fix(sessions): preserve branch lineage on reopen

hermes_state_sessions.py

@@ -448,14 +448,18 @@ class SessionSessionsMixin:
         return changed
 
     def reopen_session(self, session_id: str) -> None:
-        """Clear ended_at/end_reason so a session can be resumed; first stamp markerless legacy reset
-        children that depend on the parent's mutable end_reason (WHERE shared with the listing predicate
-        so they cannot drift)."""
+        """Clear ended_at/end_reason so a session can be resumed; first freeze only legacy reset children
+        whose creation order proves they followed the reset. The shared legacy predicate keeps list-time
+        compatibility broad, while this durable backfill must not overwrite explicit fork provenance."""
         def _do(conn):
             conn.execute(
                 "UPDATE sessions AS child SET model_config = json_set("
                 "COALESCE(child.model_config, '{}'), '$._reset_from', child.parent_session_id) "
                 f"WHERE child.parent_session_id = ? AND {_sql_json_extract('child.model_config', '$._reset_from')} IS NULL "
+                f"AND {_sql_json_extract('child.model_config', '$._branched_from')} IS NULL "
+                f"AND {_sql_json_extract('child.model_config', '$._delegate_from')} IS NULL "
+                "AND COALESCE(child.source, '') != 'tool' "
+                "AND child.started_at >= (SELECT p.ended_at FROM sessions p WHERE p.id = child.parent_session_id) "
                 f"AND {_legacy_reset_child_sql('child', _session_ids_placeholders(_RESET_END_REASONS))}",
                 (session_id, *_RESET_END_REASONS),
             )

tests/hermes_state/test_hermes_state.py

@@ -2896,6 +2896,75 @@ class TestListSessionsRich:
         ).fetchall()
         assert child_id not in {row["id"] for row in ephemeral}
 
+    def test_reopen_keeps_branch_provenance_out_of_legacy_reset_backfill(self, db):
+        """A same-key branch is never rewritten as a reset successor on reopen."""
+        lane_key = "agent:main:telegram:dm:branch"
+        db.create_session("branch_parent", "telegram", session_key=lane_key)
+        db.create_session(
+            "branch_child",
+            "telegram",
+            session_key=lane_key,
+            parent_session_id="branch_parent",
+            model_config={"_branched_from": "branch_parent"},
+        )
+        db.end_session("branch_parent", "session_switch")
+
+        db.reopen_session("branch_parent")
+
+        child = db.get_session("branch_child")
+        assert child is not None
+        assert json.loads(child["model_config"]) == {"_branched_from": "branch_parent"}
+        assert "branch_child" in [row["id"] for row in db.list_sessions_rich(source="telegram")]
+
+    def test_reopen_does_not_backfill_child_that_precedes_reset_boundary(self, db):
+        """A pre-marker branch cannot become a reset child after a later reopen cycle."""
+        lane_key = "agent:main:telegram:dm:legacy-branch"
+        db.create_session("legacy_branch_parent", "telegram", session_key=lane_key)
+        db.create_session(
+            "legacy_branch_child",
+            "telegram",
+            session_key=lane_key,
+            parent_session_id="legacy_branch_parent",
+        )
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?", (100.0, "legacy_branch_child")
+        )
+        db._conn.commit()
+        db.end_session("legacy_branch_parent", "branched")
+        db.reopen_session("legacy_branch_parent")
+        db.end_session("legacy_branch_parent", "session_switch")
+        db._conn.execute(
+            "UPDATE sessions SET ended_at = ? WHERE id = ?", (200.0, "legacy_branch_parent")
+        )
+        db._conn.commit()
+
+        db.reopen_session("legacy_branch_parent")
+
+        child = db.get_session("legacy_branch_child")
+        assert child is not None
+        assert child["model_config"] is None
+
+    def test_reopen_still_freezes_markerless_legacy_reset_child(self, db):
+        """A genuine old reset child remains visible after its parent is reopened."""
+        lane_key = "agent:main:telegram:dm:legacy-reset"
+        db.create_session("legacy_reset_parent", "telegram", session_key=lane_key)
+        db.end_session("legacy_reset_parent", "session_reset")
+        db.create_session(
+            "legacy_reset_child",
+            "telegram",
+            session_key=lane_key,
+            parent_session_id="legacy_reset_parent",
+        )
+
+        db.reopen_session("legacy_reset_parent")
+
+        child = db.get_session("legacy_reset_child")
+        assert child is not None
+        assert json.loads(child["model_config"])["_reset_from"] == "legacy_reset_parent"
+        assert "legacy_reset_child" in [
+            row["id"] for row in db.list_sessions_rich(source="telegram")
+        ]
+
     def test_reset_parent_does_not_surface_unrelated_child(self, db):
         db.create_session(
             "reset_parent",