diff --git a/src/anthropic/_utils/_transform.py b/src/anthropic/_utils/_transform.py
index d524b329..db40bff2 100644
--- a/src/anthropic/_utils/_transform.py
+++ b/src/anthropic/_utils/_transform.py
@@ -4,6 +4,8 @@
 from datetime import date, datetime
 from typing_extensions import Literal, get_args, override, get_type_hints
 
+import pydantic
+
 from ._utils import (
     is_list,
     is_mapping,
@@ -14,7 +16,7 @@
     is_annotated_type,
     strip_annotated_type,
 )
-from .._compat import is_typeddict
+from .._compat import model_dump, is_typeddict
 
 _T = TypeVar("_T")
 
@@ -165,6 +167,9 @@ def _transform_recursive(
             data = _transform_recursive(data, annotation=annotation, inner_type=subtype)
         return data
 
+    if isinstance(data, pydantic.BaseModel):
+        return model_dump(data, exclude_unset=True, exclude_defaults=True)
+
     return _transform_value(data, annotation)
 
 
diff --git a/tests/test_transform.py b/tests/test_transform.py
index b7334957..8e1d4724 100644
--- a/tests/test_transform.py
+++ b/tests/test_transform.py
@@ -1,10 +1,13 @@
 from __future__ import annotations
 
-from typing import List, Union, Optional
+from typing import Any, List, Union, Optional
 from datetime import date, datetime
 from typing_extensions import Required, Annotated, TypedDict
 
+import pytest
+
 from anthropic._utils import PropertyInfo, transform, parse_datetime
+from anthropic._models import BaseModel
 
 
 class Foo1(TypedDict):
@@ -186,3 +189,44 @@ class DateDictWithRequiredAlias(TypedDict, total=False):
 def test_datetime_with_alias() -> None:
     assert transform({"required_prop": None}, DateDictWithRequiredAlias) == {"prop": None}  # type: ignore[comparison-overlap]
     assert transform({"required_prop": date.fromisoformat("2023-02-23")}, DateDictWithRequiredAlias) == {"prop": "2023-02-23"}  # type: ignore[comparison-overlap]
+
+
+class MyModel(BaseModel):
+    foo: str
+
+
+def test_pydantic_model_to_dictionary() -> None:
+    assert transform(MyModel(foo="hi!"), Any) == {"foo": "hi!"}
+    assert transform(MyModel.construct(foo="hi!"), Any) == {"foo": "hi!"}
+
+
+def test_pydantic_empty_model() -> None:
+    assert transform(MyModel.construct(), Any) == {}
+
+
+def test_pydantic_unknown_field() -> None:
+    assert transform(MyModel.construct(my_untyped_field=True), Any) == {"my_untyped_field": True}
+
+
+def test_pydantic_mismatched_types() -> None:
+    model = MyModel.construct(foo=True)
+    with pytest.warns(UserWarning):
+        params = transform(model, Any)
+    assert params == {"foo": True}
+
+
+def test_pydantic_mismatched_object_type() -> None:
+    model = MyModel.construct(foo=MyModel.construct(hello="world"))
+    with pytest.warns(UserWarning):
+        params = transform(model, Any)
+    assert params == {"foo": {"hello": "world"}}
+
+
+class ModelNestedObjects(BaseModel):
+    nested: MyModel
+
+
+def test_pydantic_nested_objects() -> None:
+    model = ModelNestedObjects.construct(nested={"foo": "stainless"})
+    assert isinstance(model.nested, MyModel)
+    assert transform(model, Any) == {"nested": {"foo": "stainless"}}