A direct message answers in the form its recipient writes in

Lemmy 0.19 (most of the threadiverse) and Mbin take a private message only
as a ChatMessage and refuse a direct Note. A direct message to one account
elsewhere that writes to us as ChatMessages now goes out as one: to it
alone, without a mention in its text, kept on the post (Post.AsChatMessage)
so the served copy and an edit match. No software name decides it.

Live against Lemmy 0.19: alice's answer to lemmyuser's private message and
another persona's message to lemmyuser arrive (29 checks). A first message
to an account that never wrote to anyone here is still a Note, which they
refuse; G-0008 keeps that open for the owner.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -183,7 +183,14 @@ namespace PrivaPub.Federation.Rendering
 			foreach (var recipient in recipients.Where(r => !named.Contains(r.Uri)))
 				tags.Add(new JsonObject { ["type"] = "Mention", ["href"] = recipient.Uri, ["name"] = "@" + recipient.Handle });
 			var unmentioned = recipients.Where(r => !named.Contains(r.Uri)).ToList();
-			if (unmentioned.Count > 0)
+			// a ChatMessage (Pleroma's, which Lemmy 0.19 and Mbin take as their private messages) has one recipient and no
+			// mention of them in its text
+			if (post.AsChatMessage)
+			{
+				note["type"] = "ChatMessage";
+				note.Remove("cc");
+			}
+			else if (unmentioned.Count > 0)
 				note["content"] = "<p>" + string.Join(" ", unmentioned.Select(r =>
 						$"<span class=\"h-card\" translate=\"no\"><a href=\"{WebUtility.HtmlEncode(r.Uri)}\" class=\"u-url mention\">@<span>{WebUtility.HtmlEncode(r.Handle.Split('@')[0])}</span></a></span>"))
 					+ "</p>" + note["content"]!.GetValue<string>();