diff --git a/litellm/litellm_core_utils/streaming_handler.py b/litellm/litellm_core_utils/streaming_handler.py
index cdc480f610e..a0f68605229 100644
--- a/litellm/litellm_core_utils/streaming_handler.py
+++ b/litellm/litellm_core_utils/streaming_handler.py
@@ -1955,15 +1955,10 @@ class CustomStreamWrapper:
                         hasattr(processed_chunk, "usage")
                         and getattr(processed_chunk, "usage", None) is not None
                     ):
-                        # Strip usage from the outgoing chunk so
-                        # model_dump_json(exclude_none=True) drops it.
-                        # The copy in self.chunks retains usage for
-                        # calculate_total_usage().
+                        # Set usage to None so model_dump_json(exclude_none=True)
+                        # drops it. The original usage is already preserved in
+                        # self.chunks (appended above) for calculate_total_usage().
                         processed_chunk.usage = None  # type: ignore
-                        # After nullifying usage, check if the chunk has any
-                        # remaining content (delta, finish_reason, etc.).
-                        # is_model_response_stream_empty sees usage=None and
-                        # correctly skips it, only checking meaningful fields.
                         is_empty = is_model_response_stream_empty(
                             model_response=cast(ModelResponseStream, processed_chunk)
                         )
diff --git a/tests/test_litellm/proxy/middleware/test_prometheus_auth_middleware_asgi.py b/tests/test_litellm/proxy/middleware/test_prometheus_auth_middleware_asgi.py
new file mode 100644
index 00000000000..8d7af21f7b3
--- /dev/null
+++ b/tests/test_litellm/proxy/middleware/test_prometheus_auth_middleware_asgi.py
@@ -0,0 +1,24 @@
+"""
+Tests that PrometheusAuthMiddleware is a pure ASGI middleware (not BaseHTTPMiddleware).
+
+BaseHTTPMiddleware wraps streaming responses with receive_or_disconnect per chunk,
+which blocks the event loop and causes severe throughput degradation.
+"""
+from starlette.middleware.base import BaseHTTPMiddleware
+
+from litellm.proxy.middleware.prometheus_auth_middleware import PrometheusAuthMiddleware
+
+
+def test_is_not_base_http_middleware():
+    """PrometheusAuthMiddleware must NOT inherit from BaseHTTPMiddleware."""
+    assert not issubclass(PrometheusAuthMiddleware, BaseHTTPMiddleware), (
+        "PrometheusAuthMiddleware should be a pure ASGI middleware, not BaseHTTPMiddleware. "
+        "BaseHTTPMiddleware causes severe streaming performance degradation."
+    )
+
+
+def test_has_asgi_call_protocol():
+    """PrometheusAuthMiddleware must implement the ASGI __call__ protocol."""
+    assert "__call__" in PrometheusAuthMiddleware.__dict__, (
+        "PrometheusAuthMiddleware must define __call__(self, scope, receive, send)"
+    )