[Bugfix] Fix Pydantic union resolution for ResponseFunctionToolCall in Responses API (#26706)

Signed-off-by: Shai Trinczer <strinczer@icloud.com>
Co-authored-by: Chauncey <chaunceyjiang@gmail.com>
Co-authored-by: Ye (Charlotte) Qi <yeq@meta.com>

tests/entrypoints/openai/test_responses_function_call_parsing.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Test function call parsing in ResponsesRequest."""
+
+import json
+
+import pytest
+from openai.types.responses import ResponseFunctionToolCall
+
+from vllm.entrypoints.openai.protocol import ResponsesRequest
+
+
+def test_function_call_dict_converted_to_object():
+    """Test that function_call dictionaries are correctly parsed into
+    ResponseFunctionToolCall objects."""
+    # Create a request with function_call as dict
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            {
+                "type": "function_call",
+                "call_id": "fc_123",
+                "name": "get_weather",
+                "arguments": '{"location": "Boston", "unit": "celsius"}',
+            }
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify the input item is now a ResponseFunctionToolCall object
+    assert len(request.input) == 1
+    assert isinstance(request.input[0], ResponseFunctionToolCall)
+    assert request.input[0].call_id == "fc_123"
+    assert request.input[0].name == "get_weather"
+    assert request.input[0].arguments == '{"location": "Boston", "unit": "celsius"}'
+
+
+def test_direct_function_call_object_preservation():
+    """Test that ResponseFunctionToolCall objects passed directly are preserved."""
+    # Create a request with ResponseFunctionToolCall object
+    function_call = ResponseFunctionToolCall(
+        type="function_call",
+        call_id="fc_456",
+        name="get_stock_price",
+        arguments='{"symbol": "AAPL"}',
+    )
+
+    request_data = {"model": "gpt-oss", "input": [function_call]}
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify the object is preserved
+    assert len(request.input) == 1
+    assert request.input[0] is function_call
+
+
+def test_mixed_input_types_with_function_calls():
+    """Test parsing with mixed input types including function calls."""
+
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            # Valid Message type
+            {
+                "type": "message",
+                "role": "user",
+                "content": [{"type": "input_text", "text": "What's the weather?"}],
+            },
+            # Function call that should be parsed
+            {
+                "type": "function_call",
+                "call_id": "fc_789",
+                "name": "check_weather",
+                "arguments": '{"location": "NYC"}',
+            },
+            # Another function call
+            {
+                "type": "function_call",
+                "call_id": "fc_790",
+                "name": "get_time",
+                "arguments": "{}",
+            },
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify mixed types are handled correctly
+    assert len(request.input) == 3
+    # First item should be validated as Message
+    assert request.input[0]["type"] == "message"
+    # Second item should be parsed to ResponseFunctionToolCall
+    assert isinstance(request.input[1], ResponseFunctionToolCall)
+    assert request.input[1].call_id == "fc_789"
+    assert request.input[1].name == "check_weather"
+    # Third item should also be parsed to ResponseFunctionToolCall
+    assert isinstance(request.input[2], ResponseFunctionToolCall)
+    assert request.input[2].call_id == "fc_790"
+    assert request.input[2].name == "get_time"
+
+
+def test_function_call_with_complex_arguments():
+    """Test parsing function calls with complex nested arguments."""
+    complex_args = {
+        "query": "weather forecast",
+        "filters": {
+            "location": {"city": "San Francisco", "state": "CA"},
+            "timeRange": {"start": "2024-01-01", "end": "2024-01-07"},
+            "metrics": ["temperature", "humidity", "precipitation"],
+        },
+        "options": {"format": "detailed", "includeAlerts": True},
+    }
+
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            {
+                "type": "function_call",
+                "call_id": "fc_complex",
+                "name": "advanced_weather_query",
+                "arguments": json.dumps(complex_args),
+            }
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify complex arguments are preserved correctly
+    assert len(request.input) == 1
+    assert isinstance(request.input[0], ResponseFunctionToolCall)
+    assert request.input[0].call_id == "fc_complex"
+    assert request.input[0].name == "advanced_weather_query"
+
+    # Parse the arguments back to verify they're intact
+    parsed_args = json.loads(request.input[0].arguments)
+    assert parsed_args == complex_args
+
+
+def test_invalid_function_call_fallback():
+    """Test that invalid function call dictionaries fall back gracefully."""
+    # Missing required field 'call_id'
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            {"type": "function_call", "name": "incomplete_function", "arguments": "{}"}
+        ],
+    }
+
+    # This should not raise an error during model creation
+    # The validator should keep the original dict and let Pydantic
+    # handle validation
+    with pytest.raises(ValueError):
+        # Pydantic should raise a validation error for the invalid structure
+        ResponsesRequest(**request_data)
+
+
+def test_string_input_not_affected():
+    """Test that string input is not affected by the validator."""
+    request_data = {"model": "gpt-oss", "input": "This is a simple string input"}
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify string input remains unchanged
+    assert request.input == "This is a simple string input"
+
+
+def test_empty_list_input():
+    """Test that empty list input is handled correctly."""
+    request_data = {"model": "gpt-oss", "input": []}
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify empty list is preserved
+    assert request.input == []
+
+
+def test_function_call_output_not_affected():
+    """Test that FunctionCallOutput is not affected by the function_call parsing."""
+
+    # Test with FunctionCallOutput as dict (should not be parsed)
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            {
+                "type": "function_call_output",
+                "call_id": "fc_output_123",
+                "output": "The weather in Boston is 72°F and sunny.",
+            }
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    # FunctionCallOutput should remain as dict (not converted to an object)
+    assert len(request.input) == 1
+    assert isinstance(request.input[0], dict)
+    assert request.input[0]["type"] == "function_call_output"
+    assert request.input[0]["call_id"] == "fc_output_123"
+    assert request.input[0]["output"] == "The weather in Boston is 72°F and sunny."
+
+
+def test_mixed_function_call_and_output():
+    """Test that function_call is parsed while function_call_output is preserved."""
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            # This should be parsed to ResponseFunctionToolCall
+            {
+                "type": "function_call",
+                "call_id": "fc_call_456",
+                "name": "get_weather",
+                "arguments": '{"location": "NYC"}',
+            },
+            # This should remain as dict
+            {
+                "type": "function_call_output",
+                "call_id": "fc_call_456",
+                "output": "NYC weather is 68°F with light rain",
+            },
+        ],
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    assert len(request.input) == 2
+
+    # First item should be parsed to ResponseFunctionToolCall
+    assert isinstance(request.input[0], ResponseFunctionToolCall)
+    assert request.input[0].call_id == "fc_call_456"
+    assert request.input[0].name == "get_weather"
+
+    # Second item should remain as dict (FunctionCallOutput)
+    assert isinstance(request.input[1], dict)
+    assert request.input[1]["type"] == "function_call_output"
+    assert request.input[1]["call_id"] == "fc_call_456"
+    assert request.input[1]["output"] == "NYC weather is 68°F with light rain"
+
+
+def test_function_call_validation_failure_logs_debug(caplog):
+    """Test that validation failures are logged at debug level."""
+    from unittest.mock import patch
+
+    request_data = {
+        "model": "gpt-oss",
+        "input": [
+            {
+                "type": "function_call",
+                "name": "incomplete_function",
+                "arguments": "{}",  # Missing call_id
+            }
+        ],
+    }
+
+    # Mock the logger to verify debug was called
+    with patch("vllm.entrypoints.openai.protocol.logger") as mock_logger:
+        with pytest.raises(ValueError):
+            ResponsesRequest(**request_data)
+
+        # Verify debug was called with expected message
+        mock_logger.debug.assert_called_once()
+        call_args = mock_logger.debug.call_args[0][0]
+        assert "Failed to parse function_call" in call_args
+
+
+def test_validator_handles_iterator_input():
+    """Test that validator can handle ValidatorIterator input (Pydantic internal)."""
+
+    # This test simulates when Pydantic passes a ValidatorIterator instead of a list
+    # This happened with complex nested structures containing reasoning + function_call
+
+    # Create test data that would normally be a list
+    test_input_items = [
+        {
+            "type": "message",
+            "role": "user",
+            "content": [{"type": "input_text", "text": "Test"}],
+        },
+        {
+            "type": "reasoning",
+            "id": "rs_1",
+            "summary": [{"type": "summary_text", "text": "Test reasoning"}],
+            "content": [{"type": "reasoning_text", "text": "Test content"}],
+        },
+        {
+            "type": "function_call",
+            "call_id": "call_1",
+            "name": "test_function",
+            "arguments": '{"test": "value"}',
+            "id": "fc_1",
+        },
+    ]
+
+    # Mock data where input is an iterator (simulates Pydantic ValidatorIterator)
+    mock_data = {
+        "model": "test-model",
+        "input": iter(test_input_items),  # Iterator instead of list
+    }
+
+    # This should NOT raise an error with the fixed validator
+    try:
+        request = ResponsesRequest(**mock_data)
+
+        # Verify the validator processed the data correctly
+        assert len(request.input) == 3
+
+        # Verify function_call was converted to ResponseFunctionToolCall object
+        function_call_item = None
+        for item in request.input:
+            if isinstance(item, ResponseFunctionToolCall):
+                function_call_item = item
+                break
+
+        assert function_call_item is not None
+        assert function_call_item.call_id == "call_1"
+        assert function_call_item.name == "test_function"
+
+    except Exception as e:
+        pytest.fail(f"Validator should handle iterator input, but failed with: {e}")
+
+
+def test_validator_handles_empty_iterator():
+    """Test validator handles empty iterator gracefully."""
+    mock_data = {
+        "model": "test-model",
+        "input": iter([]),  # Empty iterator
+    }
+
+    request = ResponsesRequest(**mock_data)
+    assert request.input == []

vllm/entrypoints/openai/protocol.py

@@ -69,6 +69,7 @@ from pydantic import (
     ConfigDict,
     Field,
     TypeAdapter,
+    ValidationError,
     ValidationInfo,
     field_serializer,
     field_validator,
@@ -478,6 +479,48 @@ class ResponsesRequest(OpenAIBaseModel):
                 )
         return data
 
+    @model_validator(mode="before")
+    def function_call_parsing(cls, data):
+        """Parse function_call dictionaries into ResponseFunctionToolCall objects.
+        This ensures Pydantic can properly resolve union types in the input field.
+        Function calls provided as dicts are converted to ResponseFunctionToolCall
+        objects before validation, while invalid structures are left for Pydantic
+        to reject with appropriate error messages.
+        """
+
+        input_data = data.get("input")
+
+        # Early return for None, strings, or bytes
+        # (strings are iterable but shouldn't be processed)
+        if input_data is None or isinstance(input_data, (str, bytes)):
+            return data
+
+        # Convert iterators (like ValidatorIterator) to list
+        if not isinstance(input_data, list):
+            try:
+                input_data = list(input_data)
+            except TypeError:
+                # Not iterable, leave as-is for Pydantic to handle
+                return data
+
+        processed_input = []
+        for item in input_data:
+            if isinstance(item, dict) and item.get("type") == "function_call":
+                try:
+                    processed_input.append(ResponseFunctionToolCall(**item))
+                except ValidationError:
+                    # Let Pydantic handle validation for malformed function calls
+                    logger.debug(
+                        "Failed to parse function_call to ResponseFunctionToolCall, "
+                        "leaving for Pydantic validation"
+                    )
+                    processed_input.append(item)
+            else:
+                processed_input.append(item)
+
+        data["input"] = processed_input
+        return data
+
 
 class ChatCompletionRequest(OpenAIBaseModel):
     # Ordered by official OpenAI API documentation