fix(providers): support anthropic proxy v1 endpoints

tests/agent/test_anthropic_adapter.py

@@ -113,6 +113,15 @@ class TestBuildAnthropicClient:
                 "anthropic-beta": "interleaved-thinking-2025-05-14,fine-grained-tool-streaming-2025-05-14"
             }
 
+    def test_custom_base_url_strips_trailing_v1(self):
+        with patch("agent.anthropic_adapter._anthropic_sdk") as mock_sdk:
+            build_anthropic_client(
+                "sk-ant-api03-x",
+                base_url="https://proxy.example.com/anthropic/v1",
+            )
+            kwargs = mock_sdk.Anthropic.call_args[1]
+            assert kwargs["base_url"] == "https://proxy.example.com/anthropic"
+
     def test_azure_anthropic_endpoint_keeps_context_1m_beta(self):
         with patch("agent.anthropic_adapter._anthropic_sdk") as mock_sdk:
             build_anthropic_client(

tests/agent/test_auxiliary_transport_autodetect.py

@@ -39,6 +39,8 @@ def _clean_env(monkeypatch):
     ("https://api.moonshot.ai/v1", False, "Moonshot legacy"),
     ("https://api.minimax.io/anthropic", True, "MiniMax /anthropic"),
     ("https://litellm.example.com/v1/anthropic", True, "/anthropic suffix"),
+    ("https://litellm.example.com/anthropic/v1", True, "/anthropic/v1 base"),
+    ("https://litellm.example.com/anthropic/v1/models", False, "/anthropic/v1 subpath"),
     ("https://api.anthropic.com", True, "native Anthropic"),
     ("https://api.anthropic.com/v1", True, "native Anthropic /v1"),
     ("https://openrouter.ai/api/v1", False, "OpenRouter"),