fix: read the Responses delta event types off the event enum

The hand-listed set left out response.mcp_call_arguments.delta, so a turn that
streamed only MCP tool arguments and then reported an empty body still took the
no-content shortcut and forwarded those chunks unscanned.

Deriving the set from ResponsesAPIStreamEvents keeps it complete as the enum
grows, and the str guard in the reader already covers any event whose delta is
not text.

litellm/proxy/guardrails/guardrail_hooks/model_armor/model_armor.py

@@ -45,6 +45,7 @@ from litellm.types.llms.openai import (
     AllMessageValues,
     ChatCompletionToolCallChunk,
     ResponsesAPIResponse,
+    ResponsesAPIStreamEvents,
 )
 from litellm.types.utils import (
     CallTypes,
@@ -62,15 +63,10 @@ GUARDRAIL_NAME: Final = "model_armor"
 # Only these carry the finished output; response.created carries an empty body
 _RESPONSES_TERMINAL_EVENT_TYPES: Final = frozenset({"response.completed", "response.incomplete", "response.failed"})
 
-# Every event whose ``delta`` is model output already on its way to the client
+# Every event whose ``delta`` is model output already on its way to the client. Read off the event
+# enum rather than listed, so an event added there cannot quietly fall out of the scan
 _RESPONSES_DELTA_EVENT_TYPES: Final = frozenset(
-    {
-        "response.output_text.delta",
-        "response.refusal.delta",
-        "response.function_call_arguments.delta",
-        "response.custom_tool_call_input.delta",
-        "response.reasoning_summary_text.delta",
-    }
+    event.value for event in ResponsesAPIStreamEvents if event.value.endswith(".delta")
 )
 
 

tests/test_litellm/proxy/guardrails/guardrail_hooks/test_model_armor.py

@@ -4615,3 +4615,68 @@ async def test_streaming_responses_terminal_event_without_a_body_still_scans_the
     rendered = "".join(str(item) for item in delivered)
     assert "4111-1111-1111-1111" not in rendered
     assert "Streaming response blocked by Model Armor" in rendered
+
+
+@pytest.mark.asyncio
+async def test_streaming_responses_mcp_argument_deltas_are_scanned_when_the_body_is_empty():
+    """A turn that only streamed MCP tool arguments still handed the client a payload.
+
+    The delta fallback is read off the event enum rather than listed by hand, so an argument event
+    that carries no `output_text` cannot fall out of the scan.
+    """
+    from litellm.types.llms.openai import (
+        MCPCallArgumentsDeltaEvent,
+        ResponseIncompleteEvent,
+        ResponsesAPIResponse,
+        ResponsesAPIStreamEvents,
+    )
+
+    chunks = (
+        MCPCallArgumentsDeltaEvent(
+            type=ResponsesAPIStreamEvents.MCP_CALL_ARGUMENTS_DELTA,
+            output_index=0,
+            item_id="mcp_1",
+            delta='{"note": "my card is 4111-1111-1111-1111"}',
+            sequence_number=0,
+        ),
+        ResponseIncompleteEvent(
+            type=ResponsesAPIStreamEvents.RESPONSE_INCOMPLETE,
+            response=ResponsesAPIResponse(
+                id="resp_1",
+                created_at=0,
+                model="gpt-4o-mini",
+                object="response",
+                output=[],
+                parallel_tool_calls=False,
+                tool_choice="auto",
+                tools=[],
+            ),
+        ),
+    )
+    guardrail = _surface_guardrail()
+    post = _armor_post_mock(_MODEL_ARMOR_BLOCK)
+
+    with patch.object(guardrail.async_handler, "post", post):
+        delivered = await _drain_surface_hook(guardrail, chunks)
+
+    post.assert_called_once()
+    assert "4111-1111-1111-1111" in post.call_args.kwargs["json"]["modelResponseData"]["text"]
+    rendered = "".join(str(item) for item in delivered)
+    assert "4111-1111-1111-1111" not in rendered
+    assert "Streaming response blocked by Model Armor" in rendered
+
+
+def test_every_responses_delta_event_is_in_the_scanned_set():
+    """Every ``.delta`` the Responses event enum defines is model output on its way to the client."""
+    from litellm.proxy.guardrails.guardrail_hooks.model_armor.model_armor import (
+        _RESPONSES_DELTA_EVENT_TYPES,
+    )
+    from litellm.types.llms.openai import ResponsesAPIStreamEvents
+
+    missing = {
+        event.value
+        for event in ResponsesAPIStreamEvents
+        if event.value.endswith(".delta") and event.value not in _RESPONSES_DELTA_EVENT_TYPES
+    }
+    assert not missing
+    assert "response.mcp_call_arguments.delta" in _RESPONSES_DELTA_EVENT_TYPES