diff --git a/plugins/platforms/slack/adapter.py b/plugins/platforms/slack/adapter.py
index cd848c4bc5..7debfb6616 100644
--- a/plugins/platforms/slack/adapter.py
+++ b/plugins/platforms/slack/adapter.py
@@ -5959,12 +5959,20 @@ class SlackAdapter(BasePlatformAdapter):
         started (the sequential-suppression case) is left untouched.
         """
         _ts = str((event or {}).get("ts") or "")
-        _was_claimed = bool(_ts) and _ts in self._processed_message_ts
+        # getattr: bare test doubles (object.__new__) may lack the map.
+        _claims = getattr(self, "_processed_message_ts", None)
+        _was_claimed = bool(_ts) and _claims is not None and _ts in _claims
         try:
             return await self._handle_slack_message_impl(event, payload)
         except BaseException:
-            if _ts and not _was_claimed and _ts in self._processed_message_ts:
-                self._processed_message_ts.pop(_ts, None)
+            _claims = getattr(self, "_processed_message_ts", None)
+            if (
+                _ts
+                and not _was_claimed
+                and _claims is not None
+                and _ts in _claims
+            ):
+                _claims.pop(_ts, None)
                 logger.warning(
                     "[%s] handler failed after claiming ts=%s; claim released "
                     "so a retry or edit can re-drive the turn",
diff --git a/tests/gateway/test_slack_mention.py b/tests/gateway/test_slack_mention.py
index a3d3aed0b8..2a52eea832 100644
--- a/tests/gateway/test_slack_mention.py
+++ b/tests/gateway/test_slack_mention.py
@@ -263,7 +263,9 @@ def test_reaction_guard_pinned_to_production_expression():
     (back to ``is_dm or is_mentioned``, which reacts to unmentioned MPIMs)
     fails here instead of silently passing a self-referential lambda.
     """
-    src = inspect.getsource(SlackAdapter._handle_slack_message)
+    # The public method is a thin claim-release guard; the production
+    # expression lives in the impl.
+    src = inspect.getsource(SlackAdapter._handle_slack_message_impl)
     assert "(is_one_to_one_dm or is_mentioned)" in src, (
         "reaction guard no longer keys off is_one_to_one_dm — an unmentioned "
         "MPIM would react again (regression of the group-DM fix)"