diff --git a/PROGRESS_MARMOT.md b/PROGRESS_MARMOT.md
index 1331896..50c591d 100644
--- a/PROGRESS_MARMOT.md
+++ b/PROGRESS_MARMOT.md
@@ -35,10 +35,10 @@ Spec source: `~/marmot/README.md` + MIP-00..05.
 
 ## M4 — MIP-03 (group events)
 
-- [ ] Enforce kind `445` envelope validation (`#h` tag presence/shape, base64 content shape)
-- [ ] Keep relay MLS-agnostic (no MLS decrypt/inspect in relay hot path)
-- [ ] Add configurable retention policy for kind `445` traffic
-- [ ] Add tests for high-volume fanout behavior and deterministic query results
+- [x] Enforce kind `445` envelope validation (`#h` tag presence/shape, base64 content shape)
+- [x] Keep relay MLS-agnostic (no MLS decrypt/inspect in relay hot path)
+- [x] Add configurable retention policy for kind `445` traffic
+- [x] Add tests for high-volume fanout behavior and deterministic query results
 
 ## M5 — optional MIP-04 (encrypted media)
 
diff --git a/lib/parrhesia/protocol/event_validator.ex b/lib/parrhesia/protocol/event_validator.ex
index 1ef5426..b871d51 100644
--- a/lib/parrhesia/protocol/event_validator.ex
+++ b/lib/parrhesia/protocol/event_validator.ex
@@ -40,6 +40,9 @@ defmodule Parrhesia.Protocol.EventValidator do
           | :invalid_giftwrap_content
           | :missing_giftwrap_recipient_tag
           | :invalid_giftwrap_recipient_tag
+          | :missing_marmot_group_tag
+          | :invalid_marmot_group_tag
+          | :invalid_marmot_group_content
 
   @spec validate(map()) :: :ok | {:error, error_reason()}
   def validate(event) when is_map(event) do
@@ -117,7 +120,11 @@ defmodule Parrhesia.Protocol.EventValidator do
     missing_giftwrap_recipient_tag:
       "invalid: kind 1059 must include at least one recipient p tag",
     invalid_giftwrap_recipient_tag:
-      "invalid: kind 1059 recipient p tags must contain lowercase hex pubkeys"
+      "invalid: kind 1059 recipient p tags must contain lowercase hex pubkeys",
+    missing_marmot_group_tag: "invalid: kind 445 must include at least one h tag with a group id",
+    invalid_marmot_group_tag:
+      "invalid: kind 445 h tags must contain 32-byte lowercase hex group ids",
+    invalid_marmot_group_content: "invalid: kind 445 content must be non-empty base64"
   }
 
   @spec error_message(error_reason()) :: String.t()
@@ -198,6 +205,9 @@ defmodule Parrhesia.Protocol.EventValidator do
   defp validate_kind_specific(%{"kind" => 10_051} = event),
     do: validate_marmot_keypackage_relay_list(event)
 
+  defp validate_kind_specific(%{"kind" => 445} = event),
+    do: validate_marmot_group_event(event)
+
   defp validate_kind_specific(%{"kind" => 444}),
     do: {:error, :invalid_marmot_direct_welcome_event}
 
@@ -261,6 +271,14 @@ defmodule Parrhesia.Protocol.EventValidator do
     end
   end
 
+  defp validate_marmot_group_event(event) do
+    tags = Map.get(event, "tags", [])
+
+    with :ok <- validate_non_empty_base64_content(event, :invalid_marmot_group_content) do
+      validate_marmot_group_tags(tags)
+    end
+  end
+
   defp validate_giftwrap_event(event) do
     tags = Map.get(event, "tags", [])
 
@@ -269,10 +287,13 @@ defmodule Parrhesia.Protocol.EventValidator do
     end
   end
 
-  defp validate_non_empty_base64_content(event) do
+  defp validate_non_empty_base64_content(event),
+    do: validate_non_empty_base64_content(event, :invalid_marmot_keypackage_content)
+
+  defp validate_non_empty_base64_content(event, error_reason) do
     case Base.decode64(Map.get(event, "content", "")) do
       {:ok, decoded} when byte_size(decoded) > 0 -> :ok
-      _other -> {:error, :invalid_marmot_keypackage_content}
+      _other -> {:error, error_reason}
     end
   end
 
@@ -283,6 +304,21 @@ defmodule Parrhesia.Protocol.EventValidator do
     end
   end
 
+  defp validate_marmot_group_tags(tags) do
+    group_tags = Enum.filter(tags, &match_tag_name?(&1, "h"))
+
+    cond do
+      group_tags == [] ->
+        {:error, :missing_marmot_group_tag}
+
+      Enum.all?(group_tags, &valid_marmot_group_tag?/1) ->
+        :ok
+
+      true ->
+        {:error, :invalid_marmot_group_tag}
+    end
+  end
+
   defp validate_giftwrap_recipient_tags(tags) do
     recipient_tags = Enum.filter(tags, &match_tag_name?(&1, "p"))
 
@@ -405,6 +441,9 @@ defmodule Parrhesia.Protocol.EventValidator do
   defp valid_single_relay_tag?(["relay", relay_url]), do: valid_websocket_url?(relay_url)
   defp valid_single_relay_tag?(_tag), do: false
 
+  defp valid_marmot_group_tag?(["h", group_id | _rest]), do: lowercase_hex?(group_id, 32)
+  defp valid_marmot_group_tag?(_tag), do: false
+
   defp valid_giftwrap_recipient_tag?(["p", recipient_pubkey | _rest]),
     do: lowercase_hex?(recipient_pubkey, 32)
 
diff --git a/test/parrhesia/protocol/event_validator_marmot_test.exs b/test/parrhesia/protocol/event_validator_marmot_test.exs
index 53d8dce..ffbf81e 100644
--- a/test/parrhesia/protocol/event_validator_marmot_test.exs
+++ b/test/parrhesia/protocol/event_validator_marmot_test.exs
@@ -57,6 +57,57 @@ defmodule Parrhesia.Protocol.EventValidatorMarmotTest do
              Protocol.validate_event(event)
   end
 
+  test "accepts valid kind 445 Marmot group envelope" do
+    group_event =
+      valid_keypackage_event(%{
+        "kind" => 445,
+        "tags" => [["h", String.duplicate("c", 64)]],
+        "content" => Base.encode64("mls-message")
+      })
+
+    assert :ok = EventValidator.validate(group_event)
+  end
+
+  test "accepts opaque binary payload for kind 445 without MLS inspection" do
+    opaque_payload = Base.encode64(<<0, 255, 10, 42, 128, 1, 2, 3>>)
+
+    group_event =
+      valid_keypackage_event(%{
+        "kind" => 445,
+        "tags" => [["h", String.duplicate("c", 64)]],
+        "content" => opaque_payload
+      })
+
+    assert :ok = EventValidator.validate(group_event)
+  end
+
+  test "rejects malformed kind 445 Marmot group envelopes" do
+    missing_h =
+      valid_keypackage_event(%{
+        "kind" => 445,
+        "tags" => [["p", String.duplicate("c", 64)]],
+        "content" => Base.encode64("mls-message")
+      })
+
+    invalid_h =
+      valid_keypackage_event(%{
+        "kind" => 445,
+        "tags" => [["h", "not-hex"]],
+        "content" => Base.encode64("mls-message")
+      })
+
+    invalid_content =
+      valid_keypackage_event(%{
+        "kind" => 445,
+        "tags" => [["h", String.duplicate("d", 64)]],
+        "content" => "not-base64"
+      })
+
+    assert {:error, :missing_marmot_group_tag} = EventValidator.validate(missing_h)
+    assert {:error, :invalid_marmot_group_tag} = EventValidator.validate(invalid_h)
+    assert {:error, :invalid_marmot_group_content} = EventValidator.validate(invalid_content)
+  end
+
   test "rejects malformed kind 1059 wrapped welcome envelopes" do
     invalid_missing_recipient =
       valid_keypackage_event(%{
diff --git a/test/parrhesia/storage/adapters/postgres/events_query_count_test.exs b/test/parrhesia/storage/adapters/postgres/events_query_count_test.exs
index ac091f8..66f3d70 100644
--- a/test/parrhesia/storage/adapters/postgres/events_query_count_test.exs
+++ b/test/parrhesia/storage/adapters/postgres/events_query_count_test.exs
@@ -316,6 +316,36 @@ defmodule Parrhesia.Storage.Adapters.Postgres.EventsQueryCountTest do
     assert Enum.map(results, & &1["id"]) == [tie_winner_id, tie_loser_id, older["id"]]
   end
 
+  test "query/3 keeps deterministic ordering for high-volume kind 445 group traffic" do
+    group_id = String.duplicate("c", 64)
+
+    events =
+      Enum.map(1..60, fn idx ->
+        persist_event(%{
+          "kind" => 445,
+          "created_at" => 1_700_001_000 + div(idx, 3),
+          "tags" => [["h", group_id]],
+          "content" => Base.encode64("group-message-#{idx}")
+        })
+      end)
+
+    assert {:ok, results} =
+             Events.query(%{}, [%{"kinds" => [445], "#h" => [group_id]}], [])
+
+    expected_ids =
+      events
+      |> Enum.sort(fn left, right ->
+        cond do
+          left["created_at"] > right["created_at"] -> true
+          left["created_at"] < right["created_at"] -> false
+          true -> left["id"] < right["id"]
+        end
+      end)
+      |> Enum.map(& &1["id"])
+
+    assert Enum.map(results, & &1["id"]) == expected_ids
+  end
+
   test "mls keypackage relay list kind 10051 follows replaceable conflict semantics" do
     author = String.duplicate("c", 64)
 
diff --git a/test/parrhesia/storage/adapters/postgres/events_test.exs b/test/parrhesia/storage/adapters/postgres/events_test.exs
index dc0d30c..421aa0e 100644
--- a/test/parrhesia/storage/adapters/postgres/events_test.exs
+++ b/test/parrhesia/storage/adapters/postgres/events_test.exs
@@ -59,6 +59,37 @@ defmodule Parrhesia.Storage.Adapters.Postgres.EventsTest do
     assert normalized.expires_at == 1_700_000_120
   end
 
+  test "keeps explicit expiration tag for kind 445 when present" do
+    previous_features = Application.get_env(:parrhesia, :features, [])
+    previous_policies = Application.get_env(:parrhesia, :policies, [])
+
+    Application.put_env(:parrhesia, :features, Keyword.put(previous_features, :nip_ee_mls, true))
+
+    Application.put_env(
+      :parrhesia,
+      :policies,
+      Keyword.put(previous_policies, :mls_group_event_ttl_seconds, 120)
+    )
+
+    on_exit(fn ->
+      Application.put_env(:parrhesia, :features, previous_features)
+      Application.put_env(:parrhesia, :policies, previous_policies)
+    end)
+
+    event = %{
+      "id" => String.duplicate("4", 64),
+      "pubkey" => String.duplicate("5", 64),
+      "created_at" => 1_700_000_000,
+      "kind" => 445,
+      "tags" => [["expiration", "1700000900"]],
+      "content" => "mls",
+      "sig" => String.duplicate("6", 128)
+    }
+
+    assert {:ok, normalized} = Events.normalize_event(event)
+    assert normalized.expires_at == 1_700_000_900
+  end
+
   test "candidate_wins_state?/2 uses created_at then lexical id tie-break" do
     assert Events.candidate_wins_state?(
              %{created_at: 11, id: <<2>>},
diff --git a/test/parrhesia/web/conformance_test.exs b/test/parrhesia/web/conformance_test.exs
index d80c6a0..5fb9a09 100644
--- a/test/parrhesia/web/conformance_test.exs
+++ b/test/parrhesia/web/conformance_test.exs
@@ -111,7 +111,7 @@ defmodule Parrhesia.Web.ConformanceTest do
       valid_event(%{
         "kind" => 445,
         "tags" => [["h", String.duplicate("b", 64)]],
-        "content" => "commit-envelope"
+        "content" => Base.encode64("commit-envelope")
       })
 
     assert {:push, {:text, commit_ok_frame}, ^state} =
diff --git a/test/parrhesia/web/connection_test.exs b/test/parrhesia/web/connection_test.exs
index f9ed209..75604c7 100644
--- a/test/parrhesia/web/connection_test.exs
+++ b/test/parrhesia/web/connection_test.exs
@@ -158,6 +158,37 @@ defmodule Parrhesia.Web.ConnectionTest do
            ]
   end
 
+  test "malformed kind 445 envelope EVENT is rejected" do
+    previous_features = Application.get_env(:parrhesia, :features, [])
+
+    Application.put_env(:parrhesia, :features, Keyword.put(previous_features, :nip_ee_mls, true))
+
+    on_exit(fn ->
+      Application.put_env(:parrhesia, :features, previous_features)
+    end)
+
+    state = connection_state()
+
+    event =
+      valid_event()
+      |> Map.put("kind", 445)
+      |> Map.put("tags", [["h", "not-hex"]])
+      |> Map.put("content", "not-base64")
+      |> then(&Map.put(&1, "id", EventValidator.compute_id(&1)))
+
+    payload = Jason.encode!(["EVENT", event])
+
+    assert {:push, {:text, response}, ^state} =
+             Connection.handle_in({payload, [opcode: :text]}, state)
+
+    assert Jason.decode!(response) == [
+             "OK",
+             event["id"],
+             false,
+             "invalid: kind 445 content must be non-empty base64"
+           ]
+  end
+
   test "NEG sessions open and close" do
     state = connection_state()
 
@@ -205,6 +236,35 @@ defmodule Parrhesia.Web.ConnectionTest do
     assert Jason.decode!(payload) == ["EVENT", "sub-1", event]
   end
 
+  test "high-volume kind 445 fanout drains in order across batches" do
+    group_id = String.duplicate("c", 64)
+
+    state =
+      subscribed_group_connection_state(group_id,
+        max_outbound_queue: 256,
+        outbound_drain_batch_size: 16
+      )
+
+    events =
+      Enum.map(1..70, fn idx ->
+        live_group_event("group-event-#{idx}", group_id)
+      end)
+
+    fanout_events = Enum.map(events, &{"sub-group", &1})
+
+    assert {:ok, queued_state} = Connection.handle_info({:fanout_events, fanout_events}, state)
+    assert queued_state.outbound_queue_size == 70
+
+    frames = drain_all_event_frames(queued_state)
+
+    delivered_ids =
+      frames
+      |> Enum.map(fn {:text, payload} -> Jason.decode!(payload) end)
+      |> Enum.map(fn ["EVENT", "sub-group", event] -> event["id"] end)
+
+    assert delivered_ids == Enum.map(events, & &1["id"])
+  end
+
   test "outbound queue overflow closes connection when strategy is close" do
     state =
       subscribed_connection_state(
@@ -238,6 +298,16 @@ defmodule Parrhesia.Web.ConnectionTest do
     subscribed_state
   end
 
+  defp subscribed_group_connection_state(group_id, opts) do
+    state = connection_state(opts)
+    req_payload = Jason.encode!(["REQ", "sub-group", %{"kinds" => [445], "#h" => [group_id]}])
+
+    assert {:push, _, subscribed_state} =
+             Connection.handle_in({req_payload, [opcode: :text]}, state)
+
+    subscribed_state
+  end
+
   defp connection_state(opts \\ []) do
     {:ok, state} = Connection.init(Keyword.put_new(opts, :subscription_index, nil))
     state
@@ -255,6 +325,18 @@ defmodule Parrhesia.Web.ConnectionTest do
     }
   end
 
+  defp live_group_event(id, group_id) do
+    %{
+      "id" => id,
+      "pubkey" => String.duplicate("a", 64),
+      "created_at" => System.system_time(:second),
+      "kind" => 445,
+      "tags" => [["h", group_id]],
+      "content" => Base.encode64("mls-group-message"),
+      "sig" => String.duplicate("b", 128)
+    }
+  end
+
   defp valid_auth_event(challenge) do
     now = System.system_time(:second)
 
@@ -270,6 +352,31 @@ defmodule Parrhesia.Web.ConnectionTest do
     Map.put(base, "id", EventValidator.compute_id(base))
   end
 
+  defp drain_all_event_frames(state), do: drain_all_event_frames(state, [])
+
+  defp drain_all_event_frames(%{outbound_queue_size: 0}, acc) do
+    flush_drain_messages()
+    Enum.reverse(acc)
+  end
+
+  defp drain_all_event_frames(state, acc) do
+    case Connection.handle_info(:drain_outbound_queue, state) do
+      {:push, frames, next_state} ->
+        drain_all_event_frames(next_state, Enum.reverse(frames) ++ acc)
+
+      {:ok, next_state} ->
+        drain_all_event_frames(next_state, acc)
+    end
+  end
+
+  defp flush_drain_messages do
+    receive do
+      :drain_outbound_queue -> flush_drain_messages()
+    after
+      0 -> :ok
+    end
+  end
+
   defp valid_event do
     base_event = %{
       "pubkey" => String.duplicate("1", 64),