diff --git a/libs/core/langchain_core/utils/function_calling.py b/libs/core/langchain_core/utils/function_calling.py
index c664ec03e3d..aa288bedc53 100644
--- a/libs/core/langchain_core/utils/function_calling.py
+++ b/libs/core/langchain_core/utils/function_calling.py
@@ -788,9 +788,12 @@ def _recursive_set_additional_properties_false(
             schema["additionalProperties"] = False
 
         # Recursively check 'properties' and 'items' if they exist
+        if "anyOf" in schema:
+            for sub_schema in schema["anyOf"]:
+                _recursive_set_additional_properties_false(sub_schema)
         if "properties" in schema:
-            for value in schema["properties"].values():
-                _recursive_set_additional_properties_false(value)
+            for sub_schema in schema["properties"].values():
+                _recursive_set_additional_properties_false(sub_schema)
         if "items" in schema:
             _recursive_set_additional_properties_false(schema["items"])
 
diff --git a/libs/core/tests/unit_tests/utils/test_function_calling.py b/libs/core/tests/unit_tests/utils/test_function_calling.py
index 126c1753f4e..f75ae304937 100644
--- a/libs/core/tests/unit_tests/utils/test_function_calling.py
+++ b/libs/core/tests/unit_tests/utils/test_function_calling.py
@@ -497,6 +497,61 @@ def test_convert_to_openai_function_nested_strict() -> None:
     assert actual == expected
 
 
+def test_convert_to_openai_function_strict_union_of_objects_arg_type() -> None:
+    class NestedA(BaseModel):
+        foo: str
+
+    class NestedB(BaseModel):
+        bar: int
+
+    class NestedC(BaseModel):
+        baz: bool
+
+    def my_function(my_arg: Union[NestedA, NestedB, NestedC]) -> None:
+        """Dummy function."""
+
+    expected = {
+        "name": "my_function",
+        "description": "Dummy function.",
+        "parameters": {
+            "properties": {
+                "my_arg": {
+                    "anyOf": [
+                        {
+                            "properties": {"foo": {"title": "Foo", "type": "string"}},
+                            "required": ["foo"],
+                            "title": "NestedA",
+                            "type": "object",
+                            "additionalProperties": False,
+                        },
+                        {
+                            "properties": {"bar": {"title": "Bar", "type": "integer"}},
+                            "required": ["bar"],
+                            "title": "NestedB",
+                            "type": "object",
+                            "additionalProperties": False,
+                        },
+                        {
+                            "properties": {"baz": {"title": "Baz", "type": "boolean"}},
+                            "required": ["baz"],
+                            "title": "NestedC",
+                            "type": "object",
+                            "additionalProperties": False,
+                        },
+                    ]
+                }
+            },
+            "required": ["my_arg"],
+            "type": "object",
+            "additionalProperties": False,
+        },
+        "strict": True,
+    }
+
+    actual = convert_to_openai_function(my_function, strict=True)
+    assert actual == expected
+
+
 json_schema_no_description_no_params = {
     "title": "dummy_function",
 }