diff --git a/src/anthropic/__init__.py b/src/anthropic/__init__.py
index d47dbb93..dd589f66 100644
--- a/src/anthropic/__init__.py
+++ b/src/anthropic/__init__.py
@@ -11,7 +11,6 @@
     Transport,
     AsyncClient,
     AsyncStream,
-    ProxiesTypes,
     AsyncAnthropic,
     RequestOptions,
 )
diff --git a/src/anthropic/_base_client.py b/src/anthropic/_base_client.py
index e8a42612..f7d910e4 100644
--- a/src/anthropic/_base_client.py
+++ b/src/anthropic/_base_client.py
@@ -304,6 +304,8 @@ class BaseClient:
     max_retries: int
     timeout: Union[float, Timeout, None]
     _limits: httpx.Limits
+    _proxies: ProxiesTypes | None
+    _transport: Transport | None
     _strict_response_validation: bool
     _idempotency_header: str | None
 
@@ -315,6 +317,8 @@ def __init__(
         max_retries: int = DEFAULT_MAX_RETRIES,
         timeout: float | Timeout | None = DEFAULT_TIMEOUT,
         limits: httpx.Limits,
+        transport: Transport | None,
+        proxies: ProxiesTypes | None,
         custom_headers: Mapping[str, str] | None = None,
         custom_query: Mapping[str, object] | None = None,
     ) -> None:
@@ -322,6 +326,8 @@ def __init__(
         self.max_retries = max_retries
         self.timeout = timeout
         self._limits = limits
+        self._proxies = proxies
+        self._transport = transport
         self._custom_headers = custom_headers or {}
         self._custom_query = custom_query or {}
         self._strict_response_validation = _strict_response_validation
@@ -590,6 +596,11 @@ def user_agent(self) -> str:
     def base_url(self) -> URL:
         return self._client.base_url
 
+    @base_url.setter
+    def base_url(self, url: URL | str) -> None:
+        # mypy doesn't use the type from the setter
+        self._client.base_url = url  # type: ignore[assignment]
+
     @lru_cache(maxsize=None)
     def platform_headers(self) -> Dict[str, str]:
         return {
@@ -689,6 +700,8 @@ def __init__(
             version=version,
             limits=limits,
             timeout=timeout,
+            proxies=proxies,
+            transport=transport,
             max_retries=max_retries,
             custom_query=custom_query,
             custom_headers=custom_headers,
@@ -1045,6 +1058,8 @@ def __init__(
             version=version,
             limits=limits,
             timeout=timeout,
+            proxies=proxies,
+            transport=transport,
             max_retries=max_retries,
             custom_query=custom_query,
             custom_headers=custom_headers,
diff --git a/src/anthropic/_utils/__init__.py b/src/anthropic/_utils/__init__.py
index b45dc1b1..679193bd 100644
--- a/src/anthropic/_utils/__init__.py
+++ b/src/anthropic/_utils/__init__.py
@@ -1,3 +1,4 @@
+from ._proxy import LazyProxy as LazyProxy
 from ._utils import flatten as flatten
 from ._utils import is_dict as is_dict
 from ._utils import is_list as is_list
diff --git a/src/anthropic/_utils/_proxy.py b/src/anthropic/_utils/_proxy.py
new file mode 100644
index 00000000..fd85ebd5
--- /dev/null
+++ b/src/anthropic/_utils/_proxy.py
@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from typing import Generic, TypeVar, Iterable, cast
+from typing_extensions import ClassVar
+
+T = TypeVar("T")
+
+
+class LazyProxy(Generic[T], ABC):
+    """Implements data methods to pretend that an instance is another instance.
+
+    This includes forwarding attribute access and othe methods.
+    """
+
+    should_cache: ClassVar[bool] = False
+
+    def __init__(self) -> None:
+        self.__proxied: T | None = None
+
+    def __getattr__(self, attr: str) -> object:
+        return getattr(self.__get_proxied__(), attr)
+
+    def __repr__(self) -> str:
+        return repr(self.__get_proxied__())
+
+    def __str__(self) -> str:
+        return str(self.__get_proxied__())
+
+    def __dir__(self) -> Iterable[str]:
+        return self.__get_proxied__().__dir__()
+
+    @property  # type: ignore
+    def __class__(self) -> type:
+        return self.__get_proxied__().__class__
+
+    def __get_proxied__(self) -> T:
+        if not self.should_cache:
+            return self.__load__()
+
+        proxied = self.__proxied
+        if proxied is not None:
+            return proxied
+
+        self.__proxied = proxied = self.__load__()
+        return proxied
+
+    def __set_proxied__(self, value: T) -> None:
+        self.__proxied = value
+
+    def __as_proxied__(self) -> T:
+        """Helper method that returns the current proxy, typed as the loaded object"""
+        return cast(T, self)
+
+    @abstractmethod
+    def __load__(self) -> T:
+        ...