diff --git a/backend/app/api/auth.py b/backend/app/api/auth.py
index 17af9ce..0570d32 100644
--- a/backend/app/api/auth.py
+++ b/backend/app/api/auth.py
@@ -28,7 +28,7 @@ from argon2 import PasswordHasher
 from argon2.exceptions import VerifyMismatchError
 from fastapi import APIRouter, HTTPException, Request, WebSocket
 from pydantic import BaseModel, Field
-from sqlalchemy import select
+from sqlalchemy import select, update
 from sqlalchemy.exc import IntegrityError
 from starlette.websockets import WebSocketDisconnect
 
@@ -56,6 +56,26 @@ _vanished: set[str] = set()
 AUTH_GENERATION_SYNC_SECONDS = 1.0
 AUTH_GENERATION_MAX_AGE_SECONDS = 2.0
 _generations_synced_at: float | None = None
+# Один lock на ещё неизвестный узлу логин: параллельные HTTP и WS входы
+# разделяют один запрос к БД до очередной синхронизации (lct-42). Запись
+# удаляется, когда lock больше никто не ждёт: иначе словарь растёт с каждым
+# логином, который узел когда-либо проверял.
+_lookup_locks: dict[str, "_LoginLookup"] = {}
+# Логины, которых разовая проверка не нашла в users. Держатся до следующей
+# сверки: повтор старой cookie не должен давать SELECT на каждый запрос.
+_missing_logins: set[str] = set()
+
+
+class _LoginLookup:
+    __slots__ = ("lock", "users")
+
+    def __init__(self) -> None:
+        self.lock = asyncio.Lock()
+        self.users = 0
+
+
+class _AuthStateUnavailable(Exception):
+    """Разовая проверка неизвестного узлу логина не смогла обратиться к БД."""
 
 
 async def _close_revoked(ws: WebSocket) -> None:
@@ -98,6 +118,7 @@ def prime_generations(values: dict[str, int]) -> None:
     global _generations_synced_at
     _generations.clear()
     _generations.update(values)
+    _missing_logins.clear()
     _generations_synced_at = time.monotonic()
 
 
@@ -108,15 +129,38 @@ async def load_generations() -> None:
 
 
 async def sync_generations() -> None:
-    """Refresh shared account epochs and close sockets revoked on peer nodes."""
+    """Сверить версии полномочий и закрыть отозванные на другом узле сокеты."""
+    known_before_query = set(_generations)
     async with get_sessionmaker()() as db:
         rows = (await db.execute(select(User.login, User.auth_version))).all()
-    current = {login: version for login, version in rows}
+        current = {login: version for login, version in rows}
+        # Версия узла выше БД, если отзыв не удалось записать (неудачный
+        # logout) или учётку пересоздали после удаления. Без записи в БД
+        # узлы расходятся навсегда: соседний узел выдаёт cookie со старой
+        # версией, а этот её отвергает. Поднимаем БД до версии узла — отзыв
+        # сохраняется, остальные узлы догоняют за одну сверку. При гонке со
+        # снимком БД уже выше, и UPDATE ничего не меняет.
+        ahead = {
+            login: _generations[login]
+            for login, version in current.items()
+            if login in _generations and _generations[login] > version
+        }
+        if ahead:
+            try:
+                for login, local in ahead.items():
+                    await db.execute(_raise_auth_version(login, local))
+                await db.commit()
+            except Exception as exc:  # noqa: BLE001 — повторим на следующей сверке
+                log.error("не удалось записать версию полномочий узла (%s)",
+                          type(exc).__name__)
+                await db.rollback()
     for login, version in current.items():
         previous = _generations.get(login)
+        # Снимок БД мог устареть за время запроса; меньшая версия не должна
+        # отменять локальный отзыв, который уже поднял поколение.
         if previous is None:
             _generations[login] = version
-        elif previous != version:
+        elif version > previous:
             invalidate_login(login, version)
     # Account deletion is not exposed by the application. Still close active
     # sockets if an operator removes one directly from the shared directory DB.
@@ -127,13 +171,88 @@ async def sync_generations() -> None:
     # lookup fall back to 0 and re-accept cookies issued before the revocation.
     synthetic = {"dev"} if get_settings().dev_auth_bypass else set()
     _vanished.intersection_update(_generations.keys() - current.keys())
-    for login in _generations.keys() - current.keys() - synthetic - _vanished:
+    # Разовая проверка могла найти учётку после снимка этого запроса.
+    # Её отсутствие в старом снимке не означает отзыв.
+    for login in known_before_query - current.keys() - synthetic - _vanished:
         invalidate_login(login)
         _vanished.add(login)
+    # Снимок только что прочитан: учётка, созданная до него, уже в кэше.
+    _missing_logins.clear()
     global _generations_synced_at
     _generations_synced_at = time.monotonic()
 
 
+def _raise_auth_version(login: str, version: int):
+    # Только вверх: параллельная запись в БД могла уже поднять версию выше.
+    return (
+        update(User)
+        .where(User.login == login, User.auth_version < version)
+        .values(auth_version=version)
+    )
+
+
+async def _resolve_unknown_login(login: str) -> int | None:
+    """Проверить неизвестный узлу логин сразу, не дожидаясь опроса БД.
+
+    Отозванный логин остаётся в `_generations` (см. `_vanished`), поэтому
+    отсутствие в кэше означает, что этот узел ещё не видел учётку.
+    Возвращает версию или None, если строки в `users` нет. При недоступной
+    или зависшей БД вызывает `_AuthStateUnavailable`, чтобы вход был закрыт
+    с 503/1013.
+    """
+    if login in _missing_logins:
+        return None
+    entry = _lookup_locks.get(login)
+    if entry is None:
+        entry = _lookup_locks[login] = _LoginLookup()
+    entry.users += 1
+    try:
+        # Тот же предел, что у сверки: при partition handshake не ждёт
+        # таймаута TCP, а очередь за lock не растягивает ожидание сверх него.
+        async with asyncio.timeout(AUTH_GENERATION_MAX_AGE_SECONDS):
+            async with entry.lock:
+                cached = _generations.get(login)
+                if cached is not None:
+                    return cached  # параллельный запрос уже получил версию
+                if login in _missing_logins:
+                    return None
+                async with get_sessionmaker()() as db:
+                    version = await db.scalar(
+                        select(User.auth_version).where(User.login == login)
+                    )
+                # Пока шёл SELECT, локальный отзыв или синхронизация могли
+                # записать новую версию. Старый ответ не должен вернуть
+                # отозванную cookie.
+                cached = _generations.get(login)
+                if cached is not None:
+                    return cached
+                if version is None:
+                    _missing_logins.add(login)
+                    return None
+                _generations[login] = version
+                log.warning(
+                    "учётка %s найдена разовой проверкой до сверки узла",
+                    login_log_marker(login),
+                )
+                return version
+    except Exception as exc:  # noqa: BLE001 — ошибка или таймаут БД закрывают вход
+        log.error("разовая проверка полномочий не удалась (%s)", type(exc).__name__)
+        raise _AuthStateUnavailable from exc
+    finally:
+        entry.users -= 1
+        if entry.users == 0 and _lookup_locks.get(login) is entry:
+            del _lookup_locks[login]
+
+
+def login_log_marker(login: str) -> str:
+    """Метка логина для журнала без самого логина.
+
+    По ней кластерный смоук доказывает, что узел прошёл через разовую
+    проверку, а не увидел учётку обычной сверкой.
+    """
+    return hashlib.sha256(f"lct-login:{login}".encode()).hexdigest()[:12]
+
+
 async def watch_generations() -> None:
     """Poll PostgreSQL once per node so remote logout/role changes close WS."""
     while True:
@@ -151,6 +270,21 @@ async def watch_generations() -> None:
         await asyncio.sleep(AUTH_GENERATION_SYNC_SECONDS)
 
 
+async def _send_auth_state_unavailable(scope, send) -> None:
+    if scope["type"] == "websocket":
+        await send({"type": "websocket.close", "code": 1013})
+    else:
+        await send({
+            "type": "http.response.start",
+            "status": 503,
+            "headers": [(b"content-type", b"application/json")],
+        })
+        await send({
+            "type": "http.response.body",
+            "body": b'{"detail":"auth_state_unavailable"}',
+        })
+
+
 class AuthVersionMiddleware:
     """Check signed-cookie epochs against the fresh, DB-synchronized node cache."""
 
@@ -186,22 +320,17 @@ class AuthVersionMiddleware:
             synced_at is None
             or time.monotonic() - synced_at > AUTH_GENERATION_MAX_AGE_SECONDS
         ):
-            if scope["type"] == "websocket":
-                await send({"type": "websocket.close", "code": 1013})
-            else:
-                await send({
-                    "type": "http.response.start",
-                    "status": 503,
-                    "headers": [(b"content-type", b"application/json")],
-                })
-                await send({
-                    "type": "http.response.body",
-                    "body": b'{"detail":"auth_state_unavailable"}',
-                })
+            await _send_auth_state_unavailable(scope, send)
             return
 
         cookie_version = session.get("auth_generation")
         version = _generations.get(login)
+        if version is None:
+            try:
+                version = await _resolve_unknown_login(login)
+            except _AuthStateUnavailable:
+                await _send_auth_state_unavailable(scope, send)
+                return
         if version is None or cookie_version != version:
             if session is not None:
                 session.clear()
@@ -536,7 +665,20 @@ async def login(payload: LoginIn, request: Request) -> dict:
         await audit_required(user.login, user.role, "login.blocked")
         raise HTTPException(status_code=403, detail="blocked")
 
-    if _generations.get(user.login) != user.auth_version:
+    local_version = _generations.get(user.login)
+    if local_version is not None and local_version > user.auth_version:
+        # Отзыв на этом узле не дошёл до БД (неудачный logout). Сброс к версии
+        # БД вернул бы силу cookie, выданной до выхода; выдача cookie с
+        # версией узла без записи в БД развела бы узлы. Поднимаем БД.
+        try:
+            async with get_sessionmaker()() as db:
+                await db.execute(_raise_auth_version(user.login, local_version))
+                await db.commit()
+        except Exception as exc:  # noqa: BLE001 — cookie без записанной версии не выдаём
+            log.error("вход: версию полномочий не удалось записать (%s)",
+                      type(exc).__name__)
+            raise HTTPException(status_code=503, detail="auth_state_unavailable") from exc
+    elif local_version != user.auth_version:
         invalidate_login(user.login, user.auth_version)
 
     who = Principal(
diff --git a/backend/tests/test_auth_hardening.py b/backend/tests/test_auth_hardening.py
index eec963a..f116913 100644
--- a/backend/tests/test_auth_hardening.py
+++ b/backend/tests/test_auth_hardening.py
@@ -135,6 +135,229 @@ def test_generation_sync_preserves_synthetic_dev_account(monkeypatch):
     asyncio.run(run())
 
 
+@pytest.mark.asyncio
+async def test_stale_generation_snapshot_does_not_revoke_newly_resolved_login(monkeypatch):
+    snapshot_read = asyncio.Event()
+    release_snapshot = asyncio.Event()
+
+    class FakeResult:
+        def all(self):
+            return []
+
+    class FakeDb:
+        async def execute(self, _query):
+            snapshot_read.set()
+            await release_snapshot.wait()
+            return FakeResult()
+
+        async def scalar(self, _query):
+            return 0
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    auth.prime_generations({})
+    monkeypatch.setattr(auth, "get_settings", lambda: SimpleNamespace(dev_auth_bypass=False))
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    sync = asyncio.create_task(auth.sync_generations())
+    await snapshot_read.wait()
+    assert await auth._resolve_unknown_login("just-created") == 0
+    release_snapshot.set()
+    await sync
+    assert auth._generations["just-created"] == 0
+    assert "just-created" not in auth._vanished
+
+
+@pytest.mark.asyncio
+async def test_stale_generation_snapshot_does_not_restore_revoked_cookie(monkeypatch):
+    snapshot_read = asyncio.Event()
+    release_snapshot = asyncio.Event()
+
+    raised = []
+
+    class FakeResult:
+        def all(self):
+            return [("revoked", 0)]
+
+    class FakeDb:
+        async def execute(self, query):
+            if query.is_update:
+                raised.append(query.compile().params)
+                return None
+            snapshot_read.set()
+            await release_snapshot.wait()
+            return FakeResult()
+
+        async def commit(self):
+            return None
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    auth.prime_generations({"revoked": 0})
+    monkeypatch.setattr(auth, "get_settings", lambda: SimpleNamespace(dev_auth_bypass=False))
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    sync = asyncio.create_task(auth.sync_generations())
+    await snapshot_read.wait()
+    auth.invalidate_login("revoked", 1)
+    release_snapshot.set()
+    await sync
+    assert auth._generations["revoked"] == 1
+    # В БД версия уже 1 (отзыв записан до invalidate); UPDATE «только вверх»
+    # в реальной БД ничего не изменит, в снимке же он выглядит как отставание.
+    assert [params["auth_version"] for params in raised] == [1]
+
+
+def _versioned_users_db(versions: dict[str, int]):
+    """Поддельная users: сверка читает версии, UPDATE поднимает их только вверх."""
+    updates = []
+
+    class FakeResult:
+        def all(self):
+            return list(versions.items())
+
+    class FakeDb:
+        async def execute(self, query):
+            if query.is_update:
+                params = query.compile().params
+                login = next(v for k, v in params.items() if k.startswith("login"))
+                target = params["auth_version"]
+                updates.append((login, target))
+                if versions.get(login, target) < target:
+                    versions[login] = target
+                return None
+            return FakeResult()
+
+        async def scalar(self, _query):
+            raise AssertionError("login is cached, no one-shot lookup expected")
+
+        async def commit(self):
+            return None
+
+        async def rollback(self):
+            return None
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    return updates, lambda: lambda: FakeSession()
+
+
+@pytest.mark.asyncio
+async def test_failed_logout_revocation_is_written_to_db_so_nodes_converge(monkeypatch):
+    """Неудачный logout поднял версию только на этом узле (B). Сверка не
+    откатывает отзыв, а записывает его в БД: соседний узел A догоняет, и вход
+    на A после этого выдаёт cookie, которую B принимает."""
+    login = "failed-logout-user"
+    versions = {login: 4}
+    updates, sessionmaker = _versioned_users_db(versions)
+    monkeypatch.setattr(auth, "get_settings", lambda: SimpleNamespace(dev_auth_bypass=False))
+    monkeypatch.setattr(auth, "get_sessionmaker", sessionmaker)
+    auth.prime_generations({login: 4})
+    try:
+        auth.invalidate_login(login)  # ветка except в logout: БД недоступна
+        assert auth._generations[login] == 5
+
+        await auth.sync_generations()
+        assert updates == [(login, 5)]
+        assert versions[login] == 5, "revocation must reach the shared DB"
+        assert auth._generations[login] == 5, "sync must not roll back a local revocation"
+
+        # Вход на A берёт версию из БД, и теперь она совпадает с версией B.
+        assert versions[login] == auth._generations[login]
+
+        await auth.sync_generations()
+        assert updates == [(login, 5)], "converged nodes must not write again"
+    finally:
+        auth._generations.pop(login, None)
+
+
+@pytest.mark.asyncio
+async def test_recreated_account_is_raised_to_the_node_revocation_version(monkeypatch):
+    """Учётку удалили (узел отозвал её, поколение +1) и создали заново с 0.
+    Без записи в БД узел остался бы впереди навсегда."""
+    login = "recreated-user"
+    versions = {}
+    updates, sessionmaker = _versioned_users_db(versions)
+    monkeypatch.setattr(auth, "get_settings", lambda: SimpleNamespace(dev_auth_bypass=False))
+    monkeypatch.setattr(auth, "get_sessionmaker", sessionmaker)
+    auth.prime_generations({login: 2})
+    try:
+        await auth.sync_generations()
+        assert auth._generations[login] == 3
+        assert login in auth._vanished
+
+        versions[login] = 0  # оператор создал учётку заново
+        await auth.sync_generations()
+        assert updates == [(login, 3)]
+        assert versions[login] == 3
+        assert login not in auth._vanished
+    finally:
+        auth._generations.pop(login, None)
+        auth._vanished.discard(login)
+
+
+def test_login_raises_db_version_when_node_revocation_was_not_persisted(client, monkeypatch):
+    """Вход на узле, где отзыв не дошёл до БД: cookie получает версию узла,
+    а БД поднимается до неё. Сброс к версии БД вернул бы силу старой cookie."""
+    from app.config import get_settings
+
+    user = SimpleNamespace(
+        login="ahead-login", auth_provider="local", password_hash="hash",
+        blocked=False, role="instructor", full_name="Преподаватель",
+        service=None, trainee_id=None, auth_version=1,
+    )
+    updates = []
+
+    class FakeDb:
+        async def scalar(self, _statement):
+            return user
+
+        async def execute(self, query):
+            assert query.is_update
+            updates.append(query.compile().params["auth_version"])
+
+        async def commit(self):
+            return None
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    async def audit_ok(*_args, **_kwargs):
+        return True
+
+    settings = get_settings().model_copy(update={"demo_no_db": False, "ldap_enabled": False})
+    monkeypatch.setattr(auth, "get_settings", lambda: settings)
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    monkeypatch.setattr(auth, "verify_password", lambda *_args: True)
+    monkeypatch.setattr(auth, "audit", audit_ok)
+    auth._generations[user.login] = 2  # неудачный logout на этом узле
+    try:
+        response = client.post("/api/auth/login", json={"login": user.login, "password": "x"})
+        assert response.status_code == 200
+        assert updates == [2]
+        assert auth._generations[user.login] == 2, "local revocation must not be rolled back"
+        assert client.get("/api/auth/me").status_code == 200
+    finally:
+        auth._generations.pop(user.login, None)
+
+
 def test_cross_origin_browser_websocket_is_rejected_before_handshake(client):
     assert client.post("/api/auth/dev-token").status_code == 200
     with pytest.raises(WebSocketDisconnect) as exc:
@@ -410,6 +633,401 @@ def test_auth_middleware_fails_closed_when_generation_cache_is_stale_but_allows_
     asyncio.run(run())
 
 
+def test_middleware_resolves_and_caches_login_unknown_to_this_node(monkeypatch):
+    """Cluster handshake (lct-42): a login another node just authenticated is
+    fetched via a single SELECT rather than being treated as revoked, and the
+    result is cached so a second request for it does not query again."""
+    login = "peer-node-fresh-login"
+    calls = []
+
+    class FakeDb:
+        async def scalar(self, _query):
+            calls.append(1)
+            return 5
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    class InnerApp:
+        def __init__(self):
+            self.called = 0
+
+        async def __call__(self, _scope, _receive, _send):
+            self.called += 1
+
+    async def run():
+        auth.prime_generations({})
+
+        def make_scope():
+            return {
+                "type": "http", "path": "/api/admin/users",
+                "session": {
+                    "principal": {"login": login},
+                    "auth_instance": auth._INSTANCE,
+                    "auth_generation": 5,
+                },
+            }
+
+        async def receive():
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        async def send(_message):
+            return None
+
+        first = InnerApp()
+        await auth.AuthVersionMiddleware(first)(make_scope(), receive, send)
+        assert first.called == 1, "a fresh, valid epoch must reach the route"
+        assert auth._generations[login] == 5
+        assert calls == [1]
+
+        second = InnerApp()
+        await auth.AuthVersionMiddleware(second)(make_scope(), receive, send)
+        assert second.called == 1
+        assert calls == [1], "cached epoch must not trigger a second SELECT"
+
+    settings = auth.get_settings().model_copy(update={"demo_no_db": False})
+    monkeypatch.setattr(auth, "get_settings", lambda: settings)
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._generations.pop(login, None)
+        auth._lookup_locks.pop(login, None)
+
+
+def test_middleware_rejects_login_missing_from_users_via_one_shot_query(monkeypatch):
+    login = "peer-node-deleted-login"
+
+    class FakeDb:
+        async def scalar(self, _query):
+            return None
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    class InnerApp:
+        def __init__(self):
+            self.called = False
+
+        async def __call__(self, _scope, _receive, _send):
+            self.called = True
+
+    async def run():
+        auth.prime_generations({})
+        cookie_session = {
+            "principal": {"login": login},
+            "auth_instance": auth._INSTANCE,
+            "auth_generation": 3,
+        }
+        scope = {"type": "http", "path": "/api/admin/users", "session": cookie_session}
+
+        async def receive():
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        async def send(_message):
+            return None
+
+        protected = InnerApp()
+        await auth.AuthVersionMiddleware(protected)(scope, receive, send)
+        assert protected.called, "route still runs but the session was cleared below"
+        assert cookie_session == {}
+        assert login not in auth._generations
+
+    settings = auth.get_settings().model_copy(update={"demo_no_db": False})
+    monkeypatch.setattr(auth, "get_settings", lambda: settings)
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._lookup_locks.pop(login, None)
+
+
+def test_unknown_login_lookup_does_not_clobber_a_newer_local_revocation(monkeypatch):
+    """A stale SELECT reply landing after a concurrent local revoke must not
+    resurrect the revoked cookie's epoch (regression: lock only deduplicates
+    concurrent lookups, it does not order a lookup against a write)."""
+    login = "peer-node-race-login"
+    started = asyncio.Event()
+    resume = asyncio.Event()
+
+    class FakeDb:
+        async def scalar(self, _query):
+            started.set()
+            await resume.wait()
+            return 0  # the epoch as it stood before the concurrent revoke below
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    async def run():
+        auth.prime_generations({})
+        task = asyncio.ensure_future(auth._resolve_unknown_login(login))
+        await started.wait()  # the SELECT is in flight, holding the login's lock
+        auth.invalidate_login(login, 1)  # a local logout/edit races the reply
+        resume.set()
+        result = await task
+        assert result == 1, "the newer local revocation must win over the stale SELECT reply"
+        assert auth._generations[login] == 1
+
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._generations.pop(login, None)
+        auth._lookup_locks.pop(login, None)
+
+
+def test_unknown_login_lookup_deduplicates_concurrent_requests_into_one_select(monkeypatch):
+    login = "peer-node-concurrent-login"
+    calls = []
+
+    class FakeDb:
+        async def scalar(self, _query):
+            calls.append(1)
+            await asyncio.sleep(0.01)  # widen the window for a racing second caller
+            return 9
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    async def run():
+        auth.prime_generations({})
+        results = await asyncio.gather(
+            auth._resolve_unknown_login(login),
+            auth._resolve_unknown_login(login),
+            auth._resolve_unknown_login(login),
+        )
+        assert results == [9, 9, 9]
+        assert calls == [1], "concurrent lookups for one login must issue a single SELECT"
+        assert login not in auth._lookup_locks, "lock entry must not outlive its waiters"
+
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._generations.pop(login, None)
+        auth._lookup_locks.pop(login, None)
+
+
+def test_unknown_login_lookup_fails_closed_when_database_is_unreachable(monkeypatch):
+    login = "peer-node-db-outage-login"
+
+    class BrokenSession:
+        async def __aenter__(self):
+            raise OSError("database unavailable")
+
+        async def __aexit__(self, *_args):
+            return None
+
+    class InnerApp:
+        def __init__(self):
+            self.called = False
+
+        async def __call__(self, _scope, _receive, _send):
+            self.called = True
+
+    async def run():
+        auth.prime_generations({})
+        cookie_session = {
+            "principal": {"login": login},
+            "auth_instance": auth._INSTANCE,
+            "auth_generation": 0,
+        }
+
+        async def receive():
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        http_messages = []
+
+        async def http_send(message):
+            http_messages.append(message)
+
+        http_app = InnerApp()
+        await auth.AuthVersionMiddleware(http_app)(
+            {"type": "http", "path": "/api/admin/users", "session": dict(cookie_session)},
+            receive, http_send,
+        )
+        assert not http_app.called
+        assert http_messages[0]["status"] == 503
+
+        ws_messages = []
+
+        async def ws_send(message):
+            ws_messages.append(message)
+
+        ws_app = InnerApp()
+        await auth.AuthVersionMiddleware(ws_app)(
+            {"type": "websocket", "path": "/ws/control/x", "session": dict(cookie_session)},
+            receive, ws_send,
+        )
+        assert not ws_app.called
+        assert ws_messages[0] == {"type": "websocket.close", "code": 1013}
+
+    settings = auth.get_settings().model_copy(update={"demo_no_db": False})
+    monkeypatch.setattr(auth, "get_settings", lambda: settings)
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: BrokenSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._lookup_locks.pop(login, None)
+
+
+def test_unknown_login_lookup_times_out_instead_of_hanging_on_partition(monkeypatch):
+    """При partition SELECT может висеть до таймаута TCP. Разовая проверка
+    ограничена тем же порогом, что сверка, и закрывает вход 503 / 1013 —
+    и для запроса, который ждёт lock за зависшим."""
+    login = "peer-node-hanging-db-login"
+    monkeypatch.setattr(auth, "AUTH_GENERATION_MAX_AGE_SECONDS", 0.3)
+
+    class HangingDb:
+        async def scalar(self, _query):
+            await asyncio.Event().wait()
+
+    class FakeSession:
+        async def __aenter__(self):
+            return HangingDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    class InnerApp:
+        called = False
+
+        async def __call__(self, _scope, _receive, _send):
+            self.called = True
+
+    async def run():
+        auth.prime_generations({})
+        cookie_session = {
+            "principal": {"login": login},
+            "auth_instance": auth._INSTANCE,
+            "auth_generation": 0,
+        }
+
+        async def receive():
+            return {"type": "http.request", "body": b"", "more_body": False}
+
+        http_messages, ws_messages = [], []
+
+        async def http_send(message):
+            http_messages.append(message)
+
+        async def ws_send(message):
+            ws_messages.append(message)
+
+        http_app, ws_app = InnerApp(), InnerApp()
+        started = asyncio.get_running_loop().time()
+        await asyncio.wait_for(asyncio.gather(
+            auth.AuthVersionMiddleware(http_app)(
+                {"type": "http", "path": "/api/admin/users", "session": dict(cookie_session)},
+                receive, http_send,
+            ),
+            auth.AuthVersionMiddleware(ws_app)(
+                {"type": "websocket", "path": "/ws/control/x", "session": dict(cookie_session)},
+                receive, ws_send,
+            ),
+        ), timeout=2)
+        elapsed = asyncio.get_running_loop().time() - started
+        assert elapsed < 1, "a queued handshake must not wait for a second timeout"
+        assert not http_app.called and not ws_app.called
+        assert http_messages[0]["status"] == 503
+        assert ws_messages[0] == {"type": "websocket.close", "code": 1013}
+        assert login not in auth._lookup_locks, "timed-out lookup must release its lock entry"
+
+    settings = auth.get_settings().model_copy(update={"demo_no_db": False})
+    monkeypatch.setattr(auth, "get_settings", lambda: settings)
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    asyncio.run(run())
+
+
+def test_one_shot_lookup_logs_login_marker_not_login(caplog, monkeypatch):
+    """Кластерный смоук ищет эту метку в журнале узла B: она доказывает, что
+    вход прошёл через разовую проверку, а не через обычную сверку."""
+    login = "peer-node-logged-login"
+
+    class FakeDb:
+        async def scalar(self, _query):
+            return 0
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    async def run():
+        auth.prime_generations({})
+        assert await auth._resolve_unknown_login(login) == 0
+
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        with caplog.at_level("WARNING", logger="app.api.auth"):
+            asyncio.run(run())
+    finally:
+        auth._generations.pop(login, None)
+    assert auth.login_log_marker(login) in caplog.text
+    assert login not in caplog.text
+
+
+def test_missing_login_is_cached_until_next_sync_and_lock_entries_are_released(monkeypatch):
+    login = "peer-node-replayed-missing-login"
+    lookups = []
+
+    class FakeResult:
+        def all(self):
+            return []
+
+    class FakeDb:
+        async def scalar(self, _query):
+            lookups.append(1)
+            return None
+
+        async def execute(self, _query):
+            return FakeResult()
+
+    class FakeSession:
+        async def __aenter__(self):
+            return FakeDb()
+
+        async def __aexit__(self, *_args):
+            return None
+
+    async def run():
+        auth.prime_generations({})
+        assert await auth._resolve_unknown_login(login) is None
+        assert await auth._resolve_unknown_login(login) is None
+        assert lookups == [1], "a replayed cookie must not query users on every request"
+        assert auth._lookup_locks == {}
+
+        await auth.sync_generations()
+        assert await auth._resolve_unknown_login(login) is None
+        assert lookups == [1, 1], "the negative result lives only until the next sync"
+
+    monkeypatch.setattr(auth, "get_settings", lambda: SimpleNamespace(dev_auth_bypass=False))
+    monkeypatch.setattr(auth, "get_sessionmaker", lambda: lambda: FakeSession())
+    try:
+        asyncio.run(run())
+    finally:
+        auth._missing_logins.discard(login)
+
+
 def test_auth_middleware_uses_fresh_generation_cache_without_per_request_database_query(monkeypatch):
     from app.config import get_settings
 
diff --git a/scripts/test_cluster_failover.py b/scripts/test_cluster_failover.py
index c4bcb5f..342d8ac 100644
--- a/scripts/test_cluster_failover.py
+++ b/scripts/test_cluster_failover.py
@@ -72,7 +72,7 @@ def login(base: str, login_name: str, password: str) -> str:
     return cookie
 
 
-async def send_control(ws_base: str, session_id: UUID, cookie: str, payload: dict) -> None:
+async def send_control(ws_base: str, session_id: UUID, cookie: str, payload: dict) -> float:
     async with websockets.connect(
         f"{ws_base}/ws/control/{session_id}",
         additional_headers={"Cookie": cookie},
@@ -81,7 +81,9 @@ async def send_control(ws_base: str, session_id: UUID, cookie: str, payload: dic
         ping_interval=None,
     ) as socket:
         await socket.send(json.dumps(payload, ensure_ascii=False))
+        sent_at = time.monotonic()
         await asyncio.sleep(0.15)
+        return sent_at
 
 
 async def probe_ws(ws_base: str, path: str, cookie: str) -> dict:
@@ -238,7 +240,7 @@ async def main(args: argparse.Namespace) -> int:
     from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
     from sqlalchemy.pool import NullPool
 
-    from app.api.auth import hash_password
+    from app.api.auth import hash_password, login_log_marker
     from app.db.models import AuditLog, Session, Trainee, User
 
     database_url = (
@@ -251,7 +253,7 @@ async def main(args: argparse.Namespace) -> int:
     factory = async_sessionmaker(engine, expire_on_commit=False)
     observer_engine = create_async_engine(database_url, poolclass=NullPool)
     run_id = uuid4().hex[:12]
-    session_id = uuid4()
+    session_id = args.session_id or uuid4()
     trainee = Trainee(name=f"Failover smoke {run_id}")
     instructor_login = f"fo-i-{run_id}"
     trainee_login = f"fo-t-{run_id}"
@@ -263,6 +265,7 @@ async def main(args: argparse.Namespace) -> int:
     fault_stopped = False
     result: dict = {"session_id": str(session_id), "checks": {}}
     backend = f"http://127.0.0.1:{args.backend_port}"
+    result["login_node"] = "backend-a"
     ws_base = args.frontend_url.replace("https://", "wss://").replace("http://", "ws://")
     instructor_cookie = trainee_cookie = None
     try:
@@ -278,21 +281,32 @@ async def main(args: argparse.Namespace) -> int:
                      trainee_id=trainee.id, blocked=False),
             ])
             await db.commit()
+        users_committed_at = time.monotonic()
 
         instructor_cookie = login(backend, instructor_login, instructor_password)
         trainee_cookie = login(backend, trainee_login, trainee_password)
         # Учётки созданы прямо в БД, вход — на узле A, а сокет занятия Nginx
-        # может отдать узлу B. B узнаёт новый логин при очередной сверке
-        # поколений (раз в секунду); до неё middleware считает cookie отозванной.
-        for port in (args.backend_port, args.backend_b_port):
-            for cookie in (instructor_cookie, trainee_cookie):
-                if not await wait_auth_fresh(f"http://127.0.0.1:{port}", cookie):
-                    raise RuntimeError(f"backend :{port} не узнал новый логин за 10 с")
-        await send_control(ws_base, session_id, instructor_cookie, {
+        # может отдать узлу B до его сверки поколений. Ждать сверку не нужно:
+        # B проверяет неизвестный логин разовым SELECT (lct-42). Ожидание здесь
+        # лишило бы доказательности проверку ниже: опрос B сам вызвал бы разовую
+        # проверку, а за секунду B узнал бы логин сверкой.
+        control_sent_at = await send_control(ws_base, session_id, instructor_cookie, {
             "type": "scenario.start", "scenario_id": args.scenario,
             "trainee": trainee.name, "trainee_id": str(trainee.id),
             "mode": "training", "exercise": "dds",
         })
+        result["users_commit_to_control_seconds"] = round(control_sent_at - users_committed_at, 4)
+        if args.expect_initial_owner == "backend-b":
+            result["checks"]["control_sent_within_first_second"] = (
+                result["users_commit_to_control_seconds"] < 1
+            )
+            # Сверка B идёт раз в секунду в случайной фазе, и время само по себе
+            # не доказывает, что B не знал логин. Доказательство — запись B о
+            # разовой проверке именно этого логина.
+            result["checks"]["backend_b_resolved_login_one_shot"] = (
+                login_log_marker(instructor_login)
+                in compose(args, "logs", "--no-color", "backend-b")
+            )
 
         async def session_row():
             async with factory() as db:
@@ -327,6 +341,12 @@ async def main(args: argparse.Namespace) -> int:
         old_epoch = row.backend_fencing_epoch
         result["initial_owner"] = row.backend_node_id
         result["initial_epoch"] = old_epoch
+        if args.expect_initial_owner and row.backend_node_id != args.expect_initial_owner:
+            raise RuntimeError(
+                f"session routed to {row.backend_node_id}, expected {args.expect_initial_owner}"
+            )
+        if args.expect_initial_owner == "backend-b":
+            result["checks"]["login_a_initial_control_b"] = True
 
         # Commit real DDS work before the fault. These actions must survive the
         # checkpoint handoff and remain part of the final scored report.
@@ -735,4 +755,6 @@ if __name__ == "__main__":
     parser.add_argument("--reconnect-attempts", type=int, default=5)
     parser.add_argument("--rest-timeout", type=float, default=60)
     parser.add_argument("--failure-mode", choices=("kill", "partition"), default="kill")
+    parser.add_argument("--expect-initial-owner", choices=("backend-a", "backend-b"))
+    parser.add_argument("--session-id", type=UUID)
     raise SystemExit(asyncio.run(main(parser.parse_args())))