On Scripting News, Dave Winer floated an addition to the source namespace:

Thinking about adding <source:inReplyTo> to the source namespace. Its
value is a URL, by default, and has an optional isPermaLink attribute,
a boolean, to indicate if it's not a permalink. Works just like the guid
element in RSS 2.0.

Three examples below, using real comment feeds from ma.tt: the site-wide one, the Easter Thoughts per-post feed, and the EmDash Feedback per-post feed which has threaded replies.

How clients discover the per-post comments feed

In the <head> of every WordPress post, you'll find:

<link rel="alternate" type="application/rss+xml"
      title="Matt Mullenweg &raquo; Easter Thoughts Comments Feed"
      href="https://ma.tt/2026/04/easter-thoughts/feed/" />

So there are two kinds of comment feeds to consider - the site-wide one and the per-post one - plus the threaded case.

A · Site-wide comments feed

https://ma.tt/comments/feed/

Each item is a comment on a different post, so <source:inReplyTo> varies per item. Default form (no attribute → the URL is a permalink).

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"
  xmlns:content="http://purl.org/rss/1.0/modules/content/"
  xmlns:dc="http://purl.org/dc/elements/1.1/"
  xmlns:atom="http://www.w3.org/2005/Atom"
  xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
  xmlns:source="http://source.scripting.com/">  <!-- NEW namespace -->
<channel>
  <title>Comments for Matt Mullenweg</title>
  <atom:link href="https://ma.tt/comments/feed/" rel="self" type="application/rss+xml" />
  <link>https://ma.tt</link>
  <description>Unlucky in Cards</description>
  <lastBuildDate>Wed, 06 May 2026 14:46:54 +0000</lastBuildDate>

  <item>
    <title>Comment on Easter Thoughts by Faustine</title>
    <link>https://ma.tt/2026/04/easter-thoughts/#comment-606592</link>
    <dc:creator><![CDATA[Faustine]]></dc:creator>
    <pubDate>Wed, 06 May 2026 14:46:54 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151842#comment-606592</guid>
    <!-- NEW: replies to post "Easter Thoughts" -->
    <source:inReplyTo>https://ma.tt/2026/04/easter-thoughts/</source:inReplyTo>
    <description><![CDATA[Saw this originally on LinkedIn, and it definitely came off as humor to me. Thanks for sharing!]]></description>
  </item>

  <item>
    <title>Comment on EmDash Feedback by A look into EmDash CMS - Hongkiat</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606503</link>
    <dc:creator><![CDATA[A look into EmDash CMS - Hongkiat]]></dc:creator>
    <pubDate>Wed, 22 Apr 2026 13:01:06 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606503</guid>
    <!-- NEW: replies to post "EmDash Feedback" -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/</source:inReplyTo>
    <description><![CDATA[[…] as I looked at the GitHub code, read Cloudflare's announcement … an interesting […]]]></description>
  </item>

  <item>
    <title>Comment on Taxonomist by Dan Q</title>
    <link>https://ma.tt/2026/04/taxonomist/#comment-606495</link>
    <dc:creator><![CDATA[Dan Q]]></dc:creator>
    <pubDate>Sat, 18 Apr 2026 21:02:31 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151765#comment-606495</guid>
    <!-- NEW: replies to post "Taxonomist" -->
    <source:inReplyTo>https://ma.tt/2026/04/taxonomist/</source:inReplyTo>
    <description><![CDATA[I tried to do almost exactly this a year and a half ago, with mixed success … Maybe it's time to revisit!]]></description>
  </item>
</channel>
</rss>

B · Per-post comments feed (flat)

https://ma.tt/2026/04/easter-thoughts/feed/

Every item is a comment on the same post, so every <source:inReplyTo>
value is identical. This panel uses WordPress's id form (https://ma.tt/?p=151842) - mirroring the existing <guid isPermaLink="false"> lines already in this feed - so isPermaLink="false" is set.

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"
  xmlns:content="http://purl.org/rss/1.0/modules/content/"
  xmlns:dc="http://purl.org/dc/elements/1.1/"
  xmlns:atom="http://www.w3.org/2005/Atom"
  xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
  xmlns:source="http://source.scripting.com/">  <!-- NEW namespace -->
<channel>
  <title>Comments on: Easter Thoughts</title>
  <atom:link href="https://ma.tt/2026/04/easter-thoughts/feed/" rel="self" type="application/rss+xml" />
  <link>https://ma.tt/2026/04/easter-thoughts/</link>
  <description>Unlucky in Cards</description>
  <lastBuildDate>Wed, 06 May 2026 14:46:54 +0000</lastBuildDate>

  <item>
    <title>By: Faustine</title>
    <link>https://ma.tt/2026/04/easter-thoughts/#comment-606592</link>
    <dc:creator><![CDATA[Faustine]]></dc:creator>
    <pubDate>Wed, 06 May 2026 14:46:54 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151842#comment-606592</guid>
    <!-- NEW: every item replies to the same post; id form -->
    <source:inReplyTo isPermaLink="false">https://ma.tt/?p=151842</source:inReplyTo>
    <description><![CDATA[Saw this originally on LinkedIn, and it definitely came off as humor to me. Thanks for sharing!]]></description>
  </item>

  <item>
    <title>By: Jimmy</title>
    <link>https://ma.tt/2026/04/easter-thoughts/#comment-606504</link>
    <dc:creator><![CDATA[Jimmy]]></dc:creator>
    <pubDate>Wed, 22 Apr 2026 15:34:32 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151842#comment-606504</guid>
    <source:inReplyTo isPermaLink="false">https://ma.tt/?p=151842</source:inReplyTo>
    <description><![CDATA[The above-discussed semi-controversy is a good example of why a corporate executive friend says the only non-work topic he discusses at work is the weather.]]></description>
  </item>

  <item>
    <title>By: JakeJake</title>
    <link>https://ma.tt/2026/04/easter-thoughts/#comment-606501</link>
    <dc:creator><![CDATA[JakeJake]]></dc:creator>
    <pubDate>Tue, 21 Apr 2026 16:21:02 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151842#comment-606501</guid>
    <source:inReplyTo isPermaLink="false">https://ma.tt/?p=151842</source:inReplyTo>
    <description><![CDATA[Happy Easter! WordPress Core drew me deeper into the web in the first place. And in that openness, new life spreads through a shared body :)]]></description>
  </item>
</channel>
</rss>

C · Per-post comments feed (threaded)

https://ma.tt/2026/04/emdash-feedback/feed/

WordPress threading today is encoded inside the HTML of <description>: a reply's text is prefixed with In reply to <a href="…#comment-N">Author</a>. Useful for humans reading the rendered HTML; lossy for everything else - a parser has to look inside the description and apply HTML parsing rules to figure out who replied to whom.

<source:inReplyTo> makes the same information structured, sitting alongside the existing description - so clients that already parse the "In reply to" link don't break, and new clients can lift threading directly from the feed.

Reply structure across the 5 selected items below (real data, copied verbatim from the live feed):

yanislav3380   (#606352)   →  top-level (replies to the post)
├─ Alan           (#606358)   →  replies to yanislav3380
└─ Márcio Duarte  (#606374)   →  replies to yanislav3380

Iqbal          (#606397)   →  top-level (replies to the post)
└─ Alberto Prado  (#606419)   →  replies to Iqbal

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"
  xmlns:content="http://purl.org/rss/1.0/modules/content/"
  xmlns:dc="http://purl.org/dc/elements/1.1/"
  xmlns:atom="http://www.w3.org/2005/Atom"
  xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
  xmlns:source="http://source.scripting.com/">  <!-- NEW namespace -->
<channel>
  <title>Comments on: EmDash Feedback</title>
  <atom:link href="https://ma.tt/2026/04/emdash-feedback/feed/" rel="self" type="application/rss+xml" />
  <link>https://ma.tt/2026/04/emdash-feedback/</link>
  <description>Unlucky in Cards</description>
  <lastBuildDate>Wed, 22 Apr 2026 13:01:06 +0000</lastBuildDate>

  <item>
    <title>By: Alberto Prado</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606419</link>
    <dc:creator><![CDATA[Alberto Prado]]></dc:creator>
    <pubDate>Wed, 08 Apr 2026 20:55:50 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606419</guid>
    <!-- NEW: replies to Iqbal (#606397) -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/#comment-606397</source:inReplyTo>
    <description><![CDATA[In reply to &lt;a href=&quot;https://ma.tt/2026/04/emdash-feedback/#comment-606397&quot;&gt;Iqbal&lt;/a&gt;.

$2 a year is quite a deal. I used to feel proud about my $2 a month VPS where i host almost 10 wordpress websites.]]></description>
  </item>

  <item>
    <title>By: Iqbal</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606397</link>
    <dc:creator><![CDATA[Iqbal]]></dc:creator>
    <pubDate>Sun, 05 Apr 2026 13:29:04 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606397</guid>
    <!-- NEW: top-level - replies to the post -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/</source:inReplyTo>
    <description><![CDATA[That part about “a random web host in Indonesia charging 99 cents a month” made me grin  I’m actually using hosting that costs only $2 a year, and hey, that’s exactly what makes WordPress so great!]]></description>
  </item>

  <item>
    <title>By: Márcio Duarte</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606374</link>
    <dc:creator><![CDATA[Márcio Duarte]]></dc:creator>
    <pubDate>Fri, 03 Apr 2026 14:20:10 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606374</guid>
    <!-- NEW: replies to yanislav3380 (#606352) -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/#comment-606352</source:inReplyTo>
    <description><![CDATA[In reply to &lt;a href=&quot;https://ma.tt/2026/04/emdash-feedback/#comment-606352&quot;&gt;yanislav3380&lt;/a&gt;.

They did the same for 1.1.1.1, and look how far it went.]]></description>
  </item>

  <item>
    <title>By: Alan</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606358</link>
    <dc:creator><![CDATA[Alan]]></dc:creator>
    <pubDate>Fri, 03 Apr 2026 07:40:53 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606358</guid>
    <!-- NEW: replies to yanislav3380 (#606352) -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/#comment-606352</source:inReplyTo>
    <description><![CDATA[In reply to &lt;a href=&quot;https://ma.tt/2026/04/emdash-feedback/#comment-606352&quot;&gt;yanislav3380&lt;/a&gt;.

CF has a tradition of releasing real products on April 1st. So, it shouldn&#039;t be the case to judge. That being said, if anyone want to simply create a blog, that doesn&#039;t have much traffic then, it could work with this product. But considering it&#039;s on workers, each page request will be considered against workers request(I could be wrong here). Again, many WP hosting also do so.]]></description>
  </item>

  <item>
    <title>By: yanislav3380</title>
    <link>https://ma.tt/2026/04/emdash-feedback/#comment-606352</link>
    <dc:creator><![CDATA[yanislav3380]]></dc:creator>
    <pubDate>Fri, 03 Apr 2026 04:40:23 +0000</pubDate>
    <guid isPermaLink="false">https://ma.tt/?p=151783#comment-606352</guid>
    <!-- NEW: top-level - replies to the post -->
    <source:inReplyTo>https://ma.tt/2026/04/emdash-feedback/</source:inReplyTo>
    <description><![CDATA[They posted this on April 1st... I&#039;m not falling for it.]]></description>
  </item>
</channel>
</rss>