The WordPress Feed API provides functionality for both generating outgoing feeds (RSS, Atom, RDF) and consuming external feeds via SimplePie integration.

Feed Types

WordPress supports multiple feed formats out of the box:

Comment Feeds

WordPress also generates dedicated comment feeds:

Feed URLs

Standard feed endpoints:

/feed/                  # Default feed (RSS 2.0)
/feed/rss2/             # RSS 2.0 explicitly
/feed/atom/             # Atom feed
/feed/rdf/              # RSS 1.0 (RDF)
/feed/rss/              # RSS 0.92

/comments/feed/         # Site-wide comment feed
/category/{slug}/feed/  # Category feed
/tag/{slug}/feed/       # Tag feed
/author/{name}/feed/    # Author feed
/{post-slug}/feed/      # Post comment feed

SimplePie Integration

WordPress uses SimplePie for consuming external feeds. The integration consists of:

Core Classes

Cache Behavior

Feed caching uses WordPress transients with multisite-aware storage:

• Default lifetime: 12 hours (43200 seconds)
• Storage: Site transients (*_site_transient())
• Transient names: feed_{hash} for data, feed_mod_{hash} for modification time

Feed Generation Flow

1. Request hits feed endpoint (e.g., /feed/)
2. WordPress loads the appropriate template (feed-rss2.php)
3. Template sets Content-Type header via feed_content_type()
4. Template loops through posts using have_posts() / the_post()
5. Feed-specific functions format content (e.g., the_title_rss(), the_content_feed())

Feed Consumption Flow

1. Call fetch_feed( $url ) with feed URL(s)
2. SimplePie instance is created and configured
3. WordPress HTTP API fetches the feed
4. Content is sanitized via WP_SimplePie_Sanitize_KSES
5. Data is cached in transients
6. Returns SimplePieSimplePie object or WP_Error

Key Concepts

Enclosures

Media attachments (audio, video) stored in post meta as enclosure:

URL
Size (bytes)
MIME type

Functions rss_enclosure() and atom_enclosure() output appropriate XML.

Content vs Excerpt

The rss_use_excerpt option controls feed content:

• Enabled: Only excerpt in <description>
• Disabled: Excerpt in <description>, full content in <content:encoded>

Atom Text Constructs

prep_atom_text_construct() determines content type per RFC 4287:

• text – No markup
• html – Tag soup (CDATA wrapped)
• xhtml – Well-formed XML (wrapped in <div xmlns="...">)

Multisite Considerations

As of WordPress 6.9, feed caching uses global cache via *_site_transient() functions, ensuring consistent caching across multisite networks.

Security

• Content is sanitized via wp_kses_post() when consuming feeds
• Enclosures validate MIME types against get_allowed_mime_types()
• Password-protected posts exclude content from feeds
• URLs are escaped with esc_url() throughout

Related Files

wp-includes/
├── feed.php                           # Core feed functions
├── feed-atom.php                      # Atom post template
├── feed-atom-comments.php             # Atom comment template
├── feed-rdf.php                       # RDF/RSS 1.0 template
├── feed-rss.php                       # RSS 0.92 template
├── feed-rss2.php                      # RSS 2.0 post template
├── feed-rss2-comments.php             # RSS 2.0 comment template
├── class-wp-feed-cache.php            # Legacy cache (deprecated)
├── class-wp-feed-cache-transient.php  # Transient cache handler
├── class-wp-simplepie-file.php        # HTTP file fetcher
├── class-wp-simplepie-sanitize-kses.php  # KSES sanitizer
└── class-simplepie.php                # SimplePie autoloader