fix(session): require user anchor for delivery retention

nanobot/session/manager.py

@@ -352,12 +352,14 @@ class Session:
 
         start_idx = max(0, len(self.messages) - max_messages)
         if extend_to_user:
-            start_idx = next(
+            recovered_user = next(
                 (i for i in range(start_idx, -1, -1) if self.messages[i].get("role") == "user"),
-                start_idx,
+                None,
             )
-            if start_idx > 0 and self.messages[start_idx - 1].get("_channel_delivery"):
-                start_idx -= 1
+            if recovered_user is not None:
+                start_idx = recovered_user
+                if start_idx > 0 and self.messages[start_idx - 1].get("_channel_delivery"):
+                    start_idx -= 1
 
         retained = self.messages[start_idx:]
 

tests/agent/test_session_retention.py

@@ -120,6 +120,20 @@ def test_retain_extend_to_user_matches_get_history_boundary():
     assert _contents(session.messages) == _contents(expected)
 
 
+def test_retain_extend_to_user_does_not_extend_delivery_only_tail():
+    session = Session(key="test:extend-no-user")
+    for i in range(4):
+        session.messages.append(_delivery(f"notification {i}"))
+
+    session.retain_recent_legal_suffix(3, extend_to_user=True)
+
+    assert _contents(session.messages) == [
+        "notification 1",
+        "notification 2",
+        "notification 3",
+    ]
+
+
 # --- Only the immediately-preceding delivery is part of the anchor ---