diff --git a/astrbot/core/utils/session_waiter.py b/astrbot/core/utils/session_waiter.py
index b327a61843..0f4b9fe50d 100644
--- a/astrbot/core/utils/session_waiter.py
+++ b/astrbot/core/utils/session_waiter.py
@@ -97,8 +97,8 @@ def filter(self, event: AstrMessageEvent) -> str:
 
 class DefaultSessionFilter(SessionFilter):
     def filter(self, event: AstrMessageEvent) -> str:
-        """默认实现，返回统一消息来源字符串作为会话标识符"""
-        return event.unified_msg_origin
+        """Return a session identifier scoped to the current sender and chat."""
+        return f"{event.unified_msg_origin}:{event.get_sender_id()}"
 
 
 class SessionWaiter:
diff --git a/tests/unit/test_session_waiter.py b/tests/unit/test_session_waiter.py
new file mode 100644
index 0000000000..a9b8fb416b
--- /dev/null
+++ b/tests/unit/test_session_waiter.py
@@ -0,0 +1,39 @@
+from unittest.mock import MagicMock
+
+from astrbot.core.utils.session_waiter import DefaultSessionFilter
+
+
+def _event(umo: str, sender_id: str) -> MagicMock:
+    event = MagicMock()
+    event.unified_msg_origin = umo
+    event.get_sender_id.return_value = sender_id
+    return event
+
+
+def test_default_filter_isolates_group_members() -> None:
+    session_filter = DefaultSessionFilter()
+    umo = "qq:GroupMessage:123"
+
+    first_member = session_filter.filter(_event(umo, "user-1"))
+    second_member = session_filter.filter(_event(umo, "user-2"))
+
+    assert first_member != second_member
+
+
+def test_default_filter_keeps_same_sender_in_same_chat() -> None:
+    session_filter = DefaultSessionFilter()
+    umo = "qq:GroupMessage:123"
+
+    first_message = session_filter.filter(_event(umo, "user-1"))
+    next_message = session_filter.filter(_event(umo, "user-1"))
+
+    assert first_message == next_message
+
+
+def test_default_filter_isolates_same_sender_across_chats() -> None:
+    session_filter = DefaultSessionFilter()
+
+    first_chat = session_filter.filter(_event("qq:GroupMessage:123", "user-1"))
+    second_chat = session_filter.filter(_event("qq:GroupMessage:456", "user-1"))
+
+    assert first_chat != second_chat