diff --git a/libs/partners/openai/langchain_openai/chat_models/_client_utils.py b/libs/partners/openai/langchain_openai/chat_models/_client_utils.py
index 4a0efce9e1bfe..40c672a3c4285 100644
--- a/libs/partners/openai/langchain_openai/chat_models/_client_utils.py
+++ b/libs/partners/openai/langchain_openai/chat_models/_client_utils.py
@@ -11,10 +11,12 @@
 import asyncio
 import inspect
 import os
+import weakref
 from collections.abc import Awaitable, Callable
 from functools import lru_cache
 from typing import Any, cast
 
+import httpx
 import openai
 from pydantic import SecretStr
 
@@ -46,6 +48,44 @@ def __del__(self) -> None:
             pass
 
 
+class _LoopAwareAsyncHttpxClientProxy:
+    """A proxy for async httpx clients that maintains per-event-loop instances.
+
+    httpx.AsyncClient connections are bound to the event loop they were created
+    on. This proxy is cached at the (base_url, timeout) level via @lru_cache,
+    preserving fast O(1) init-time lookup. Internally it stores one
+    _AsyncHttpxClientWrapper per event loop in a WeakValueDictionary so that
+    clients are automatically garbage-collected when their loop closes.
+
+    This avoids the 'Event loop is closed' RuntimeError that occurs when a
+    process-global @lru_cache'd client is reused across different event loops
+    (e.g. in multi-threaded environments, Celery workers, or sequential
+    asyncio.run() calls).
+    """
+
+    def __init__(self, base_url: str | None, timeout: Any) -> None:
+        self._base_url = base_url
+        self._timeout = timeout
+        self._loop_clients: weakref.WeakValueDictionary[
+            int, _AsyncHttpxClientWrapper
+        ] = weakref.WeakValueDictionary()
+
+    def get_client(self) -> _AsyncHttpxClientWrapper:
+        """Return the cached client for the current event loop, creating one if needed."""
+        try:
+            loop = asyncio.get_running_loop()
+            loop_id = id(loop)
+        except RuntimeError:
+            # No running event loop; return a fresh client (won't be cached).
+            return _build_async_httpx_client(self._base_url, self._timeout)
+
+        client = self._loop_clients.get(loop_id)
+        if client is None:
+            client = _build_async_httpx_client(self._base_url, self._timeout)
+            self._loop_clients[loop_id] = client
+        return client
+
+
 def _build_sync_httpx_client(
     base_url: str | None, timeout: Any
 ) -> _SyncHttpxClientWrapper:
@@ -76,10 +116,16 @@ def _cached_sync_httpx_client(
 
 
 @lru_cache
-def _cached_async_httpx_client(
+def _cached_async_httpx_client_proxy(
     base_url: str | None, timeout: Any
-) -> _AsyncHttpxClientWrapper:
-    return _build_async_httpx_client(base_url, timeout)
+) -> _LoopAwareAsyncHttpxClientProxy:
+    """Return a per-(base_url, timeout) proxy cached by @lru_cache.
+
+    The proxy itself is lightweight and @lru_cache'd, so init-time overhead is
+    identical to the original implementation. The proxy internally dispatches
+    to a per-event-loop client, preventing cross-loop connection reuse.
+    """
+    return _LoopAwareAsyncHttpxClientProxy(base_url, timeout)
 
 
 def _get_default_httpx_client(
@@ -100,16 +146,22 @@ def _get_default_httpx_client(
 def _get_default_async_httpx_client(
     base_url: str | None, timeout: Any
 ) -> _AsyncHttpxClientWrapper:
-    """Get default httpx client.
+    """Get default async httpx client, scoped to the current event loop.
 
-    Uses cached client unless timeout is `httpx.Timeout`, which is not hashable.
+    Async httpx clients are bound to the event loop they were created on, so
+    they cannot be safely shared across different event loops. This function
+    returns a client that is cached per-loop to avoid 'Event loop is closed'
+    errors in multi-threaded or multi-loop environments (e.g. Celery, sequential
+    asyncio.run() calls, or multi-threaded FastAPI handlers).
+
+    Uses a fresh (uncached) client when timeout is not hashable.
     """
     try:
         hash(timeout)
     except TypeError:
         return _build_async_httpx_client(base_url, timeout)
     else:
-        return _cached_async_httpx_client(base_url, timeout)
+        return _cached_async_httpx_client_proxy(base_url, timeout).get_client()
 
 
 def _resolve_sync_and_async_api_keys(
@@ -117,8 +169,8 @@ def _resolve_sync_and_async_api_keys(
 ) -> tuple[str | None | Callable[[], str], str | Callable[[], Awaitable[str]]]:
     """Resolve sync and async API key values.
 
-    Because OpenAI and AsyncOpenAI clients support either sync or async callables for
-    the API key, we need to resolve separate values here.
+    Because OpenAI and AsyncOpenAI clients support either sync or async callables
+    for the API key, we need to resolve separate values here.
     """
     if isinstance(api_key, SecretStr):
         sync_api_key_value: str | None | Callable[[], str] = api_key.get_secret_value()
@@ -138,5 +190,4 @@ async def async_api_key_wrapper() -> str:
                 )
 
             async_api_key_value = async_api_key_wrapper
-
     return sync_api_key_value, async_api_key_value