A public reply names the author it answers in cc

A reply whose text had lost the @name of the author it answers (decePub
prefills it, a reader may delete it) was addressed to nobody on the
author's server. PrivaPub delivered it to the author's inbox anyway, but
GoToSocial keeps only what is addressed to someone there, so the reply
never appeared under the post. A public or unlisted reply to a remote
post now names its parent's author in cc, as Pleroma does; the reply
already says whom it answers, so nothing new is revealed. The parent
author's actor id is kept on the reply (InReplyToActorURI).

Found by decePub's end-to-end tests on the town.

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

PrivaPub.Tests/Http/MastodonStatusesTests.cs

@@ -163,6 +163,24 @@ namespace PrivaPub.Tests.Http
 			Assert.Equal(post.ID, status.Text("id"));
 		}
 
+		// a reader who deletes the @name decePub prefills still answers its author: GoToSocial dropped such a reply, addressed
+		// to nobody there (found by decePub's e2e tests on the town)
+		[Fact]
+		public async Task A_reply_without_the_authors_name_still_names_the_author_in_cc()
+		{
+			var since = DateTime.UtcNow.AddSeconds(-1);
+			var alice = await _host.Mastodon("alice");
+			var bob = new RemoteActor(_peer, "bob");
+			_peer.WebFinger(bob);
+			var parent = await _host.PublicPostFrom(bob, alice);
+
+			var reply = await alice.Status("no name in this one", ("in_reply_to_id", parent.ID));
+
+			Assert.Empty(reply["mentions"]!.AsArray());
+			var create = Assert.Single(await bob.Delivered(since), d => d.Type() == "Create");
+			Assert.Contains(bob.Id, Strings(create["object"]!["cc"]));
+		}
+
 		[Fact]
 		public async Task A_poll_is_created_validated_and_federated_as_a_question()
 		{

PrivaPub/Domain/Statuses/StatusService.cs

@@ -198,6 +198,7 @@ namespace PrivaPub.Domain.Statuses
 				AnsweringToPostId = parent?.ID,
 				InReplyToURI = parent?.ObjectURI ?? (IsRemoteUri(draft.InReplyTo) ? draft.InReplyTo : default),
 				InReplyToAccountId = parent?.AuthorAccountId ?? parent?.GroupUserId,
+				InReplyToActorURI = parent is { IsFederatedCopy: true } ? parent.ActorURI : default,
 				IsLocalOnly = isLocalOnly,
 				ActorURI = author.Uri,
 				Poll = draft.Poll == default ? default : _polls.Create(draft.Poll),

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -146,6 +146,11 @@ namespace PrivaPub.Federation.Rendering
 			};
 			if (group is { IsCircle: false })
 				cc.Add(group.Uri);
+			// a public reply names the author it answers in cc even when the @name was deleted from its text, as Pleroma does:
+			// GoToSocial keeps only what is addressed to someone there, and the reply already says whom it answers
+			if (post.Visibility is PostVisibility.Public or PostVisibility.Unlisted && !string.IsNullOrEmpty(post.InReplyToActorURI)
+				&& !mentions.Any(m => m.GetValue<string>() == post.InReplyToActorURI))
+				cc.Add(post.InReplyToActorURI);
 
 			var note = NoteBody(post, author, to, cc, inReplyTo);
 			if (group != default)

PrivaPub/Models/Post/Post.cs

@@ -10,6 +10,7 @@ namespace PrivaPub.Models.Post
 		public string AnsweringToPostId { get; set; }//the parent's Post.ID when we hold it
 		public string InReplyToURI { get; set; }
 		public string InReplyToAccountId { get; set; }
+		public string InReplyToActorURI { get; set; }//a local reply's remote parent author, named in its cc
 		public string GroupId { get; set; }
 		public string ConversationId { get; set; }//DmGroup.ID of a direct post
 		public string AudienceURI { get; set; }//a remote community the post was made in