Why custom post type URLs return 404 until permalinks are re-saved, and how to automate the fix

Custom post type URLs often return 404 errors after registration because WordPress stores its compiled rewrite rules in the database and does not rebuild them when code changes. Only a flush, such as re-saving Settings → Permalinks, refreshes those rules. Flushing on every request is wasteful, and flushing only on plugin activation misses theme switches, deploys, and other changes. The author's approach is to hash the registered rewrite data on the wp_loaded hook and perform a soft flush only when that fingerprint changes. This keeps the per-request cost to one option read and one hash. The article also notes a complication with plugins that register post types only in the admin area.