fix(proxy): write the Prisma engine through the public setter, not the mangled name

`_write_engine` assigned directly to `prisma_client._Prisma__engine`, the
name Prisma <0.13 used internally for the engine attribute. Prisma 0.13+
is a `__slots__` class that no longer has that attribute at all, only a
public `_engine` property/setter backed by `_internal_engine`. Writing to
the mangled name on a modern client raises `AttributeError`, which
`connect()`'s `backoff` decorator retries indefinitely, spinning CPU and
respawning Prisma engine subprocesses without ever connecting.

Use the public `_engine` setter instead, which exists on both the legacy
and current Prisma internal layouts.

Reproduced locally, prisma==0.15.0: router at 196-293% CPU with a
continuous `AttributeError: 'Prisma' object has no attribute
'_Prisma__engine'` retry loop; 0% after the fix, connects immediately.

litellm/proxy/db/prisma_client.py

@@ -53,10 +53,7 @@ class _PrismaEngine(Protocol):
 
 
 class _PrismaClient(Protocol):
-    _Prisma__engine: _PrismaEngine
-
-    @property
-    def _engine(self) -> _PrismaEngine: ...
+    _engine: _PrismaEngine
 
 
 class _PrismaDrainTracker:
@@ -240,7 +237,9 @@ class PrismaWrapper:
 
     @staticmethod
     def _write_engine(prisma_client: _PrismaClient, engine: _PrismaEngine) -> None:
-        prisma_client._Prisma__engine = engine
+        # Public setter: prisma <0.13 stored the engine on the mangled `Prisma.__engine`,
+        # 0.13+ on `BasePrisma._internal_engine`. Both are __slots__ classes.
+        prisma_client._engine = engine  # rebind-ok: third-party Prisma client instance, not a local param we own
 
     def _instrument_prisma_client(self, prisma_client: _PrismaClient) -> _PrismaDrainTracker | None:
         from prisma.errors import ClientNotConnectedError

tests/test_litellm/proxy/db/test_prisma_client.py

@@ -171,6 +171,35 @@ def test_get_engine_pid_returns_zero_for_disconnected_client(disconnected_prisma
     assert wrapper._get_engine_pid() == 0
 
 
+def test_write_engine_uses_the_public_setter_not_the_mangled_name():
+    """Real Prisma clients (>=0.13) are __slots__ classes exposing only a public
+    `_engine` property/setter, with no `_Prisma__engine` slot. Writing to the
+    mangled name raises AttributeError instead of storing the engine."""
+
+    class _FakeSlotsBasedPrismaClient:
+        __slots__ = ("_internal_engine",)
+
+        def __init__(self):
+            self._internal_engine = None
+
+        @property
+        def _engine(self):
+            if self._internal_engine is None:
+                raise AttributeError("not connected")
+            return self._internal_engine
+
+        @_engine.setter
+        def _engine(self, engine):
+            self._internal_engine = engine
+
+    client = _FakeSlotsBasedPrismaClient()
+    sentinel = object()
+
+    PrismaWrapper._write_engine(client, sentinel)
+
+    assert PrismaWrapper._read_engine(client) is sentinel
+
+
 @pytest.mark.asyncio
 async def test_recreate_prisma_client_recovers_from_disconnected_client(
     mock_prisma_binary, disconnected_prisma

tests/test_litellm/proxy/db/test_prisma_planned_engine_restart.py

@@ -60,7 +60,7 @@ def _make_wrapper(engine_pid: int = 111, iam: bool = False) -> PrismaWrapper:
     prisma = GeneratedPrisma(use_dotenv=False)
     engine = MagicMock()
     engine.process.pid = engine_pid
-    setattr(prisma, "_Prisma__engine", engine)
+    prisma._engine = engine
     return PrismaWrapper(original_prisma=prisma, iam_token_db_auth=iam)
 
 
@@ -158,7 +158,7 @@ def _token_db_url(created: datetime, expires_in: int = 900) -> str:
 def test_wrapper_instruments_generated_prisma_engine() -> None:
     prisma = GeneratedPrisma(use_dotenv=False)
     engine = MagicMock()
-    setattr(prisma, "_Prisma__engine", engine)
+    prisma._engine = engine
 
     wrapper = PrismaWrapper(original_prisma=prisma, iam_token_db_auth=False)