feat(relay): flat-DM liveliness — status anchors to the triggering ts, replies stay flat (QA-8)

Victor's correction: flat DMs CAN have a live thinking status. setStatus
on the triggering message's ts renders '… thinking'/per-tool phrases in
that message's thread-footer space and clears without leaving a message
artifact. Native suppresses this because ITS reply routing could inherit
the activated thread; the relay lane's flat-mode sends strip their
anchors explicitly (QA-6/7), so the status anchor cannot leak into reply
placement — proven by the new leak-guard test.

send_typing/stop_typing now anchor the status in flat mode too, gated by
platforms.relay.extra.slack.flat_dm_status (default ON; false restores
the fully anchorless posture). Thread mode unchanged.

gateway/relay/adapter.py

@@ -311,6 +311,24 @@ class RelayAdapter(BasePlatformAdapter):
         except Exception:  # noqa: BLE001 - config shape is operator-owned
             return True
 
+    def _flat_dm_status_enabled(self) -> bool:
+        """Liveliness in flat-DM mode: anchor the STATUS (not the reply) to the
+        triggering message's ts.
+
+        ``assistant.threads.setStatus`` on a message ts renders "… thinking" in
+        that message's thread-footer space and vanishes on clear — no message
+        artifact. Native suppresses this in flat mode because ITS response
+        routing could inherit the activated thread; the relay lane's sends are
+        explicitly flat in flat mode (QA-6/7 anchor strip), so the status
+        anchor cannot leak into reply placement here. Default ON — flat DMs
+        get a live status billboard while replies still post at the DM root.
+        Opt out: platforms.relay.extra.slack.flat_dm_status: false.
+        """
+        try:
+            return bool(self._relay_slack_extra().get("flat_dm_status", True))
+        except Exception:  # noqa: BLE001 - config shape is operator-owned
+            return True
+
     def _stamp_slack_session_thread(self, event) -> None:
         """Native session-keying parity for fronted Slack (QA-3).
 
@@ -1023,9 +1041,17 @@ class RelayAdapter(BasePlatformAdapter):
             and self._platform_by_chat.get(str(chat_id)) == Platform.SLACK.value
             and self._chat_type_by_chat.get(str(chat_id)) == "dm"
         ):
-            reply_in_thread = self._effective_reply_in_thread()
             anchor = self._last_inbound_ts_by_chat.get(str(chat_id))
-            if reply_in_thread and anchor:
+            # Thread mode: status targets the per-message thread (QA-1).
+            # Flat mode: the status can STILL anchor to the triggering ts —
+            # setStatus renders in the footer space and clears without a
+            # message artifact, and flat sends strip their anchors (QA-6/7)
+            # so reply placement cannot inherit it. Gated separately
+            # (flat_dm_status, default on) for a clean opt-out.
+            if anchor and (
+                self._effective_reply_in_thread()
+                or self._flat_dm_status_enabled()
+            ):
                 md["thread_id"] = anchor
         # Rich status parity (QA-1): run.py's live-status lane stashes the
         # current per-tool phrase via set_status_text() (base class store).
@@ -1084,9 +1110,11 @@ class RelayAdapter(BasePlatformAdapter):
             not (md.get("thread_id") or md.get("thread_ts"))
             and self._chat_type_by_chat.get(str(chat_id)) == "dm"
         ):
-            reply_in_thread = self._effective_reply_in_thread()
             anchor = self._last_inbound_ts_by_chat.get(str(chat_id))
-            if reply_in_thread and anchor:
+            if anchor and (
+                self._effective_reply_in_thread()
+                or self._flat_dm_status_enabled()
+            ):
                 md["thread_id"] = anchor
         try:
             await self._transport.send_outbound(

tests/gateway/relay/test_relay_slack_prompt_dm_root.py

@@ -277,15 +277,42 @@ async def test_typing_synthesizes_thread_anchor_in_thread_mode():
 
 
 @pytest.mark.asyncio
-async def test_typing_keeps_no_anchor_in_flat_mode():
-    """Flat mode: no synthetic thread for the status either (#18859)."""
+async def test_typing_flat_mode_status_anchors_to_trigger_ts_by_default():
+    """Flat-DM liveliness: the STATUS still anchors to the triggering ts
+    (renders in the footer space, no message artifact) while replies stay
+    flat — QA-6/7 strip send anchors, so placement cannot inherit this."""
     adapter, stub = _wire_with_ts("D1", "dm", "1700.0042")
     adapter.config.extra = {"reply_in_thread": False}
     await adapter.send_typing("D1", metadata=None)
     typing = [f for f in stub.sent if f["op"] == "typing"]
+    assert typing and typing[-1]["metadata"].get("thread_id") == "1700.0042"
+
+
+@pytest.mark.asyncio
+async def test_typing_flat_mode_opt_out_drops_anchor():
+    """flat_dm_status: false restores the fully-anchorless flat posture."""
+    adapter, stub = _wire_with_ts("D1", "dm", "1700.0042")
+    adapter.config.extra = {
+        "slack": {"reply_in_thread": False, "flat_dm_status": False}
+    }
+    await adapter.send_typing("D1", metadata=None)
+    typing = [f for f in stub.sent if f["op"] == "typing"]
     assert typing and "thread_id" not in typing[-1]["metadata"]
 
 
+@pytest.mark.asyncio
+async def test_flat_mode_sends_stay_flat_with_status_anchor_active():
+    """The liveliness anchor must NOT leak into reply placement: sends in
+    flat mode still strip the synthetic anchor (QA-6/7 contract)."""
+    adapter, stub = _wire_with_ts("D1", "dm", "1700.0042")
+    adapter.config.extra = {"reply_in_thread": False}
+    await adapter.send_typing("D1", metadata=None)
+    await adapter.send("D1", "the answer", reply_to="1700.0042")
+    frame = [f for f in stub.sent if f["op"] == "send"][-1]
+    assert frame["reply_to"] is None
+    assert "thread_id" not in (frame["metadata"] or {})
+
+
 @pytest.mark.asyncio
 async def test_typing_honours_real_thread_anchor():
     """Metadata that already names a thread wins over the synthetic cache."""