Do some transmogrifying for the output.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -1,5 +1,6 @@
 defmodule Pleroma.Web.ActivityPub.ActivityPub do
   alias Pleroma.{Activity, Repo, Object, Upload, User, Notification}
+  alias Pleroma.Web.ActivityPub.Transmogrifier
   import Ecto.Query
   import Pleroma.Web.ActivityPub.Utils
   require Logger
@@ -242,15 +243,9 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
     end
   end
 
-  # TODO: Extract to own module, align as close to Mastodon format as possible.
-  def sanitize_outgoing_activity_data(data) do
-    data
-    |> Map.put("@context", "https://www.w3.org/ns/activitystreams")
-  end
-
   def publish(actor, activity) do
     remote_users = Pleroma.Web.Salmon.remote_users(activity)
-    data = sanitize_outgoing_activity_data(activity.data)
+    {:ok, data} = Transmogrifier.prepare_outgoing(activity.data)
     Enum.each remote_users, fn(user) ->
       if user.info["ap_enabled"] do
         inbox = user.info["source_data"]["inbox"]

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -38,7 +38,38 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
     end
   end
 
-  def prepare_incoming(_) do
-    :error
+  @doc
+  """
+  internal -> Mastodon
+  """
+  def prepare_outgoing(%{"type" => "Create", "object" => %{"type" => "Note"} = object} = data) do
+    object = object
+    |> add_mention_tags
+    |> add_attributed_to
+
+    data = data
+    |> Map.put("object", object)
+    |> Map.put("@context", "https://www.w3.org/ns/activitystreams")
+
+    {:ok, data}
+  end
+
+  def add_mention_tags(object) do
+    mentions = object["to"]
+    |> Enum.map(fn (ap_id) -> User.get_cached_by_ap_id(ap_id) end)
+    |> Enum.filter(&(&1))
+    |> Enum.map(fn(user) -> %{"type" => "mention", "href" => user.ap_id, "name" => "@#{user.nickname}"} end)
+
+    tags = object["tags"] || []
+
+    object
+    |> Map.put("tags", tags ++ mentions)
+  end
+
+  def add_attributed_to(object) do
+    attributedTo = object["attributedTo"] || object["actor"]
+
+    object
+    |> Map.put("attributedTo", attributedTo)
   end
 end

lib/pleroma/web/common_api/common_api.ex

@@ -61,7 +61,7 @@ defmodule Pleroma.Web.CommonAPI do
          cw <- data["spoiler_text"],
          object <- make_note_data(user.ap_id, to, context, content_html, attachments, inReplyTo, tags, cw),
          object <- Map.put(object, "emoji", Formatter.get_emoji(status) |> Enum.reduce(%{}, fn({name, file}, acc) -> Map.put(acc, name, "#{Pleroma.Web.Endpoint.static_url}#{file}") end)) do
-      res = ActivityPub.create(%{to: to, actor: user, context: context, object: object})
+      res = ActivityPub.create(%{to: to, actor: user, context: context, object: object, additional: %{"cc" => to}})
       User.increase_note_count(user)
       res
     end