diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
index 8e7c8506278..6f1a7718fc5 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
@@ -23,7 +23,6 @@ from ...common_utils import (
     optionally_handle_anthropic_oauth,
 )
 
-DEFAULT_ANTHROPIC_API_BASE = "https://api.anthropic.com"
 DEFAULT_ANTHROPIC_API_VERSION = "2023-06-01"
 
 
@@ -127,7 +126,7 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         litellm_params: dict,
         stream: Optional[bool] = None,
     ) -> str:
-        api_base = api_base or DEFAULT_ANTHROPIC_API_BASE
+        api_base = AnthropicModelInfo.get_api_base(api_base)
         if not api_base.endswith("/v1/messages"):
             api_base = f"{api_base}/v1/messages"
         return api_base
diff --git a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
index dd5c49258ee..22470b93540 100644
--- a/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
+++ b/tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py
@@ -1107,3 +1107,27 @@ class TestPassthroughAuthToken:
 
         assert updated_headers["x-api-key"] == FAKE_REGULAR_KEY
         assert "authorization" not in updated_headers
+
+    def test_passthrough_get_complete_url_honours_base_url_env(self):
+        """get_complete_url should use ANTHROPIC_BASE_URL when api_base is None."""
+        from unittest.mock import patch as mock_patch
+
+        from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+            AnthropicMessagesConfig,
+        )
+
+        config = AnthropicMessagesConfig()
+        with mock_patch.dict(
+            "os.environ",
+            {"ANTHROPIC_BASE_URL": "https://custom.example.com"},
+            clear=True,
+        ):
+            url = config.get_complete_url(
+                api_base=None,
+                api_key=FAKE_REGULAR_KEY,
+                model="claude-sonnet-4-5-20250929",
+                optional_params={},
+                litellm_params={},
+            )
+
+        assert url == "https://custom.example.com/v1/messages"