From fe945f9d512c7d4c5cea96fa45b3557b71dd6e8b Mon Sep 17 00:00:00 2001
From: Marco Molinati <marco.molinati@elmec.it>
Date: Tue, 1 Sep 2026 16:53:49 +0200
Subject: [PATCH] [Bugfix][Frontend] Flatten list-form function_call_output on
 Responses API

`ResponsesRequest.input` validates items against the OpenAI SDK types, where
`FunctionCallOutput.output` is `Union[str, ResponseFunctionCallOutputItemListParam]`,
so a tool result may legitimately arrive as a list of content parts. The Harmony
conversion passed that value straight into `Message.from_author_and_content`,
which expects a string, and the request failed with:

    1 validation error for Message
    content.0
      Input should be a valid dictionary or instance of Content
      [type=model_type, input_value=[{'text': '...', 'type': 'input_text'}], input_type=list]

The Chat Completions path already flattens list content for the tool role
(`parser/harmony_utils.py`, added in #25593 for #25001). Apply the same
`flatten_input_text_content` call to the Responses path so both surfaces accept
either form. A string output is returned unchanged and an empty list becomes an
empty string, matching the Chat Completions behaviour.

Signed-off-by: Marco Molinati <marco.molinati@elmec.it>
---
 .../openai/responses/test_harmony_utils.py    | 63 +++++++++++++++++++
 vllm/entrypoints/openai/responses/harmony.py  |  2 +-
 2 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/tests/entrypoints/openai/responses/test_harmony_utils.py b/tests/entrypoints/openai/responses/test_harmony_utils.py
index bd4a46741d88..dd391fda27eb 100644
--- a/tests/entrypoints/openai/responses/test_harmony_utils.py
+++ b/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."""
 
diff --git a/vllm/entrypoints/openai/responses/harmony.py b/vllm/entrypoints/openai/responses/harmony.py
index 60a9f65f4486..e2e4be3c0fc3 100644
--- a/vllm/entrypoints/openai/responses/harmony.py
+++ b/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")