Rewrite feed images. Keep the feed's identity intact.

Make the behaviour opt-in

Add a checkbox whose manifest default is the boolean false, not the string "false". Publii supplies manifest values to plugin configuration; a string is truthy in JavaScript and can silently turn an intended default-off feature on.

{
  "name": "cdnFeedImages",
  "label": "Use CDN for feed images",
  "value": false,
  "type": "checkbox"
}

Register the XML modifier

Register feedXmlOutput alongside any existing HTML modifier. Publii renders the feed, runs the registered XML modifiers sequentially, passes each modifier's result to the next, and only then saves feed.xml.

addModifiers() {
  this.API.addModifier(
    'feedXmlOutput',
    this.modifyFeedXmlOutput.bind(this),
    999,
    this
  );
}

modifyFeedXmlOutput(renderer, xmlCode) {
  if (!this.config.cdnFeedImages) return xmlCode;
  return rewriteFeedImageUrls(xmlCode, renderer.siteConfig.domain, this.config.url);
}

Return the original string directly while disabled. That makes byte-for-byte preservation both possible and easy to prove.

Rewrite attributes, not every occurrence of the domain

Do not run a global siteDomain → cdnDomain replacement over the XML. Publii's default template uses the site domain in the feed self link and feed ID, while each post URL appears in both its entry link and ID. Those are identity and navigation values, not image delivery URLs.

Limit the transformation to url on media:content, plus src and every candidate in srcset on img. Confirm the old URL belongs to the exact origin before replacing it; a similarly named host such as assets.example.com must not match example.com accidentally.

Respect XML outside and HTML inside

The document is Atom XML, but its summaries and content can contain HTML inside CDATA. Parsing the entire response as HTML and serialising it back can change the XML declaration, namespaces, casing, CDATA, or envelope. Use an XML-aware transform, or narrowly handle the intended tag attributes while preserving every other byte.

Likewise, do not rewrite every XML attribute named url. An unrelated enclosure url may describe audio or another resource with different delivery rules.