[responsesAPI] add better error messaging for long prompts (#25724)

Signed-off-by: Andrew Xia <axia@meta.com>
Signed-off-by: Andrew Xia <axia@fb.com>
Co-authored-by: Andrew Xia <axia@fb.com>

tests/entrypoints/openai/test_serving_responses.py

@@ -8,9 +8,10 @@ import pytest
 import pytest_asyncio
 
 from vllm.entrypoints.context import ConversationContext
-from vllm.entrypoints.openai.protocol import ResponsesRequest
+from vllm.entrypoints.openai.protocol import ErrorResponse, ResponsesRequest
 from vllm.entrypoints.openai.serving_responses import OpenAIServingResponses
 from vllm.entrypoints.tool_server import ToolServer
+from vllm.inputs.data import TokensPrompt as EngineTokensPrompt
 
 
 class MockConversationContext(ConversationContext):
@@ -127,3 +128,63 @@ class TestInitializeToolSessions:
 
         # Verify that init_tool_sessions was called
         assert mock_context.init_tool_sessions_called
+
+
+class TestValidateGeneratorInput:
+    """Test class for _validate_generator_input method"""
+
+    @pytest_asyncio.fixture
+    async def serving_responses_instance(self):
+        """Create a real OpenAIServingResponses instance for testing"""
+        # Create minimal mocks for required dependencies
+        engine_client = MagicMock()
+        engine_client.get_model_config = AsyncMock()
+
+        model_config = MagicMock()
+        model_config.hf_config.model_type = "test"
+        model_config.get_diff_sampling_param.return_value = {}
+
+        models = MagicMock()
+
+        # Create the actual instance
+        instance = OpenAIServingResponses(
+            engine_client=engine_client,
+            model_config=model_config,
+            models=models,
+            request_logger=None,
+            chat_template=None,
+            chat_template_content_format="auto",
+        )
+
+        # Set max_model_len for testing
+        instance.max_model_len = 100
+
+        return instance
+
+    def test_validate_generator_input(self, serving_responses_instance):
+        """Test _validate_generator_input with valid prompt length"""
+        # Create an engine prompt with valid length (less than max_model_len)
+        valid_prompt_token_ids = list(range(5))  # 5 tokens < 100 max_model_len
+        engine_prompt = EngineTokensPrompt(
+            prompt_token_ids=valid_prompt_token_ids)
+
+        # Call the method
+        result = serving_responses_instance._validate_generator_input(
+            engine_prompt)
+
+        # Should return None for valid input
+        assert result is None
+
+        # create an invalid engine prompt
+        invalid_prompt_token_ids = list(
+            range(200))  # 100 tokens >= 100 max_model_len
+        engine_prompt = EngineTokensPrompt(
+            prompt_token_ids=invalid_prompt_token_ids)
+
+        # Call the method
+        result = serving_responses_instance._validate_generator_input(
+            engine_prompt)
+
+        # Should return an ErrorResponse
+        assert result is not None
+        assert isinstance(result, ErrorResponse)