fix(prometheus): never label deployment failures api_provider=None

When a resolved deployment failed without custom_llm_provider on
litellm_params (e.g. a fallback target), the failure counter emitted the
literal label value api_provider="None", creating a phantom ~100%-failure
series for any alert grouped by provider.

Fall back to the existing best-effort resolver
(_extract_api_provider_from_request_data, already used by the client-side
failure hook) and coalesce to "" — the same pattern the
set_deployment_partial_outage call a few lines below already uses.

Fixes #38531

litellm/integrations/prometheus.py

@@ -2620,7 +2620,18 @@ class PrometheusLogger(CustomLogger):
                 label_litellm_model_name = litellm_model_name
                 label_model_id = model_id
                 label_api_base = api_base
-                label_api_provider = llm_provider
+                # custom_llm_provider can be absent from litellm_params when a
+                # resolved deployment fails (e.g. fallbacks); fall back to the
+                # best-effort resolver and coalesce to "" so the label never
+                # serializes as the literal "None" (same pattern as
+                # set_deployment_partial_outage below).
+                label_api_provider = (  # pyright: ignore[reportUnknownVariableType]  # bare-dict kwargs, same pattern as every label reader in this handler
+                    llm_provider
+                    or self._extract_api_provider_from_request_data(
+                        request_kwargs  # pyright: ignore[reportUnknownArgumentType, reportUnknownMemberType]
+                    )
+                    or ""
+                )
                 label_requested_model = model_group or litellm_model_name
             else:
                 label_litellm_model_name = ""

tests/test_litellm/integrations/test_prometheus_deployment_failure_api_provider.py

@@ -0,0 +1,83 @@
+"""
+Regression tests for #38531: the deployment-failure counter must never emit
+api_provider="None". When a resolved deployment fails without
+custom_llm_provider on litellm_params, fall back to the best-effort resolver
+and coalesce to "" so alerting never sees a phantom all-failure series.
+"""
+
+import pytest
+from prometheus_client import REGISTRY
+
+from litellm.integrations.prometheus import PrometheusLogger
+
+
+@pytest.fixture(scope="function")
+def prometheus_logger():
+    collectors = list(REGISTRY._collector_to_names.keys())
+    for collector in collectors:
+        REGISTRY.unregister(collector)
+    return PrometheusLogger()
+
+
+def _failure_samples(logger: PrometheusLogger):
+    """Collected samples of the deployment-failure counter, as (labels, value)."""
+    samples = []
+    for metric in logger.litellm_deployment_failure_responses.collect():
+        for sample in metric.samples:
+            if sample.name == "litellm_deployment_failure_responses_total":
+                samples.append((dict(sample.labels), sample.value))
+    return samples
+
+
+def _request_kwargs(model: str, standard_custom_provider: str | None = None) -> dict:
+    standard_logging_object = {
+        "model_id": "resolved-deployment-id",
+        "model_group": "my-model-group",
+        "api_base": "https://example.com",
+        "metadata": {},
+    }
+    if standard_custom_provider is not None:
+        standard_logging_object["custom_llm_provider"] = standard_custom_provider
+    return {
+        "model": model,
+        "exception": Exception("upstream 503"),
+        "litellm_params": {
+            # no "custom_llm_provider" key — the #38531 scenario
+            "metadata": {},
+        },
+        "standard_logging_object": standard_logging_object,
+    }
+
+
+def test_missing_provider_on_resolved_deployment_never_labels_none(prometheus_logger):
+    """custom_llm_provider absent and the model name is not provider-prefixed:
+    the label must be "" (not the literal "None")."""
+
+    prometheus_logger.set_llm_deployment_failure_metrics(_request_kwargs("my-fallback-model"))
+
+    samples = _failure_samples(prometheus_logger)
+    assert len(samples) == 1
+    assert samples[0][0]["api_provider"] == ""
+
+
+def test_missing_provider_infers_from_provider_prefixed_model(prometheus_logger):
+    """The resolver can still infer the provider from a prefixed model name."""
+
+    prometheus_logger.set_llm_deployment_failure_metrics(_request_kwargs("openai/gpt-4o-mini"))
+
+    samples = _failure_samples(prometheus_logger)
+    assert len(samples) == 1
+    assert samples[0][0]["api_provider"] == "openai"
+
+
+def test_explicit_provider_still_wins(prometheus_logger):
+    """Behavior is unchanged when custom_llm_provider is present."""
+
+    kwargs = _request_kwargs("openai/gpt-4o-mini")
+    kwargs["litellm_params"]["custom_llm_provider"] = "azure"
+
+    prometheus_logger.set_llm_deployment_failure_metrics(kwargs)
+
+    samples = _failure_samples(prometheus_logger)
+    assert len(samples) == 1
+    assert samples[0][0]["api_provider"] == "azure"