Add validation to reject non-text content in system messages (#34072)

Signed-off-by: Varun Chawla <varun_6april@hotmail.com>

tests/entrypoints/openai/test_chat_error.py

@@ -4,7 +4,7 @@
 from dataclasses import dataclass, field
 from http import HTTPStatus
 from typing import Any
-from unittest.mock import AsyncMock, MagicMock
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -233,3 +233,140 @@ async def test_chat_error_stream():
         f"Expected error message in chunks: {chunks}"
     )
     assert chunks[-1] == "data: [DONE]\n\n"
+
+
+@pytest.mark.parametrize(
+    "image_content",
+    [
+        [{"type": "image_url", "image_url": {"url": "https://example.com/image.jpg"}}],
+        [{"image_url": {"url": "https://example.com/image.jpg"}}],
+    ],
+)
+def test_system_message_warns_on_image(image_content):
+    """Test that system messages with image content trigger a warning."""
+    with patch(
+        "vllm.entrypoints.openai.chat_completion.protocol.logger"
+    ) as mock_logger:
+        ChatCompletionRequest(
+            model=MODEL_NAME,
+            messages=[
+                {
+                    "role": "system",
+                    "content": image_content,
+                }
+            ],
+        )
+
+    mock_logger.warning_once.assert_called()
+    call_args = str(mock_logger.warning_once.call_args)
+    assert "System messages should only contain text" in call_args
+    assert "image_url" in call_args
+
+
+def test_system_message_accepts_text():
+    """Test that system messages can contain text content."""
+    # Should not raise an exception
+    request = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[
+            {"role": "system", "content": "You are a helpful assistant."},
+        ],
+    )
+    assert request.messages[0]["role"] == "system"
+
+
+def test_system_message_accepts_text_array():
+    """Test that system messages can contain an array with text content."""
+    # Should not raise an exception
+    request = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[
+            {
+                "role": "system",
+                "content": [{"type": "text", "text": "You are a helpful assistant."}],
+            },
+        ],
+    )
+    assert request.messages[0]["role"] == "system"
+
+
+def test_user_message_accepts_image():
+    """Test that user messages can still contain image content."""
+    # Should not raise an exception
+    request = ChatCompletionRequest(
+        model=MODEL_NAME,
+        messages=[
+            {
+                "role": "user",
+                "content": [
+                    {"type": "text", "text": "What's in this image?"},
+                    {
+                        "type": "image_url",
+                        "image_url": {"url": "https://example.com/image.jpg"},
+                    },
+                ],
+            },
+        ],
+    )
+    assert request.messages[0]["role"] == "user"
+
+
+@pytest.mark.parametrize(
+    "audio_content",
+    [
+        [
+            {
+                "type": "input_audio",
+                "input_audio": {"data": "base64data", "format": "wav"},
+            }
+        ],
+        [{"input_audio": {"data": "base64data", "format": "wav"}}],
+    ],
+)
+def test_system_message_warns_on_audio(audio_content):
+    """Test that system messages with audio content trigger a warning."""
+    with patch(
+        "vllm.entrypoints.openai.chat_completion.protocol.logger"
+    ) as mock_logger:
+        ChatCompletionRequest(
+            model=MODEL_NAME,
+            messages=[
+                {
+                    "role": "system",
+                    "content": audio_content,
+                }
+            ],
+        )
+
+    mock_logger.warning_once.assert_called()
+    call_args = str(mock_logger.warning_once.call_args)
+    assert "System messages should only contain text" in call_args
+    assert "input_audio" in call_args
+
+
+@pytest.mark.parametrize(
+    "video_content",
+    [
+        [{"type": "video_url", "video_url": {"url": "https://example.com/video.mp4"}}],
+        [{"video_url": {"url": "https://example.com/video.mp4"}}],
+    ],
+)
+def test_system_message_warns_on_video(video_content):
+    """Test that system messages with video content trigger a warning."""
+    with patch(
+        "vllm.entrypoints.openai.chat_completion.protocol.logger"
+    ) as mock_logger:
+        ChatCompletionRequest(
+            model=MODEL_NAME,
+            messages=[
+                {
+                    "role": "system",
+                    "content": video_content,
+                }
+            ],
+        )
+
+    mock_logger.warning_once.assert_called()
+    call_args = str(mock_logger.warning_once.call_args)
+    assert "System messages should only contain text" in call_args
+    assert "video_url" in call_args

vllm/entrypoints/openai/chat_completion/protocol.py

@@ -674,3 +674,52 @@ class ChatCompletionRequest(OpenAIBaseModel):
                 "Parameter 'cache_salt' must be a non-empty string if provided."
             )
         return data
+
+    @model_validator(mode="before")
+    @classmethod
+    def check_system_message_content_type(cls, data):
+        """Warn if system messages contain non-text content.
+
+        According to OpenAI API spec, system messages can only be of type
+        'text'. We log a warning instead of rejecting to avoid breaking
+        users who intentionally send multimodal system messages.
+        See: https://platform.openai.com/docs/api-reference/chat/create#chat_create-messages-system_message
+        """
+        if not isinstance(data, dict):
+            return data
+        messages = data.get("messages", [])
+        for msg in messages:
+            # Check if this is a system message
+            if isinstance(msg, dict) and msg.get("role") == "system":
+                content = msg.get("content")
+
+                # If content is a list (multimodal format)
+                if isinstance(content, list):
+                    for part in content:
+                        if isinstance(part, dict):
+                            part_type = part.get("type")
+                            # Infer type when 'type' field is not explicit
+                            if part_type is None:
+                                if "image_url" in part or "image_pil" in part:
+                                    part_type = "image_url"
+                                elif "image_embeds" in part:
+                                    part_type = "image_embeds"
+                                elif "audio_url" in part:
+                                    part_type = "audio_url"
+                                elif "input_audio" in part:
+                                    part_type = "input_audio"
+                                elif "audio_embeds" in part:
+                                    part_type = "audio_embeds"
+                                elif "video_url" in part:
+                                    part_type = "video_url"
+
+                            # Warn about non-text content in system messages
+                            if part_type and part_type != "text":
+                                logger.warning_once(
+                                    "System messages should only contain text "
+                                    "content according to the OpenAI API spec. "
+                                    "Found content type: '%s'.",
+                                    part_type,
+                                )
+
+        return data