diff --git a/src/anthropic/_utils/_transform.py b/src/anthropic/_utils/_transform.py
index dc497ea3..d953505f 100644
--- a/src/anthropic/_utils/_transform.py
+++ b/src/anthropic/_utils/_transform.py
@@ -168,7 +168,7 @@ def _transform_recursive(
         return data
 
     if isinstance(data, pydantic.BaseModel):
-        return model_dump(data, exclude_unset=True, exclude_defaults=True)
+        return model_dump(data, exclude_unset=True)
 
     return _transform_value(data, annotation)
 
diff --git a/tests/test_transform.py b/tests/test_transform.py
index 5f9abd21..cfcf870e 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -237,3 +237,29 @@ def test_pydantic_nested_objects() -> None:
     model = ModelNestedObjects.construct(nested={"foo": "stainless"})
     assert isinstance(model.nested, MyModel)
     assert transform(model, Any) == {"nested": {"foo": "stainless"}}
+
+
+class ModelWithDefaultField(BaseModel):
+    foo: str
+    with_none_default: Union[str, None] = None
+    with_str_default: str = "foo"
+
+
+def test_pydantic_default_field() -> None:
+    # should be excluded when defaults are used
+    model = ModelWithDefaultField.construct()
+    assert model.with_none_default is None
+    assert model.with_str_default == "foo"
+    assert transform(model, Any) == {}
+
+    # should be included when the default value is explicitly given
+    model = ModelWithDefaultField.construct(with_none_default=None, with_str_default="foo")
+    assert model.with_none_default is None
+    assert model.with_str_default == "foo"
+    assert transform(model, Any) == {"with_none_default": None, "with_str_default": "foo"}
+
+    # should be included when a non-default value is explicitly given
+    model = ModelWithDefaultField.construct(with_none_default="bar", with_str_default="baz")
+    assert model.with_none_default == "bar"
+    assert model.with_str_default == "baz"
+    assert transform(model, Any) == {"with_none_default": "bar", "with_str_default": "baz"}