fix(anthropic): honor custom-base bearer auth for messages

litellm/llms/anthropic/experimental_pass_through/messages/transformation.py

@@ -172,7 +172,11 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
         headers, api_key = optionally_handle_anthropic_oauth(headers=headers, api_key=api_key)
 
         if "x-api-key" not in headers and "authorization" not in headers:
-            auth_header = AnthropicModelInfo.get_auth_header(api_key)
+            auth_header = AnthropicModelInfo.get_auth_header(
+                api_key=api_key,
+                api_base=api_base if api_base is not None else litellm_params.get("api_base"),
+                use_bearer_for_custom_base=bool(litellm_params.get("use_bearer_for_custom_base", False)),
+            )
             if auth_header is not None:
                 headers.update(auth_header)
         if "anthropic-version" not in headers:

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -386,6 +386,29 @@ class TestPassthroughOAuth:
         assert updated_headers["x-api-key"] == FAKE_REGULAR_KEY
         assert "authorization" not in updated_headers
 
+    def test_passthrough_custom_base_uses_bearer_from_litellm_params(self):
+        from litellm.llms.anthropic.experimental_pass_through.messages.transformation import (
+            AnthropicMessagesConfig,
+        )
+
+        config = AnthropicMessagesConfig()
+
+        updated_headers, _ = config.validate_anthropic_messages_environment(
+            headers={},
+            model="claude-sonnet-4-5-20250929",
+            messages=[{"role": "user", "content": "Hello"}],
+            optional_params={},
+            litellm_params={
+                "api_base": "https://custom-gateway.example.com",
+                "use_bearer_for_custom_base": True,
+            },
+            api_key="custom-api-key",
+            api_base=None,
+        )
+
+        assert updated_headers["authorization"] == "Bearer custom-api-key"
+        assert "x-api-key" not in updated_headers
+
 
 class TestIsAnthropicOAuthKey:
     """Tests for is_anthropic_oauth_key helper function."""