diff --git a/core/models.py b/core/models.py
index b21b1af..fe85d36 100644
--- a/core/models.py
+++ b/core/models.py
@@ -11,10 +11,10 @@ from datetime import datetime
 from typing import Any
 
 
-VALID_TASK_STATUSES = [
+VALID_TASK_STATUSES = frozenset({
     "pending", "in_progress", "review", "done",
     "blocked", "decomposed", "cancelled", "revising",
-]
+})
 
 VALID_COMPLETION_MODES = {"auto_complete", "review"}
 
@@ -274,19 +274,29 @@ def get_children(conn: sqlite3.Connection, task_id: str) -> list[dict]:
     return _rows_to_list(rows)
 
 
-def has_open_children(conn: sqlite3.Connection, task_id: str) -> bool:
+def has_open_children(conn: sqlite3.Connection, task_id: str, visited: set[str] | None = None) -> bool:
     """Recursively check if task has any open (not done/cancelled) descendants."""
+    if visited is None:
+        visited = set()
+    if task_id in visited:
+        return False
+    visited = visited | {task_id}
     children = get_children(conn, task_id)
     for child in children:
         if child["status"] not in ("done", "cancelled"):
             return True
-        if has_open_children(conn, child["id"]):
+        if has_open_children(conn, child["id"], visited):
             return True
     return False
 
 
-def _check_parent_completion(conn: sqlite3.Connection, task_id: str) -> None:
+def _check_parent_completion(conn: sqlite3.Connection, task_id: str, visited: set[str] | None = None) -> None:
     """Cascade-check upward: if parent is 'revising' and all children closed → promote to 'done'."""
+    if visited is None:
+        visited = set()
+    if task_id in visited:
+        return
+    visited = visited | {task_id}
     task = get_task(conn, task_id)
     if not task:
         return
@@ -305,7 +315,7 @@ def _check_parent_completion(conn: sqlite3.Connection, task_id: str) -> None:
             (now, now, parent_id),
         )
         conn.commit()
-        _check_parent_completion(conn, parent_id)
+        _check_parent_completion(conn, parent_id, visited)
 
 
 VALID_TASK_SORT_FIELDS = frozenset({
diff --git a/tests/test_models.py b/tests/test_models.py
index 2da8ea2..3c7ede5 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -932,3 +932,80 @@ def test_get_pipeline_logs_ordered_asc(pipeline_conn):
     logs = models.get_pipeline_logs(db, pid)
     ids = [log["id"] for log in logs]
     assert ids == sorted(ids)
+
+
+# ---------------------------------------------------------------------------
+# KIN-UI-018: Защита от circular references в has_open_children /
+#              _check_parent_completion (decision #816, #817)
+# ---------------------------------------------------------------------------
+
+def test_circular_reference_protection_has_open_children_returns_false(conn):
+    """KIN-UI-018 (decision #816): has_open_children возвращает False при циклической ссылке A→B→A.
+
+    Задачи A и B создаются напрямую в БД с взаимными parent_task_id.
+    Ожидаемый результат: False (не True, не RecursionError).
+    """
+    models.create_project(conn, "p1", "P1", "/p1")
+    # Создаём задачи без parent сначала
+    models.create_task(conn, "P1-CYC-A", "p1", "Task A")
+    models.create_task(conn, "P1-CYC-B", "p1", "Task B")
+    # Устанавливаем цикл напрямую в БД, минуя валидацию API
+    conn.execute("UPDATE tasks SET parent_task_id = 'P1-CYC-B' WHERE id = 'P1-CYC-A'")
+    conn.execute("UPDATE tasks SET parent_task_id = 'P1-CYC-A' WHERE id = 'P1-CYC-B'")
+    conn.commit()
+
+    result_a = models.has_open_children(conn, "P1-CYC-A")
+    result_b = models.has_open_children(conn, "P1-CYC-B")
+
+    assert result_a is False
+    assert result_b is False
+
+
+def test_circular_reference_protection_check_parent_completion_returns_without_error(conn):
+    """KIN-UI-018 (decision #817): _check_parent_completion не падает и не зависает при цикле A→B→A.
+
+    Задачи A и B в статусе 'revising' с взаимными parent_task_id.
+    Ожидаемый результат: возврат без RecursionError, статус задач не изменился.
+    """
+    models.create_project(conn, "p1", "P1", "/p1")
+    models.create_task(conn, "P1-CPC-A", "p1", "Task A", status="revising")
+    models.create_task(conn, "P1-CPC-B", "p1", "Task B", status="revising")
+    # Устанавливаем цикл напрямую в БД
+    conn.execute("UPDATE tasks SET parent_task_id = 'P1-CPC-B' WHERE id = 'P1-CPC-A'")
+    conn.execute("UPDATE tasks SET parent_task_id = 'P1-CPC-A' WHERE id = 'P1-CPC-B'")
+    conn.commit()
+
+    # Не должно бросить RecursionError или зависнуть
+    models._check_parent_completion(conn, "P1-CPC-A")
+    models._check_parent_completion(conn, "P1-CPC-B")
+
+    # Статусы не изменились — цикл обнаружен и прерван
+    task_a = models.get_task(conn, "P1-CPC-A")
+    task_b = models.get_task(conn, "P1-CPC-B")
+    assert task_a["status"] == "revising"
+    assert task_b["status"] == "revising"
+
+
+# ---------------------------------------------------------------------------
+# KIN-UI-020: VALID_TASK_STATUSES — frozenset membership checks
+# ---------------------------------------------------------------------------
+
+def test_valid_task_statuses_is_frozenset():
+    """KIN-UI-020: VALID_TASK_STATUSES должен быть frozenset, не list."""
+    assert isinstance(models.VALID_TASK_STATUSES, frozenset)
+
+
+@pytest.mark.parametrize("status", [
+    "pending", "in_progress", "review", "done",
+    "blocked", "decomposed", "cancelled", "revising",
+])
+def test_valid_task_statuses_membership(status):
+    """KIN-UI-020: каждый валидный статус присутствует в VALID_TASK_STATUSES (membership check)."""
+    assert status in models.VALID_TASK_STATUSES
+
+
+def test_invalid_status_not_in_valid_task_statuses():
+    """KIN-UI-020: невалидный статус отсутствует в VALID_TASK_STATUSES."""
+    assert "invalid_status" not in models.VALID_TASK_STATUSES
+    assert "" not in models.VALID_TASK_STATUSES
+    assert "active" not in models.VALID_TASK_STATUSES
diff --git a/tests/test_revising_status.py b/tests/test_revising_status.py
index bffa5e7..0fd7fff 100644
--- a/tests/test_revising_status.py
+++ b/tests/test_revising_status.py
@@ -252,21 +252,14 @@ def test_list_tasks_parent_none_filter(proj):
 # 8. API tests
 # ===========================================================================
 
-def test_patch_task_revising_status(client, tmp_path):
+def test_patch_task_revising_status(client):
     """PATCH /api/tasks/{id} status=done с открытым child → ответ содержит status='revising'."""
-    import web.api as api_module
-    from core.db import init_db
-    from core import models as m
-
     client.post("/api/projects", json={"id": "p1", "name": "P1", "path": "/p1"})
     r = client.post("/api/tasks", json={"project_id": "p1", "title": "Parent Task"})
     parent_id = r.json()["id"]
 
-    # Seed child directly via models (TaskCreate API has no parent_task_id field)
-    conn = init_db(api_module.DB_PATH)
-    m.create_task(conn, parent_id + "a", "p1", "Child Task",
-                  parent_task_id=parent_id, status="pending")
-    conn.close()
+    client.post("/api/tasks", json={"project_id": "p1", "title": "Child Task",
+                                    "parent_task_id": parent_id})
 
     r = client.patch(f"/api/tasks/{parent_id}", json={"status": "done"})
 
@@ -276,19 +269,14 @@ def test_patch_task_revising_status(client, tmp_path):
 
 def test_get_children_endpoint(client):
     """GET /api/tasks/{id}/children возвращает список прямых дочерних задач."""
-    import web.api as api_module
-    from core.db import init_db
-    from core import models as m
-
     client.post("/api/projects", json={"id": "p1", "name": "P1", "path": "/p1"})
     r = client.post("/api/tasks", json={"project_id": "p1", "title": "Parent Task"})
     parent_id = r.json()["id"]
 
-    # Seed children via models
-    conn = init_db(api_module.DB_PATH)
-    m.create_task(conn, parent_id + "a", "p1", "Child A", parent_task_id=parent_id)
-    m.create_task(conn, parent_id + "b", "p1", "Child B", parent_task_id=parent_id)
-    conn.close()
+    client.post("/api/tasks", json={"project_id": "p1", "title": "Child A",
+                                    "parent_task_id": parent_id})
+    client.post("/api/tasks", json={"project_id": "p1", "title": "Child B",
+                                    "parent_task_id": parent_id})
 
     r = client.get(f"/api/tasks/{parent_id}/children")
 
@@ -307,19 +295,13 @@ def test_get_children_endpoint_not_found(client):
 
 def test_list_tasks_parent_filter_api(client):
     """GET /api/tasks?parent_task_id={id} фильтрует задачи по родителю."""
-    import web.api as api_module
-    from core.db import init_db
-    from core import models as m
-
     client.post("/api/projects", json={"id": "p1", "name": "P1", "path": "/p1"})
     r = client.post("/api/tasks", json={"project_id": "p1", "title": "Parent Task"})
     parent_id = r.json()["id"]
     client.post("/api/tasks", json={"project_id": "p1", "title": "Other Root Task"})
 
-    # Seed child via models
-    conn = init_db(api_module.DB_PATH)
-    m.create_task(conn, parent_id + "a", "p1", "Child Task", parent_task_id=parent_id)
-    conn.close()
+    client.post("/api/tasks", json={"project_id": "p1", "title": "Child Task",
+                                    "parent_task_id": parent_id})
 
     r = client.get(f"/api/tasks?parent_task_id={parent_id}")
 
diff --git a/web/api.py b/web/api.py
index 6da7021..ff6b510 100644
--- a/web/api.py
+++ b/web/api.py
@@ -1864,7 +1864,8 @@ def send_chat_message(project_id: str, body: ChatMessageIn):
 
 DIST = Path(__file__).parent / "frontend" / "dist"
 
-app.mount("/assets", StaticFiles(directory=str(DIST / "assets")), name="assets")
+if (DIST / "assets").exists():
+    app.mount("/assets", StaticFiles(directory=str(DIST / "assets")), name="assets")
 
 
 @app.get("/{path:path}")