diff --git a/litellm/proxy/guardrails/guardrail_hooks/model_armor/model_armor.py b/litellm/proxy/guardrails/guardrail_hooks/model_armor/model_armor.py
index 419eb93f506..ab7b86f44c8 100644
--- a/litellm/proxy/guardrails/guardrail_hooks/model_armor/model_armor.py
+++ b/litellm/proxy/guardrails/guardrail_hooks/model_armor/model_armor.py
@@ -849,16 +849,20 @@ class ModelArmorGuardrail(CustomGuardrail, VertexBase):
         )
 
         # Collect all chunks
-        all_chunks: Final[list[ModelResponseStream]] = []
+        all_chunks: Final[list[object]] = []
         async for chunk in response:
             all_chunks.append(chunk)
 
-        # /v1/messages arrives as raw SSE frames, which stream_chunk_builder cannot assemble
+        # /v1/messages arrives as raw SSE frames and /v1/responses as Responses API event
+        # objects; stream_chunk_builder can assemble neither
         raw_sse: Final = is_raw_sse_stream(all_chunks)
+        chat_stream: Final = bool(all_chunks) and all(isinstance(chunk, ModelResponseStream) for chunk in all_chunks)
         assembled_response: Final = (
             assemble_anthropic_sse_stream(all_chunks, restore_identity=True)
             if raw_sse
             else stream_chunk_builder(chunks=all_chunks)
+            if chat_stream
+            else None
         )
 
         if isinstance(assembled_response, ModelResponse):
@@ -963,6 +967,11 @@ class ModelArmorGuardrail(CustomGuardrail, VertexBase):
             ):
                 yield error_frame
             return
+        elif not chat_stream:
+            verbose_proxy_logger.warning(
+                "Model Armor: streaming response contained unsupported event objects "
+                "(e.g. /v1/responses events); output scanning was skipped for this response."
+            )
 
         # Return original chunks if no sanitization needed
         for chunk in all_chunks:
diff --git a/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_model_armor.py b/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_model_armor.py
index a6ffc9503e2..812bc1f0eaa 100644
--- a/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_model_armor.py
+++ b/tests/test_litellm/proxy/guardrails/guardrail_hooks/test_model_armor.py
@@ -652,7 +652,7 @@ def _sse_armor_guardrail(**kwargs: object) -> ModelArmorGuardrail:
 
 
 async def _drain_armor_streaming_hook(
-    guardrail: ModelArmorGuardrail, chunks: tuple[bytes, ...] = _ANTHROPIC_SSE_CHUNKS
+    guardrail: ModelArmorGuardrail, chunks: tuple[object, ...] = _ANTHROPIC_SSE_CHUNKS
 ) -> list[object]:
     async def _stream():
         for chunk in chunks:
@@ -764,6 +764,27 @@ async def test_streaming_hook_masks_raw_anthropic_sse():
     assert b"123-45-6789" not in body
 
 
+@pytest.mark.asyncio
+async def test_streaming_hook_passes_through_responses_api_events():
+    """/v1/responses streams deliver event objects stream_chunk_builder cannot assemble.
+
+    Regression for `500 Error building chunks for logging/streaming usage calculation`.
+    """
+    from litellm.types.llms.openai import GenericEvent, ResponsesAPIStreamEvents
+
+    guardrail = _sse_armor_guardrail()
+    events = (
+        GenericEvent(type=ResponsesAPIStreamEvents.RESPONSE_CREATED),
+        GenericEvent(type=ResponsesAPIStreamEvents.RESPONSE_COMPLETED),
+    )
+
+    with patch.object(guardrail.async_handler, "post", AsyncMock()) as mock_post:
+        delivered = await _drain_armor_streaming_hook(guardrail, chunks=events)
+
+    mock_post.assert_not_called()
+    assert tuple(delivered) == events
+
+
 @pytest.mark.asyncio
 async def test_streaming_hook_fails_closed_on_unparseable_raw_sse():
     guardrail = _sse_armor_guardrail()