fix(aiohttp): prevent closing shared ClientSession in AiohttpTransport (#21117)

When a shared ClientSession is passed to LiteLLMAiohttpTransport,
calling aclose() on the transport would close the shared session,
breaking other clients still using it.

Add owns_session parameter (default True for backwards compatibility)
to AiohttpTransport and LiteLLMAiohttpTransport. When a shared session
is provided in http_handler.py, owns_session=False is set to prevent
the transport from closing a session it does not own.

This aligns AiohttpTransport with the ownership pattern already used
in AiohttpHandler (aiohttp_handler.py).

litellm/llms/custom_httpx/aiohttp_transport.py

@@ -119,8 +119,13 @@ class AiohttpResponseStream(httpx.AsyncByteStream):
 
 
 class AiohttpTransport(httpx.AsyncBaseTransport):
-    def __init__(self, client: Union[ClientSession, Callable[[], ClientSession]]) -> None:
+    def __init__(
+        self,
+        client: Union[ClientSession, Callable[[], ClientSession]],
+        owns_session: bool = True,
+    ) -> None:
         self.client = client
+        self._owns_session = owns_session
 
         #########################################################
         # Class variables for proxy settings
@@ -128,7 +133,7 @@ class AiohttpTransport(httpx.AsyncBaseTransport):
         self.proxy_cache: Dict[str, Optional[str]] = {}
 
     async def aclose(self) -> None:
-        if isinstance(self.client, ClientSession):
+        if self._owns_session and isinstance(self.client, ClientSession):
             await self.client.close()
 
 
@@ -144,10 +149,11 @@ class LiteLLMAiohttpTransport(AiohttpTransport):
         self,
         client: Union[ClientSession, Callable[[], ClientSession]],
         ssl_verify: Optional[Union[bool, ssl.SSLContext]] = None,
+        owns_session: bool = True,
     ):
         self.client = client
         self._ssl_verify = ssl_verify  # Store for per-request SSL override
-        super().__init__(client=client)
+        super().__init__(client=client, owns_session=owns_session)
         # Store the client factory for recreating sessions when needed
         if callable(client):
             self._client_factory = client

litellm/llms/custom_httpx/http_handler.py

@@ -866,6 +866,7 @@ class AsyncHTTPHandler:
             return LiteLLMAiohttpTransport(
                 client=shared_session,
                 ssl_verify=ssl_for_transport,
+                owns_session=False,
             )
 
         # Create new session only if none provided or existing one is invalid

tests/test_litellm/llms/custom_httpx/test_aiohttp_transport.py

@@ -12,10 +12,42 @@ sys.path.insert(0, os.path.abspath("../../../.."))  # Adds the parent directory
 
 from litellm.llms.custom_httpx.aiohttp_transport import (
     AiohttpResponseStream,
+    AiohttpTransport,
     LiteLLMAiohttpTransport,
 )
 
 
+@pytest.mark.asyncio
+async def test_aclose_does_not_close_shared_session():
+    """Test that aclose() does not close a session it does not own (shared session)."""
+    session = aiohttp.ClientSession()
+    try:
+        transport = LiteLLMAiohttpTransport(client=session, owns_session=False)
+        await transport.aclose()
+        assert not session.closed, "Shared session should not be closed by transport"
+    finally:
+        await session.close()
+
+
+@pytest.mark.asyncio
+async def test_aclose_closes_owned_session():
+    """Test that aclose() closes a session it owns."""
+    session = aiohttp.ClientSession()
+    transport = LiteLLMAiohttpTransport(client=session, owns_session=True)
+    await transport.aclose()
+    assert session.closed, "Owned session should be closed by transport"
+
+
+@pytest.mark.asyncio
+async def test_owns_session_defaults_to_true():
+    """Test that owns_session defaults to True for backwards compatibility."""
+    session = aiohttp.ClientSession()
+    transport = AiohttpTransport(client=session)
+    assert transport._owns_session is True
+    await transport.aclose()
+    assert session.closed
+
+
 class MockAiohttpResponse:
     """Mock aiohttp ClientResponse for testing"""