chore(weave): Remove trailing slashes on base urls before submitting custom providers to litellm

tests/trace_server/test_custom_provider.py

@@ -477,6 +477,87 @@ def test_custom_provider_ollama_model(client):
             _secret_fetcher_context.reset(token)
 
 
+def test_custom_provider_trailing_slash_normalization(client):
+    """Test that trailing slashes in base_url are stripped to prevent redirect issues.
+
+    When a base_url has a trailing slash, HTTP servers often redirect to the
+    canonical URL without the slash using 301/302. These redirects cause most
+    HTTP clients to change POST to GET, breaking the API call.
+
+    This test verifies that trailing slashes are stripped before making the request.
+    """
+    is_sqlite = client_is_sqlite(client)
+    if is_sqlite:
+        # no need to test in sqlite
+        return
+
+    # Create provider ID and model ID for testing
+    provider_id = f"test-trailing-slash-{uuid.uuid4()}"
+    model_id = "test-model"
+    model_name = f"custom::{provider_id}::{model_id}"
+
+    # Create a Provider object with a trailing slash in base_url
+    provider_obj = create_provider_obj(
+        project_id=client._project_id(),
+        provider_id=provider_id,
+        base_url="http://localhost:11434/",  # Note the trailing slash
+        api_key_name="TEST_API_KEY",
+        extra_headers={},
+    )
+
+    # Create a ProviderModel object
+    provider_model_obj = create_provider_model_obj(
+        project_id=client._project_id(),
+        provider_id=provider_id,
+        model_id=model_id,
+        model_name=model_id,
+    )
+
+    # Mock responses for obj_read calls
+    mock_obj_read = create_mock_obj_read(provider_obj, provider_model_obj)
+
+    # Create test input
+    inputs = {
+        "model": model_name,
+        "messages": [{"role": "user", "content": "Hello, world!"}],
+    }
+
+    # Mock response from LiteLLM
+    mock_response = create_mock_completion_response(
+        model_name=model_id,
+        content="Hello!",
+    )
+
+    with with_tracing_disabled():
+        mock_secret_fetcher, token = setup_test_environment()
+        try:
+            with patch(
+                "weave.trace_server.clickhouse_trace_server_batched.ClickHouseTraceServer.obj_read"
+            ) as mock_read:
+                mock_read.side_effect = mock_obj_read
+                with patch("litellm.completion") as mock_completion:
+                    mock_completion.return_value = ModelResponse.model_validate(
+                        mock_response
+                    )
+                    client.server.completions_create(
+                        tsi.CompletionsCreateReq.model_validate(
+                            {
+                                "project_id": client._project_id(),
+                                "inputs": inputs,
+                            }
+                        )
+                    )
+
+            # Verify the trailing slash was stripped from api_base
+            mock_completion.assert_called_once()
+            call_args = mock_completion.call_args[1]
+            assert call_args["api_base"] == "http://localhost:11434", (
+                f"Expected trailing slash to be stripped. Got '{call_args['api_base']}'"
+            )
+        finally:
+            _secret_fetcher_context.reset(token)
+
+
 def test_get_custom_provider_info():
     """Test the get_custom_provider_info function directly."""
     # Set up test data

weave/trace_server/llm_completion.py

@@ -231,6 +231,11 @@ def lite_llm_completion(
     extra_headers: dict[str, str] | None = None,
     return_type: str | None = None,
 ) -> tsi.CompletionsCreateRes:
+    # Normalize base_url to prevent issues with trailing slashes causing redirects
+    # that change POST to GET (HTTP 301/302 redirect behavior)
+    if base_url:
+        base_url = base_url.rstrip("/")
+
     # Setup provider-specific credentials and model modifications
     (
         aws_access_key_id,
@@ -534,6 +539,11 @@ def lite_llm_completion_stream(
     follows the non-streaming version: any exception is surfaced to the caller
     as a single error chunk and the iterator terminates.
     """
+    # Normalize base_url to prevent issues with trailing slashes causing redirects
+    # that change POST to GET (HTTP 301/302 redirect behavior)
+    if base_url:
+        base_url = base_url.rstrip("/")
+
     # Setup provider-specific credentials and model modifications
     (
         aws_access_key_id,