diff --git a/src/anthropic/_utils/_sync.py b/src/anthropic/_utils/_sync.py
index 8b3aaf2..ad7ec71 100644
--- a/src/anthropic/_utils/_sync.py
+++ b/src/anthropic/_utils/_sync.py
@@ -7,16 +7,20 @@
 from typing import Any, TypeVar, Callable, Awaitable
 from typing_extensions import ParamSpec
 
+import anyio
+import sniffio
+import anyio.to_thread
+
 T_Retval = TypeVar("T_Retval")
 T_ParamSpec = ParamSpec("T_ParamSpec")
 
 
 if sys.version_info >= (3, 9):
-    to_thread = asyncio.to_thread
+    _asyncio_to_thread = asyncio.to_thread
 else:
     # backport of https://docs.python.org/3/library/asyncio-task.html#asyncio.to_thread
     # for Python 3.8 support
-    async def to_thread(
+    async def _asyncio_to_thread(
         func: Callable[T_ParamSpec, T_Retval], /, *args: T_ParamSpec.args, **kwargs: T_ParamSpec.kwargs
     ) -> Any:
         """Asynchronously run function *func* in a separate thread.
@@ -34,6 +38,17 @@ async def to_thread(
         return await loop.run_in_executor(None, func_call)
 
 
+async def to_thread(
+    func: Callable[T_ParamSpec, T_Retval], /, *args: T_ParamSpec.args, **kwargs: T_ParamSpec.kwargs
+) -> T_Retval:
+    if sniffio.current_async_library() == "asyncio":
+        return await _asyncio_to_thread(func, *args, **kwargs)
+
+    return await anyio.to_thread.run_sync(
+        functools.partial(func, *args, **kwargs),
+    )
+
+
 # inspired by `asyncer`, https://github.com/tiangolo/asyncer
 def asyncify(function: Callable[T_ParamSpec, T_Retval]) -> Callable[T_ParamSpec, Awaitable[T_Retval]]:
     """
diff --git a/tests/test_client.py b/tests/test_client.py
index f60e2fb..a29e65d 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -23,6 +23,7 @@
 
 from anthropic import Anthropic, AsyncAnthropic, APIResponseValidationError
 from anthropic._types import Omit
+from anthropic._utils import maybe_transform
 from anthropic._models import BaseModel, FinalRequestOptions
 from anthropic._constants import RAW_RESPONSE_HEADER
 from anthropic._streaming import Stream, AsyncStream
@@ -33,6 +34,7 @@
     BaseClient,
     make_request_options,
 )
+from anthropic.types.message_create_params import MessageCreateParamsNonStreaming
 
 from .utils import update_env
 
@@ -738,15 +740,18 @@ def test_retrying_timeout_errors_doesnt_leak(self, respx_mock: MockRouter) -> No
                 "/v1/messages",
                 body=cast(
                     object,
-                    dict(
-                        max_tokens=1024,
-                        messages=[
-                            {
-                                "role": "user",
-                                "content": "Hello, Claude",
-                            }
-                        ],
-                        model="claude-3-5-sonnet-latest",
+                    maybe_transform(
+                        dict(
+                            max_tokens=1024,
+                            messages=[
+                                {
+                                    "role": "user",
+                                    "content": "Hello, Claude",
+                                }
+                            ],
+                            model="claude-3-5-sonnet-latest",
+                        ),
+                        MessageCreateParamsNonStreaming,
                     ),
                 ),
                 cast_to=httpx.Response,
@@ -765,15 +770,18 @@ def test_retrying_status_errors_doesnt_leak(self, respx_mock: MockRouter) -> Non
                 "/v1/messages",
                 body=cast(
                     object,
-                    dict(
-                        max_tokens=1024,
-                        messages=[
-                            {
-                                "role": "user",
-                                "content": "Hello, Claude",
-                            }
-                        ],
-                        model="claude-3-5-sonnet-latest",
+                    maybe_transform(
+                        dict(
+                            max_tokens=1024,
+                            messages=[
+                                {
+                                    "role": "user",
+                                    "content": "Hello, Claude",
+                                }
+                            ],
+                            model="claude-3-5-sonnet-latest",
+                        ),
+                        MessageCreateParamsNonStreaming,
                     ),
                 ),
                 cast_to=httpx.Response,
@@ -1618,15 +1626,18 @@ async def test_retrying_timeout_errors_doesnt_leak(self, respx_mock: MockRouter)
                 "/v1/messages",
                 body=cast(
                     object,
-                    dict(
-                        max_tokens=1024,
-                        messages=[
-                            {
-                                "role": "user",
-                                "content": "Hello, Claude",
-                            }
-                        ],
-                        model="claude-3-5-sonnet-latest",
+                    maybe_transform(
+                        dict(
+                            max_tokens=1024,
+                            messages=[
+                                {
+                                    "role": "user",
+                                    "content": "Hello, Claude",
+                                }
+                            ],
+                            model="claude-3-5-sonnet-latest",
+                        ),
+                        MessageCreateParamsNonStreaming,
                     ),
                 ),
                 cast_to=httpx.Response,
@@ -1645,15 +1656,18 @@ async def test_retrying_status_errors_doesnt_leak(self, respx_mock: MockRouter)
                 "/v1/messages",
                 body=cast(
                     object,
-                    dict(
-                        max_tokens=1024,
-                        messages=[
-                            {
-                                "role": "user",
-                                "content": "Hello, Claude",
-                            }
-                        ],
-                        model="claude-3-5-sonnet-latest",
+                    maybe_transform(
+                        dict(
+                            max_tokens=1024,
+                            messages=[
+                                {
+                                    "role": "user",
+                                    "content": "Hello, Claude",
+                                }
+                            ],
+                            model="claude-3-5-sonnet-latest",
+                        ),
+                        MessageCreateParamsNonStreaming,
                     ),
                 ),
                 cast_to=httpx.Response,