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.Tests/Federation/InboundRoutingTests.cs

@@ -106,6 +106,49 @@ namespace PrivaPub.Tests.Federation
 			Assert.NotNull(message.ConversationId);
 		}
 
+		// Lemmy 0.19 and Mbin take a private message only as a ChatMessage: someone who writes to us that way is answered in
+		// kind, anyone else with a Note
+		[Fact]
+		public async Task A_direct_message_answers_in_the_form_its_recipient_writes_in()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var lemmy = new RemoteActor(_harness.Peer, "pm");
+			var mastodon = new RemoteActor(_harness.Peer, "dm");
+			async Task<Post> Received(RemoteActor sender, string type)
+			{
+				var id = NewId(sender, "messages");
+				await _harness.Deliver(sender, "/human-centipede", new JsonObject
+				{
+					["id"] = NewId(sender, "activities/create"), ["type"] = "Create", ["actor"] = sender.Id, ["to"] = new JsonArray(alice.Uri),
+					["object"] = new JsonObject
+					{
+						["id"] = id, ["type"] = type, ["attributedTo"] = sender.Id, ["to"] = new JsonArray(alice.Uri),
+						["content"] = "<p>psst</p>", ["published"] = DateTime.UtcNow.ToString("O")
+					}
+				});
+				return await DB.Default.Find<Post>().Match(p => p.ObjectURI == id).ExecuteSingleAsync(token);
+			}
+			async Task<JsonObject> Answer(RemoteActor to, Post message)
+			{
+				var answer = await _harness.Statuses.Publish(alice, new PrivaPub.Domain.Statuses.StatusDraft
+				{
+					Text = "psst back", Visibility = PostVisibility.Direct, ConversationId = message.ConversationId
+				}, token);
+				Assert.True(answer.Ok, answer.Error);
+				return (JsonObject)(await _harness.Outgoing(to.Id + "/inbox")).Last(a => a["type"]!.GetValue<string>() == "Create")["object"]!;
+			}
+
+			var chat = await Answer(lemmy, await Received(lemmy, "ChatMessage"));
+			var note = await Answer(mastodon, await Received(mastodon, "Note"));
+
+			Assert.Equal("ChatMessage", chat["type"]!.GetValue<string>());
+			Assert.Equal(lemmy.Id, Assert.Single(chat["to"]!.AsArray())!.GetValue<string>());
+			Assert.Null(chat["cc"]);
+			Assert.DoesNotContain("h-card", chat["content"]!.GetValue<string>());
+			Assert.Equal("Note", note["type"]!.GetValue<string>());
+		}
+
 		[Fact]
 		public async Task An_unreachable_key_asks_the_sender_to_retry_instead_of_refusing()
 		{