters = $_SERVER[QUERY_STRING];

            $redirect_from = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] === 'on' ? "https" : "http") . "://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";
            $redirect_to = get_permalink($post_id) . (!empty($parameters) ? ("?" . $parameters) : "");
            
            // Prevent loops
            if($redirect_from !== $redirect_to) {
                wp_redirect($redirect_to, 301);
                exit;
            }

        }
    }
}