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diff --git a/lib/pleroma/web/activity_pub/mrf/ensure_re_prepended.ex b/lib/pleroma/web/activity_pub/mrf/ensure_re_prepended.ex
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+++ b/lib/pleroma/web/activity_pub/mrf/ensure_re_prepended.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.ActivityPub.MRF.EnsureRePrepended do
+  alias Pleroma.Object
+
+  @moduledoc "Ensure a re: is prepended on replies to a post with a Subject"
+  @behaviour Pleroma.Web.ActivityPub.MRF.Policy
+
+  @reply_prefix Regex.compile!("^re:[[:space:]]*", [:caseless])
+
+  def history_awareness, do: :auto
+
+  def filter_by_summary(
+        %{data: %{"summary" => parent_summary}} = _in_reply_to,
+        %{"summary" => child_summary} = child
+      )
+      when not is_nil(child_summary) and byte_size(child_summary) > 0 and
+             not is_nil(parent_summary) and byte_size(parent_summary) > 0 do
+    if (child_summary == parent_summary and not Regex.match?(@reply_prefix, child_summary)) or
+         (Regex.match?(@reply_prefix, parent_summary) &&
+            Regex.replace(@reply_prefix, parent_summary, "") == child_summary) do
+      Map.put(child, "summary", "re: " <> child_summary)
+    else
+      child
+    end
+  end
+
+  def filter_by_summary(_in_reply_to, child), do: child
+
+  def filter(%{"type" => type, "object" => child_object} = object)
+      when type in ["Create", "Update"] and is_map(child_object) do
+    child =
+      child_object["inReplyTo"]
+      |> Object.normalize(fetch: false)
+      |> filter_by_summary(child_object)
+
+    object = Map.put(object, "object", child)
+
+    {:ok, object}
+  end
+
+  def filter(object), do: {:ok, object}
+
+  def describe, do: {:ok, %{}}
+end