[Bugfix][Frontend] Accept list-form function_call_output in Responses API

tests/entrypoints/openai/responses/test_harmony_utils.py

@@ -14,6 +14,7 @@
 
 from vllm.entrypoints.openai.responses.harmony import (
     harmony_to_response_output,
+    response_input_to_harmony,
     response_previous_input_to_harmony,
 )
 
@@ -93,6 +94,68 @@ def test_tool_message_with_empty_content(self):
         assert messages[0].content[0].text == ""
 
 
+class TestResponseInputToHarmony:
+    """Tests for the Responses API response_input_to_harmony function."""
+
+    @staticmethod
+    def _tool_call() -> ResponseFunctionToolCall:
+        return ResponseFunctionToolCall(
+            type="function_call",
+            call_id="call_1",
+            name="get_weather",
+            arguments='{"city": "Paris"}',
+        )
+
+    def test_function_call_output_with_string_output(self):
+        """Test parsing a function call output with string output."""
+        response_msg = {
+            "type": "function_call_output",
+            "call_id": "call_1",
+            "output": "The weather in Paris is sunny, 18°C",
+        }
+
+        msg = response_input_to_harmony(response_msg, [self._tool_call()])
+
+        assert msg.author.role == Role.TOOL
+        assert msg.author.name == "functions.get_weather"
+        assert msg.content[0].text == "The weather in Paris is sunny, 18°C"
+
+    def test_function_call_output_with_list_output(self):
+        """Test parsing a function call output with a content part list.
+
+        The Responses API accepts `output` as either a string or a list of
+        content parts, so the parts must be flattened into a single string
+        before the Harmony message is built.
+        """
+        response_msg = {
+            "type": "function_call_output",
+            "call_id": "call_1",
+            "output": [
+                {"type": "input_text", "text": "The weather in Paris "},
+                {"type": "input_text", "text": "is sunny, 18°C"},
+            ],
+        }
+
+        msg = response_input_to_harmony(response_msg, [self._tool_call()])
+
+        assert msg.author.role == Role.TOOL
+        assert msg.author.name == "functions.get_weather"
+        assert msg.content[0].text == "The weather in Paris is sunny, 18°C"
+
+    def test_function_call_output_with_empty_list_output(self):
+        """Test parsing a function call output with an empty content list."""
+        response_msg = {
+            "type": "function_call_output",
+            "call_id": "call_1",
+            "output": [],
+        }
+
+        msg = response_input_to_harmony(response_msg, [self._tool_call()])
+
+        assert msg.author.role == Role.TOOL
+        assert msg.content[0].text == ""
+
+
 class TestHarmonyToResponseOutput:
     """Tests for harmony_to_response_output function."""
 

vllm/entrypoints/openai/responses/harmony.py

@@ -187,7 +187,7 @@ def response_input_to_harmony(
             raise ValueError(f"No call message found for {call_id}")
         msg = Message.from_author_and_content(
             Author.new(Role.TOOL, f"functions.{call_response.name}"),
-            response_msg["output"],
+            flatten_input_text_content(response_msg["output"]) or "",
         )
         msg = msg.with_channel("commentary")
         msg = msg.with_recipient("assistant")