fix(proxy): keep extra chunk fields on the streaming fast path

_fast_serialize_simple_model_response_stream writes a fixed set of fields, so a field set on the chunk outside its schema (model_extra), for example by an async_post_call_streaming_iterator_hook, was silently dropped from plain text chunks. Fall back to the canonical serializer when the chunk carries an extra the fast payload does not write; service_tier is still served on the fast path.

litellm/proxy/proxy_server.py

@@ -9565,6 +9565,14 @@ def _fast_serialize_simple_model_response_stream(
     ):
         return None
 
+    # Fields set on the chunk outside its schema (e.g. by a streaming iterator
+    # hook) live in ``model_extra``. The payload below only writes
+    # ``service_tier``, so any other extra would be silently dropped here
+    # while the canonical serializer keeps it.
+    model_extra: Final = getattr(chunk, "model_extra", None)
+    if model_extra and any(key != "service_tier" for key in model_extra):
+        return None
+
     choices: Final = getattr(chunk, "choices", None)
     if not isinstance(choices, list) or len(choices) != 1:
         return None

tests/unit/proxy/proxy_server/test_streaming_helpers.py

@@ -550,6 +550,21 @@ def test_fast_serialize_simple_model_response_stream_with_usage_returns_none_inv
     assert _fast_serialize_simple_model_response_stream(chunk) is None
 
 
+def test_fast_serialize_simple_model_response_stream_with_extra_field_returns_none():
+    """A field a callback attaches to the chunk is not in the fast payload, so
+    the fast path must bail rather than drop it."""
+    chunk = _simple_chunk()
+    chunk.gateway_metadata = {"served_by": "fallback-model"}
+    assert _fast_serialize_simple_model_response_stream(chunk) is None
+
+
+def test_serialize_streaming_chunk_keeps_extra_field():
+    chunk = _simple_chunk()
+    chunk.gateway_metadata = {"served_by": "fallback-model"}
+    payload = json.loads(_serialize_streaming_chunk(chunk))
+    assert payload["gateway_metadata"] == {"served_by": "fallback-model"}
+
+
 # ---------------------------------------------------------------------------
 # _serialize_streaming_chunk
 # ---------------------------------------------------------------------------