diff --git a/src/anthropic/_base_client.py b/src/anthropic/_base_client.py
index a430b2ea..5e69696e 100644
--- a/src/anthropic/_base_client.py
+++ b/src/anthropic/_base_client.py
@@ -331,7 +331,7 @@ def __init__(
         self,
         *,
         version: str,
-        base_url: str,
+        base_url: str | URL,
         _strict_response_validation: bool,
         max_retries: int = DEFAULT_MAX_RETRIES,
         timeout: float | Timeout | None = DEFAULT_TIMEOUT,
@@ -739,7 +739,7 @@ def __init__(
         self,
         *,
         version: str,
-        base_url: str,
+        base_url: str | URL,
         max_retries: int = DEFAULT_MAX_RETRIES,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
         transport: Transport | None = None,
@@ -1145,7 +1145,7 @@ def __init__(
         self,
         *,
         version: str,
-        base_url: str,
+        base_url: str | URL,
         _strict_response_validation: bool,
         max_retries: int = DEFAULT_MAX_RETRIES,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
diff --git a/src/anthropic/_client.py b/src/anthropic/_client.py
index 2c0b88d3..07b32a97 100644
--- a/src/anthropic/_client.py
+++ b/src/anthropic/_client.py
@@ -4,7 +4,7 @@
 
 import os
 import asyncio
-from typing import Union, Mapping, Optional
+from typing import Union, Mapping
 
 import httpx
 from tokenizers import Tokenizer  # type: ignore[import]
@@ -64,7 +64,7 @@ def __init__(
         *,
         api_key: str | None = None,
         auth_token: str | None = None,
-        base_url: Optional[str] = None,
+        base_url: str | httpx.URL | None = None,
         timeout: Union[float, Timeout, None, NotGiven] = NOT_GIVEN,
         max_retries: int = DEFAULT_MAX_RETRIES,
         default_headers: Mapping[str, str] | None = None,
@@ -176,7 +176,7 @@ def copy(
         *,
         api_key: str | None = None,
         auth_token: str | None = None,
-        base_url: str | None = None,
+        base_url: str | httpx.URL | None = None,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
         http_client: httpx.Client | None = None,
         connection_pool_limits: httpx.Limits | None = None,
@@ -316,7 +316,7 @@ def __init__(
         *,
         api_key: str | None = None,
         auth_token: str | None = None,
-        base_url: Optional[str] = None,
+        base_url: str | httpx.URL | None = None,
         timeout: Union[float, Timeout, None, NotGiven] = NOT_GIVEN,
         max_retries: int = DEFAULT_MAX_RETRIES,
         default_headers: Mapping[str, str] | None = None,
@@ -428,7 +428,7 @@ def copy(
         *,
         api_key: str | None = None,
         auth_token: str | None = None,
-        base_url: str | None = None,
+        base_url: str | httpx.URL | None = None,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
         http_client: httpx.AsyncClient | None = None,
         connection_pool_limits: httpx.Limits | None = None,