diff --git a/src/anthropic/_base_client.py b/src/anthropic/_base_client.py
index 39c311fd..c8e190d1 100644
--- a/src/anthropic/_base_client.py
+++ b/src/anthropic/_base_client.py
@@ -880,9 +880,9 @@ def __exit__(
     def _prepare_options(
         self,
         options: FinalRequestOptions,  # noqa: ARG002
-    ) -> None:
+    ) -> FinalRequestOptions:
         """Hook for mutating the given options"""
-        return None
+        return options
 
     def _prepare_request(
         self,
@@ -962,7 +962,7 @@ def _request(
         input_options = model_copy(options)
 
         cast_to = self._maybe_override_cast_to(cast_to, options)
-        self._prepare_options(options)
+        options = self._prepare_options(options)
 
         retries = self._remaining_retries(remaining_retries, options)
         request = self._build_request(options)
@@ -1457,9 +1457,9 @@ async def __aexit__(
     async def _prepare_options(
         self,
         options: FinalRequestOptions,  # noqa: ARG002
-    ) -> None:
+    ) -> FinalRequestOptions:
         """Hook for mutating the given options"""
-        return None
+        return options
 
     async def _prepare_request(
         self,
@@ -1544,7 +1544,7 @@ async def _request(
         input_options = model_copy(options)
 
         cast_to = self._maybe_override_cast_to(cast_to, options)
-        await self._prepare_options(options)
+        options = await self._prepare_options(options)
 
         retries = self._remaining_retries(remaining_retries, options)
         request = self._build_request(options)
diff --git a/src/anthropic/_compat.py b/src/anthropic/_compat.py
index 74c7639b..c919b5ad 100644
--- a/src/anthropic/_compat.py
+++ b/src/anthropic/_compat.py
@@ -118,10 +118,10 @@ def get_model_fields(model: type[pydantic.BaseModel]) -> dict[str, FieldInfo]:
     return model.__fields__  # type: ignore
 
 
-def model_copy(model: _ModelT) -> _ModelT:
+def model_copy(model: _ModelT, *, deep: bool = False) -> _ModelT:
     if PYDANTIC_V2:
-        return model.model_copy()
-    return model.copy()  # type: ignore
+        return model.model_copy(deep=deep)
+    return model.copy(deep=deep)  # type: ignore
 
 
 def model_json(model: pydantic.BaseModel, *, indent: int | None = None) -> str: