fix: handle string callback values in prometheus multiproc setup

When callbacks are configured as a plain string (e.g., `callbacks: "my_callback"`)
instead of a list, the proxy crashes on startup with:
  TypeError: can only concatenate str (not "list") to str

Normalize each callback setting to a list before concatenating.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

litellm/proxy/proxy_cli.py

@@ -340,9 +340,16 @@ class ProxyInitializationHelpers:
             return
 
         # Check if prometheus is in any callback list
+        # Each setting can be a list or a single string; normalize to list
         callbacks = litellm_settings.get("callbacks") or []
         success_callbacks = litellm_settings.get("success_callback") or []
         failure_callbacks = litellm_settings.get("failure_callback") or []
+        if isinstance(callbacks, str):
+            callbacks = [callbacks]
+        if isinstance(success_callbacks, str):
+            success_callbacks = [success_callbacks]
+        if isinstance(failure_callbacks, str):
+            failure_callbacks = [failure_callbacks]
         all_callbacks = callbacks + success_callbacks + failure_callbacks
         if "prometheus" not in all_callbacks:
             return

tests/test_litellm/proxy/test_prometheus_cleanup.py

@@ -67,6 +67,30 @@ class TestMaybeSetupPrometheusMultiprocDir:
             assert os.environ["PROMETHEUS_MULTIPROC_DIR"] == custom_dir
             assert os.path.isdir(custom_dir)
 
+    @pytest.mark.parametrize(
+        "litellm_settings",
+        [
+            {"callbacks": "prometheus"},
+            {"success_callback": "prometheus"},
+            {"failure_callback": "prometheus"},
+            {"callbacks": "custom_callback"},  # string but not prometheus
+        ],
+    )
+    def test_handles_string_callbacks(self, litellm_settings):
+        """When callbacks are specified as a string instead of a list, should not crash."""
+        with patch.dict(os.environ, {}, clear=False):
+            os.environ.pop("PROMETHEUS_MULTIPROC_DIR", None)
+            os.environ.pop("prometheus_multiproc_dir", None)
+
+            # Should not raise TypeError
+            ProxyInitializationHelpers._maybe_setup_prometheus_multiproc_dir(
+                num_workers=4,
+                litellm_settings=litellm_settings,
+            )
+
+            # Cleanup
+            os.environ.pop("PROMETHEUS_MULTIPROC_DIR", None)
+
     @pytest.mark.parametrize(
         "num_workers, litellm_settings",
         [