diff --git a/plugins/platforms/mattermost/adapter.py b/plugins/platforms/mattermost/adapter.py
index bc6288871b22e..4e80958b35165 100644
--- a/plugins/platforms/mattermost/adapter.py
+++ b/plugins/platforms/mattermost/adapter.py
@@ -753,6 +753,21 @@ class MattermostAdapter(BasePlatformAdapter):
                 import aiohttp
                 if isinstance(exc, aiohttp.WSServerHandshakeError) and exc.status in {401, 403}:
                     logger.error("Mattermost WS auth failed (HTTP %d) — stopping reconnect", exc.status)
+                    # Escalate through the fatal-error hook instead of a bare
+                    # return: the old silent exit left _running True, so
+                    # is_connected() kept reporting healthy while the listener
+                    # was dead and the gateway was never told (OOF-156 class).
+                    # Type-based only — the substring fallback that used to sit
+                    # below this branch misclassified transient errors whose
+                    # message merely contained "401" (#80489).
+                    self._set_fatal_error(
+                        "mattermost_auth_error",
+                        f"Mattermost WebSocket authentication rejected (HTTP {exc.status}). "
+                        "The bot token is invalid, revoked, or lacks permission — check "
+                        "MATTERMOST_TOKEN and the bot account in the System Console.",
+                        retryable=False,
+                    )
+                    await self._notify_fatal_error()
                     return
                 logger.warning("Mattermost WS error: %s — reconnecting in %.0fs", exc, delay)
 
diff --git a/tests/gateway/test_ws_auth_retry.py b/tests/gateway/test_ws_auth_retry.py
index 9f8b3ce04b13e..f07bba85630dc 100644
--- a/tests/gateway/test_ws_auth_retry.py
+++ b/tests/gateway/test_ws_auth_retry.py
@@ -19,8 +19,11 @@ class TestMattermostWSAuthRetry:
     """gateway/platforms/mattermost.py — _ws_loop()"""
 
     def test_401_handshake_stops_reconnect(self):
-        """A WSServerHandshakeError with status 401 should stop the loop."""
+        """A WSServerHandshakeError with status 401 should stop the loop —
+        AND escalate through the fatal-error hook (a bare return used to
+        leave _running True: dead listener, healthy-looking adapter)."""
         import aiohttp
+        from gateway.config import Platform
 
         exc = aiohttp.WSServerHandshakeError(
             request_info=MagicMock(),
@@ -34,6 +37,14 @@ class TestMattermostWSAuthRetry:
 
         adapter = MattermostAdapter.__new__(MattermostAdapter)
         adapter._closing = False
+        adapter._running = True
+        adapter.platform = Platform.MATTERMOST
+        notified = []
+
+        async def fatal_handler(a):
+            notified.append(a)
+
+        adapter._fatal_error_handler = fatal_handler
 
         call_count = 0
 
@@ -44,10 +55,20 @@ class TestMattermostWSAuthRetry:
 
         adapter._ws_connect_and_listen = fake_connect
 
-        asyncio.run(adapter._ws_loop())
+        async def run():
+            await adapter._ws_loop()
+            # Let the detached fatal-handler task complete.
+            await asyncio.sleep(0)
+
+        asyncio.run(run())
 
         # Should have attempted once and stopped, not retried
         assert call_count == 1
+        # Escalated: fatal error recorded, _running cleared, handler notified.
+        assert adapter._fatal_error_code == "mattermost_auth_error"
+        assert adapter._fatal_error_retryable is False
+        assert adapter._running is False
+        assert notified == [adapter]
 
     def test_transient_401_substring_does_not_stop_reconnect(self):
         """A transient exception whose stringified message merely contains
diff --git a/tests/gateway/test_ws_auth_retry_verifier_probe.py b/tests/gateway/test_ws_auth_retry_verifier_probe.py
index c5389bc8a8b85..514e62a1565e1 100644
--- a/tests/gateway/test_ws_auth_retry_verifier_probe.py
+++ b/tests/gateway/test_ws_auth_retry_verifier_probe.py
@@ -25,7 +25,11 @@ implementer thought to test:
 import asyncio
 from unittest.mock import AsyncMock, MagicMock, patch
 
-import aiohttp
+import pytest
+
+# Optional dep: CI installs [all] extras, but local envs may lack aiohttp —
+# skip cleanly instead of crashing collection.
+aiohttp = pytest.importorskip("aiohttp")
 
 from plugins.platforms.mattermost.adapter import MattermostAdapter
 
@@ -33,6 +37,14 @@ from plugins.platforms.mattermost.adapter import MattermostAdapter
 def _make_adapter(closing: bool = False) -> MattermostAdapter:
     adapter = MattermostAdapter.__new__(MattermostAdapter)
     adapter._closing = closing
+    # The genuine-auth path now escalates via _set_fatal_error +
+    # _notify_fatal_error (OOF-156 follow-up), which touch these attributes
+    # that __init__ would normally provide.
+    from gateway.config import Platform
+
+    adapter.platform = Platform.MATTERMOST
+    adapter._running = True
+    adapter._fatal_error_handler = None
     return adapter