diff --git a/tests/test_litellm/passthrough/test_stream_flush_buffer.py b/tests/test_litellm/passthrough/test_stream_flush_buffer.py
new file mode 100644
index 00000000000..f638c2eca76
--- /dev/null
+++ b/tests/test_litellm/passthrough/test_stream_flush_buffer.py
@@ -0,0 +1,55 @@
+"""Stream / EventStream envelope tests for passthrough flush plumbing."""
+
+import base64
+import json
+
+import pytest
+
+
+def test_event_stream_message_to_response_dict_has_no_chunk_key():
+    """
+    ``EventStreamMessage`` is botocore's decoded *frame* object. ``to_response_dict()``
+    only exposes ``status_code``, ``headers``, and ``body`` (``body`` == ``payload`` bytes).
+
+    A ``"chunk"`` member (Bedrock ResponseStream union) appears only *after*
+    ``EventStreamJSONParser.parse(response_dict, ...)`` on ``common_utils`` — not on this dict.
+    """
+    from botocore.eventstream import EventStreamMessage, MessagePrelude
+
+    prelude = MessagePrelude(total_length=256, headers_length=0, crc=0)
+    headers = {":message-type": "event"}
+    outer_json = {
+        "chunk": {
+            "bytes": base64.b64encode(b'{"delta":{"text":"x"}}').decode("ascii"),
+        }
+    }
+    payload = json.dumps(outer_json).encode("utf-8")
+    event = EventStreamMessage(prelude, headers, payload, crc=0)
+
+    response_dict = event.to_response_dict()
+
+    assert "chunk" not in response_dict
+    assert set(response_dict.keys()) == {"status_code", "headers", "body"}
+    assert response_dict["body"] is event.payload
+
+    # Inner service JSON can still use a ``chunk`` key inside ``body`` bytes.
+    loaded = json.loads(response_dict["body"].decode("utf-8"))
+    assert "chunk" in loaded
+
+
+@pytest.mark.parametrize("message_type", ["error", "exception"])
+def test_event_stream_message_error_types_response_dict_has_no_chunk_key(
+    message_type: str,
+):
+    from botocore.eventstream import EventStreamMessage, MessagePrelude
+
+    prelude = MessagePrelude(total_length=128, headers_length=0, crc=0)
+    headers = {":message-type": message_type}
+    payload = b'{"message":"oops"}'
+    event = EventStreamMessage(prelude, headers, payload, crc=0)
+
+    response_dict = event.to_response_dict()
+
+    assert "chunk" not in response_dict
+    assert response_dict["status_code"] == 400
+    assert set(response_dict.keys()) == {"status_code", "headers", "body"}
diff --git a/tests/test_litellm/passthrough/test_streaming_interrupt_spend_tracking.py b/tests/test_litellm/passthrough/test_streaming_interrupt_spend_tracking.py
index 455b6789b4e..1b9e6205d54 100644
--- a/tests/test_litellm/passthrough/test_streaming_interrupt_spend_tracking.py
+++ b/tests/test_litellm/passthrough/test_streaming_interrupt_spend_tracking.py
@@ -7,6 +7,14 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import httpx
 import pytest
 
+from litellm.passthrough.stream_flush_buffer import RawBytesStreamBuffer
+
+
+def _provider_config_with_raw_buffer():
+    pc = MagicMock()
+    pc.build_stream_flush_buffer.return_value = RawBytesStreamBuffer()
+    return pc
+
 
 def _make_streaming_response(chunks: List[bytes]):
     mock = MagicMock(spec=httpx.Response)