WordPress Filesystem API

WP abstracts filesystem access because on shared hosts the web user often can't write plugin/theme/core paths directly — FTP/SSH credentials are required. WP_Filesystem picks the right transport (direct when PHP can write as the file owner, otherwise ssh2 / ftpext / ftpsockets) and exposes a uniform method set. Plugins that call file_put_contents() directly outside writable uploads fail on those hosts; plugins that use WP_Filesystem work across more hosting setups.

When to use this skill

Trigger when ANY of the following is true:

• A plugin needs to write files outside wp-content/uploads/ — generated CSS, bundled asset extraction, log files, exported reports, cache, mu-plugins install.
• Code references WP_Filesystem, request_filesystem_credentials, get_filesystem_method, FS_METHOD, FS_CHMOD_FILE, FS_CHMOD_DIR, $wp_filesystem, WP_Filesystem_Direct, WP_Filesystem_SSH2, WP_Filesystem_FTPext, WP_Filesystem_ftpsockets.
• Code uses file_put_contents / fwrite / fopen / unlink / mkdir / rmdir for plugin-owned files outside uploads.
• The user reports: "works on my localhost, fails on the shared host", "the file doesn't write", "users see an FTP prompt out of nowhere".

The bootstrap — five lines, in this exact order

// 1. Load the API. NOT autoloaded.
require_once ABSPATH . 'wp-admin/includes/file.php';