[Bugfix] Responses API assistant EasyInputMessageParam input

tests/entrypoints/openai/responses/test_function_call_parsing.py

@@ -5,7 +5,7 @@
 import json
 
 import pytest
-from openai.types.responses import ResponseFunctionToolCall
+from openai.types.responses import ResponseFunctionToolCall, ResponseOutputMessage
 
 from vllm.entrypoints.openai.responses.protocol import ResponsesRequest
 
@@ -328,3 +328,52 @@ def test_validator_handles_empty_iterator():
 
     request = ResponsesRequest(**mock_data)
     assert request.input == []
+
+
+def test_assistant_string_content_stays_easyinput():
+    """EasyInput assistant message with plain string content is not
+    coerced into a ResponseOutputMessage."""
+    request_data = {
+        "model": "test-model",
+        "input": [
+            {"type": "message", "role": "assistant", "content": "hello"},
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    item = request.input[0]
+    assert isinstance(item, dict), (
+        "String-content assistant message should remain a dict (EasyInput), "
+        f"got {type(item)}"
+    )
+    assert item.get("content") == "hello"
+    assert "id" not in item
+    assert "status" not in item
+
+
+def test_assistant_output_style_content_coerced():
+    """Assistant message whose content is output-message-shaped (list of
+    output_text items) should be coerced to ResponseOutputMessage."""
+    request_data = {
+        "model": "test-model",
+        "input": [
+            {
+                "type": "message",
+                "role": "assistant",
+                "content": [{"type": "output_text", "text": "world"}],
+            },
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    item = request.input[0]
+    assert isinstance(item, ResponseOutputMessage), (
+        "Output-style assistant message should be coerced to "
+        f"ResponseOutputMessage, got {type(item)}"
+    )
+    assert item.content[0].text == "world"
+    assert item.content[0].annotations == []
+    assert item.status == "completed"
+    assert item.id.startswith("msg_")

tests/entrypoints/openai/responses/test_responses_utils.py

@@ -782,6 +782,16 @@ class TestConstructChatMessagesCombinePolicy:
                 ["call_123", "call_456"],
                 id="reasoning-output-tool-call",
             ),
+            pytest.param(
+                [
+                    make_reasoning_item(content_text="Let me think"),
+                    {"type": "message", "role": "assistant", "content": "Hello"},
+                ],
+                "Hello",
+                "Let me think",
+                None,
+                id="reasoning-easyinput-assistant",
+            ),
         ],
     )
     def test_assistant_side_items_merge_until_tool_output(

vllm/entrypoints/openai/responses/protocol.py

@@ -525,31 +525,38 @@ def input_item_parsing(cls, data):
                     processed_input.append(item)
 
             elif item_type == "message" and item.get("role") == "assistant":
+                content = item.get("content")
+                if not isinstance(content, list):
+                    # String content is a valid EasyInputMessageParam,
+                    # do not coerce it to ResponseOutputMessage
+                    processed_input.append(item)
+                    continue
+
+                original_item = item
                 item = dict(item)
                 if "id" not in item:
                     item["id"] = f"msg_{random_uuid()}"
                 if "status" not in item:
                     item["status"] = "completed"
                 # ResponseOutputText requires annotations
-                if isinstance(item.get("content"), list):
-                    new_content = []
-                    for c in item["content"]:
-                        if (
-                            isinstance(c, dict)
-                            and c.get("type") == "output_text"
-                            and "annotations" not in c
-                        ):
-                            c = {**c, "annotations": []}
-                        new_content.append(c)
-                    item["content"] = new_content
+                new_content = []
+                for c in content:
+                    if (
+                        isinstance(c, dict)
+                        and c.get("type") == "output_text"
+                        and "annotations" not in c
+                    ):
+                        c = {**c, "annotations": []}
+                    new_content.append(c)
+                item["content"] = new_content
                 try:
                     processed_input.append(ResponseOutputMessage(**item))
                 except ValidationError:
                     logger.debug(
                         "Failed to parse assistant message to ResponseOutputMessage, "
                         "leaving for Pydantic validation"
                     )
-                    processed_input.append(item)
+                    processed_input.append(original_item)
 
             else:
                 processed_input.append(item)

vllm/entrypoints/openai/responses/utils.py

@@ -306,6 +306,20 @@ def _construct_message_from_response_item(
             content=item.get("output"),
             tool_call_id=item.get("call_id"),
         )
+    elif isinstance(item, dict) and item.get("role") == "assistant":
+        content = item.get("content")
+        text: str | None = None
+        if isinstance(content, str):
+            text = content
+        elif isinstance(content, list) and content:
+            text = content[0].get("text")
+        if text is not None:
+            if prev_assistant_msg:
+                previous_content = prev_assistant_msg.get("content")
+                if previous_content is None:
+                    prev_assistant_msg["content"] = text
+                    return None
+            return {"role": "assistant", "content": text}
     return item  # type: ignore[arg-type]