diff --git a/libs/partners/openai/langchain_openai/chat_models/base.py b/libs/partners/openai/langchain_openai/chat_models/base.py
index 8e2473fe221..efaff493f4b 100644
--- a/libs/partners/openai/langchain_openai/chat_models/base.py
+++ b/libs/partners/openai/langchain_openai/chat_models/base.py
@@ -14,6 +14,7 @@
 from __future__ import annotations
 
 import base64
+import inspect
 import json
 import logging
 import os
@@ -1184,6 +1185,20 @@ class BaseChatOpenAI(BaseChatModel):
             sync_api_key_value, async_api_key_value = _resolve_sync_and_async_api_keys(
                 self.openai_api_key
             )
+            # Surface the sync/async constraint at construction time so callers
+            # don't first discover it on a runtime invocation. An async-callable
+            # api_key means the sync client cannot be built, and sync entry
+            # points (invoke/stream) will fail with a clearer ValueError later.
+            if inspect.iscoroutinefunction(self.openai_api_key):
+                warnings.warn(
+                    "An async callable was provided for `openai_api_key`. "
+                    "The sync client will not be available; only async entry "
+                    "points (`ainvoke`, `astream`, ...) will work. To use sync "
+                    "methods, provide a string or a sync callable for the API "
+                    "key.",
+                    UserWarning,
+                    stacklevel=2,
+                )
 
         client_params: dict = {
             "organization": self.openai_organization,
diff --git a/libs/partners/openai/tests/unit_tests/chat_models/test_base.py b/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
index 64d6237d60c..355decaa121 100644
--- a/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
+++ b/libs/partners/openai/tests/unit_tests/chat_models/test_base.py
@@ -3803,3 +3803,31 @@ def test_defer_loading_in_responses_api_payload() -> None:
     assert weather_tool["defer_loading"] is True
     assert weather_tool["type"] == "function"
     assert {"type": "tool_search"} in result["tools"]
+
+
+def test_async_callable_api_key_warns_at_construction() -> None:
+    """An async callable api_key should warn at construction time."""
+
+    async def async_api_key() -> str:
+        return "sk-test"
+
+    with pytest.warns(UserWarning, match="async callable"):
+        llm = ChatOpenAI(model="gpt-4o-mini", openai_api_key=async_api_key)
+
+    assert llm.client is None
+    assert llm.root_client is None
+    assert llm.async_client is not None
+
+    with pytest.raises(ValueError, match="Sync client is not available"):
+        llm.invoke("hello")
+
+
+def test_sync_callable_api_key_does_not_warn() -> None:
+    """A sync callable api_key must not trigger the async-callable warning."""
+
+    def sync_api_key() -> str:
+        return "sk-test"
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", UserWarning)
+        ChatOpenAI(model="gpt-4o-mini", openai_api_key=sync_api_key)