test(anthropic): add unit test coverage for count_tokens custom api_base endpoint resolution

tests/test_anthropic_count_tokens.py

@@ -0,0 +1,32 @@
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from litellm.llms.anthropic.count_tokens.handler import AnthropicCountTokensHandler
+
+
+@pytest.mark.asyncio
+async def test_anthropic_count_tokens_custom_api_base():
+    handler = AnthropicCountTokensHandler()
+
+    mock_response = AsyncMock()
+    mock_response.status_code = 200
+    mock_response.json.return_value = {"input_tokens": 10}
+
+    with patch("litellm.llms.anthropic.count_tokens.handler.get_async_httpx_client") as mock_client_factory:
+        mock_client = AsyncMock()
+        mock_client.post.return_value = mock_response
+        mock_client_factory.return_value = mock_client
+
+        # Test custom api_base URL resolution and formatting
+        res = await handler.handle_count_tokens_request(
+            model="claude-3-5-sonnet-20241022",
+            messages=[{"role": "user", "content": "hello"}],
+            api_key="test-key",
+            api_base="https://custom-proxy.internal.com/v1",
+        )
+
+        assert res["input_tokens"] == 10
+        mock_client.post.assert_called_once()
+        called_url = mock_client.post.call_args[0][0]
+        assert called_url == "https://custom-proxy.internal.com/v1/messages/count_tokens"