fix(core): preserve default_factory when generating tool call schema (#35550)

libs/core/langchain_core/utils/pydantic.py

@@ -242,7 +242,12 @@ def _create_subset_model_v2(
     for field_name in field_names:
         field = model.model_fields[field_name]
         description = descriptions_.get(field_name, field.description)
-        field_info = FieldInfoV2(description=description, default=field.default)
+        field_kwargs: dict[str, Any] = {"description": description}
+        if field.default_factory is not None:
+            field_kwargs["default_factory"] = field.default_factory
+        else:
+            field_kwargs["default"] = field.default
+        field_info = FieldInfoV2(**field_kwargs)
         if field.metadata:
             field_info.metadata = field.metadata
         fields[field_name] = (field.annotation, field_info)

libs/core/tests/unit_tests/test_tools.py

@@ -3636,3 +3636,20 @@ def test_tool_args_schema_falsy_defaults() -> None:
     # Invoke with only required argument - falsy defaults should be applied
     result = config_tool.invoke({"name": "test"})
     assert result == "name=test, enabled=False, count=0, prefix=''"
+
+
+def test_tool_default_factory_not_required() -> None:
+    """Fields with default_factory should not appear in required."""
+
+    class Args(BaseModel):
+        """Hello."""
+
+        names: list[str] = Field(default_factory=list, description="Some names")
+
+    @tool(args_schema=Args)
+    def some_func(names: list[str] | None = None) -> None:
+        """Do something."""
+
+    schema = convert_to_openai_tool(some_func)
+    params = schema["function"]["parameters"]
+    assert "names" not in params.get("required", [])

libs/core/tests/unit_tests/utils/test_pydantic.py

@@ -186,3 +186,22 @@ def test_create_model_v2() -> None:
         foo.model_json_schema()
 
     assert list(record) == []
+
+
+def test_create_subset_model_v2_preserves_default_factory() -> None:
+    """Fields with default_factory should not be marked as required."""
+
+    class Original(BaseModel):
+        required_field: str
+        names: list[str] = Field(default_factory=list, description="Some names")
+        mapping: dict[str, int] = Field(default_factory=dict, description="A mapping")
+
+    subset = _create_subset_model_v2(
+        "Subset",
+        Original,
+        ["required_field", "names", "mapping"],
+    )
+    schema = subset.model_json_schema()
+    assert schema.get("required") == ["required_field"]
+    assert "names" not in schema.get("required", [])
+    assert "mapping" not in schema.get("required", [])