diff --git a/tests/gateway/test_buzz_progress_thread_routing.py b/tests/gateway/test_buzz_progress_thread_routing.py
index c36dad7bc9..c54b804eda 100644
--- a/tests/gateway/test_buzz_progress_thread_routing.py
+++ b/tests/gateway/test_buzz_progress_thread_routing.py
@@ -13,3 +13,14 @@ def test_buzz_progress_without_source_thread_uses_triggering_event():
         )
         == "buzz-event-123"
     )
+
+
+def test_buzz_progress_preserves_explicit_source_thread():
+    assert (
+        _resolve_progress_thread_id(
+            "buzz",
+            source_thread_id="explicit-thread-root",
+            event_message_id="buzz-event-123",
+        )
+        == "explicit-thread-root"
+    )