fix(python-sdk): preserve reused session ancestry

python/sdk/src/deepseek_harness/client.py

@@ -445,7 +445,7 @@ class HarnessClient:
             self._notification_subscribers.pop(subscription_id, None)
 
     def _record_session_relationship_locked(self, notification: Notification) -> None:
-        if notification.method not in {"subagent.started", "subagent.finished"}:
+        if notification.method != "subagent.started":
             return
         parent_id = notification.payload.get("parentSessionId")
         child_id = notification.payload.get("childSessionId")
@@ -461,15 +461,18 @@ class HarnessClient:
     def _notification_belongs_to_session_tree(self, session_id: str) -> NotificationFilter:
         def belongs(notification: Notification) -> bool:
             payload = notification.payload
-            related_ids = (
-                payload.get("sessionId"),
-                payload.get("parentSessionId"),
-                payload.get("childSessionId"),
-            )
-            return any(
+            if notification.method in {"subagent.started", "subagent.finished"}:
+                parent_id = payload.get("parentSessionId")
+                if (
+                    isinstance(parent_id, str)
+                    and self._session_is_descendant_of(parent_id, session_id)
+                ):
+                    return True
+                return payload.get("childSessionId") == session_id
+            related_id = payload.get("sessionId")
+            return (
                 isinstance(related_id, str)
                 and self._session_is_descendant_of(related_id, session_id)
-                for related_id in related_ids
             )
 
         return belongs

python/sdk/tests/test_client.py

@@ -9,7 +9,7 @@ from pathlib import Path
 
 import pytest
 
-from deepseek_harness import DeepSeekHarness, HarnessClient, HarnessConfig
+from deepseek_harness import DeepSeekHarness, HarnessClient, HarnessConfig, Notification
 
 
 def test_high_level_sdk_runs_turn_and_collects_final_response(tmp_path: Path) -> None:
@@ -442,6 +442,65 @@ def test_session_subscription_keeps_descendant_relationships_across_subscription
     assert client._notifications.qsize() == 0
 
 
+def test_session_subscription_preserves_reused_child_ancestry_after_late_finish() -> None:
+    client = HarnessClient()
+    old_seen: list[Notification] = []
+    new_seen: list[Notification] = []
+    with (
+        client.subscribe_session_notifications("old-parent") as old_subscription,
+        client.subscribe_session_notifications("new-parent") as new_subscription,
+    ):
+        client._handle_message({
+            "jsonrpc": "2.0",
+            "method": "subagent.started",
+            "params": {"parentSessionId": "old-parent", "childSessionId": "reused-child"},
+        })
+        old_subscription.drain(old_seen.append)
+        new_subscription.drain(new_seen.append)
+        assert [notification.method for notification in old_seen] == ["subagent.started"]
+        assert new_seen == []
+
+        client._handle_message({
+            "jsonrpc": "2.0",
+            "method": "subagent.started",
+            "params": {"parentSessionId": "new-parent", "childSessionId": "reused-child"},
+        })
+        old_subscription.drain(old_seen.append)
+        new_subscription.drain(new_seen.append)
+        assert [notification.method for notification in new_seen] == ["subagent.started"]
+
+        client._handle_message({
+            "jsonrpc": "2.0",
+            "method": "subagent.finished",
+            "params": {"parentSessionId": "old-parent", "childSessionId": "reused-child"},
+        })
+        old_subscription.drain(old_seen.append)
+        new_subscription.drain(new_seen.append)
+        assert [notification.method for notification in old_seen] == [
+            "subagent.started",
+            "subagent.finished",
+        ]
+        assert [notification.method for notification in new_seen] == ["subagent.started"]
+
+        client._handle_message({
+            "jsonrpc": "2.0",
+            "method": "session.event",
+            "params": {"sessionId": "reused-child", "event": {"type": "assistant/message"}},
+        })
+        old_subscription.drain(old_seen.append)
+        new_subscription.drain(new_seen.append)
+
+    assert [notification.method for notification in old_seen] == [
+        "subagent.started",
+        "subagent.finished",
+    ]
+    assert [notification.method for notification in new_seen] == [
+        "subagent.started",
+        "session.event",
+    ]
+    assert client._notifications.qsize() == 0
+
+
 def test_client_contains_notification_filter_failure_to_its_subscription(tmp_path: Path) -> None:
     script = tmp_path / "fake_bridge.py"
     script.write_text(