A community announces back to its author's own server

Lemmy keeps its user's post in a community pending until the community announces it back. A community hosted here
announced only to its followers, so a post from a server where nobody follows the community stayed pending there; the
author's own server (its shared inbox) now gets the announces too. INTEROP records, confirmed live, why Lemmy refuses
our Flags (an Application reporter, no `to`, an array object). FEDERATION.md's custom emoji line is put back before the
replies paragraph.

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

FEDERATION.md

@@ -140,7 +140,8 @@ A group is either a **community** or a **circle**.
 
 - A **community** follows [FEP-1b12](https://codeberg.org/fediverse/fep/src/branch/main/fep/1b12/fep-1b12.md). Posts
   addressed to it (in `to`, `cc` or `audience`) are accepted according to its posting policy (followers, anyone, or
-  moderators only) and the group `Announce`s the whole activity, with `audience` set, to its followers. A new post is
+  moderators only) and the group `Announce`s the whole activity, with `audience` set, to its followers and to the
+  author's own server, whether anyone there follows it or not (Lemmy keeps its user's post pending until then). A new post is
   also announced as an object so Mastodon shows it. Updates and deletes of community content are announced too. A
   top-level post is a `Page` with a `name`. Members are counted at `/flock`; moderators are listed at `/wardens`, which
   the actor's `attributedTo` points to, with `postingRestrictedToMods` as Lemmy expects. A mention of a community posts into it.
@@ -291,13 +292,13 @@ a post first met after its edits has them, and an edit that carries them brings
 - **Shown** to clients as GoToSocial's `interaction_policy` (`can_favourite`, `can_reply`, `can_reblog`).
 - Our own posts state only `canQuote`: anyone may reply to, like and boost them.
 
-**Custom emoji** (`Emoji` tags) are read on posts, display names, bios and profile fields, at most 64 per object.
 **Replies its author approves** (FEP-5624's `canReply`, as PeerTube sets it on a video whose comments are moderated; a
 `null` one says nothing): a persona it names, or that the post mentions, may reply, and the reply waits
 (`privapub.approval: pending`), its `Create` going to the author alone. The author's `ApproveReply`, signed by the author
 and naming the post answered, lets it out to its audience with `replyApproval`; a `RejectReply` leaves it ours alone
 (`rejected`). An empty `canReply` refuses every reply (422).
 
+**Custom emoji** (`Emoji` tags) are read on posts, display names, bios and profile fields, at most 64 per object.
 **Profiles** keep their header, profile fields, `manuallyApprovesFollowers`, `published`, `movedTo`, `indexable`,
 `memorial` and avatar and header descriptions. A post's title becomes `name` and is also the first, bold line of `content`, because Mastodon does not show
 `name`. A content warning without its own text uses the title, or "Content warning".

PrivaPub.Tests/Federation/GroupTests.cs

@@ -125,6 +125,21 @@ namespace PrivaPub.Tests.Federation
 			Assert.True(await DB.Default.Find<Post>().Match(p => p.GroupId == community.Id && p.ActorURI == stranger.Id).ExecuteAnyAsync(token));
 		}
 
+		// Lemmy keeps its user's post pending until the community announces it back, whether anyone there follows the
+		// community or not
+		[Fact]
+		public async Task A_post_into_a_community_is_announced_back_to_its_authors_server_though_nobody_there_follows_it()
+		{
+			var (_, owner) = await _harness.Persona("owner");
+			var (_, community) = await Group(GroupKind.Community, PostingPolicy.Anyone, owner.Id);
+			var stranger = new RemoteActor(_harness.Peer, "stranger", sharedInbox: true);
+
+			await _harness.Deliver(stranger, "/human-centipede", Create(stranger, new[] { community.Uri, Addressing.Public }, community.Uri));
+
+			var announces = (await _harness.Outgoing(stranger.SharedInbox)).Where(a => a["actor"]!.GetValue<string>() == community.Uri).ToList();
+			Assert.Contains(announces, a => a["object"] is JsonObject inner && inner["type"]!.GetValue<string>() == "Create");
+		}
+
 		[Fact]
 		public async Task Mentioning_a_community_posts_into_it_as_a_titled_page()
 		{

PrivaPub/Federation/Outbox/GroupDistributor.cs

@@ -1,6 +1,7 @@
 using MongoDB.Entities;
 
 using PrivaPub.Federation.Actors;
+using PrivaPub.Federation.Objects;
 using PrivaPub.Federation.Rendering;
 using PrivaPub.Models.Federation;
 
@@ -18,6 +19,16 @@ namespace PrivaPub.Federation.Outbox
 	public class GroupDistributor : IGroupDistributor
 	{
 		// so the announce's id resolves (PeasantsController.Grunt); a repeat of the same announce keeps the first
+		// a remote author's server's inbox (its shared one when it has one); none for our own personas
+		static async Task<string[]> AuthorsInbox(JsonObject activity, CancellationToken token)
+		{
+			if (ActivityJson.Id(activity["actor"]) is not { } actorUri)
+				return default;
+			var author = await DB.Default.Find<Models.User.ForeignAvatar>().Match(a => a.ActorURI == actorUri).ExecuteFirstAsync(token);
+			var inbox = string.IsNullOrEmpty(author?.SharedInboxURL) ? author?.InboxURL : author.SharedInboxURL;
+			return string.IsNullOrEmpty(inbox) ? default : new[] { inbox };
+		}
+
 		static async Task Keep(LocalActor group, JsonObject announce, string objectUri, CancellationToken token)
 		{
 			var body = (JsonObject)announce.DeepClone();
@@ -63,16 +74,18 @@ namespace PrivaPub.Federation.Outbox
 				["audience"] = group.Uri,
 				["object"] = embedded
 			};
-			// Every follower, the author's own server included: Lemmy 1.0 keeps its user's post pending until the community
+			// Every follower, and the author's own server whether anyone there follows the community or not: Lemmy 1.0 keeps
-			// announces it back, and clears that before it answers the echo 400 ("Object is not remote").
+			// its user's post pending until the community announces it back, and clears that before it answers the echo 400
-			await _delivery.EnqueueToFollowers(group, announce, token);
+			// ("Object is not remote").
+			var authorsServer = await AuthorsInbox(activity, token);
+			await _delivery.EnqueueToFollowers(group, announce, token, authorsServer);
 			await Keep(group, announce, objectUri, token);
 			// a new post or comment is also boosted as itself: microblogs (Mastodon, Akkoma, Misskey) drop an Announce of an
 			// activity, so without it their members never see what is said in the community; Lemmy answers it 400, harmlessly
 			if (isNewPost && !string.IsNullOrEmpty(objectUri))
 			{
 				var boost = ActivityPubRenderer.Announce(group, objectUri, $"announce-object-{key}");
-				await _delivery.EnqueueToFollowers(group, boost, token);
+				await _delivery.EnqueueToFollowers(group, boost, token, authorsServer);
 				await Keep(group, boost, objectUri, token);
 			}
 		}

docs/INTEROP.md

@@ -503,7 +503,9 @@ on a Page: pins live in `featured`, locks in `Lock`.
 - **Anti-spam:** activities for a community are accepted only if a local user follows it.
 - **Actors:** a Create, vote, moderation action or Flag must come from a Person, Service or Organization. **A Group or
   Application actor fails to parse**, so moderation comes from the moderator Person. Our Flags, sent by an
-  Application instance actor, probably fail *(unconfirmed)*.
+  Application instance actor with no `to` and an array `object` (the account and the post), are refused: confirmed live
+  2026-10-06, Lemmy 1.0.0-beta.2 answers 400 and keeps no report. Reaching its moderators needs an anonymous
+  `Service`-typed reporter, `to: [the post's community]` and the post alone as `object`.
 - **Flags:** exactly one `to` (community or site); `object` a URL or an array; the reason in `summary` or `content`.
 - **Moderation trust:** an action is trusted when it is on the community's or the object's host (FEP-fe34), or when its
   actor is in the moderators list Lemmy fetched.

docs/ROADMAP.md

@@ -661,7 +661,8 @@ it, raw where it doesn't.
     ban of a persona refuses its posts there and shows as `blocked_by`), featured, moderators, `Warn`, `Resolve`;
   - votes in and out; link posts; flairs; `Feed` actors; community polls; post `Move`;
   - the outbound shape Lemmy requires;
-  - communities we host announce to the author's own instance too;
+  - communities we host announce to the author's own instance too: **done 2026-10-06** (its server's shared inbox,
+    whether anyone there follows the community or not: Lemmy keeps its user's post pending until it is announced back);
   - Flags from a `Service`-typed reporter actor.
 - **GoToSocial interaction policies** (**done 2026-10-05**, `InteractionApprovals`): stored and shown as
   `interaction_policy`; `ReplyRequest`/`LikeRequest`/`AnnounceRequest` where approval is needed; `Accept{result}`