diff --git a/tests/entrypoints/openai/test_responses_function_call_parsing.py b/tests/entrypoints/openai/test_responses_function_call_parsing.py
new file mode 100644
index 000000000000..a0039462a0d7
--- /dev/null
+++ b/tests/entrypoints/openai/test_responses_function_call_parsing.py
@@ -0,0 +1,135 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Unit tests for ResponsesRequest function call parsing.
+
+This test module ensures that function call dictionaries are properly parsed
+as ResponseFunctionToolCall objects when creating ResponsesRequest instances,
+preventing regression of the function call parsing issue.
+"""
+
+import pytest
+from openai.types.responses.response_function_tool_call import (
+    ResponseFunctionToolCall)
+
+from vllm.entrypoints.openai.protocol import ResponsesRequest
+
+
+def test_function_call_dict_parsing():
+    """Test that function call dictionaries are parsed as 
+    ResponseFunctionToolCall objects."""
+    # Function call data as it would come from JSON/API
+    function_call_dict = {
+        'arguments': '{"location": "Tokyo"}',
+        'call_id': 'call_a90e65e81cd84553855dde1733e77749',
+        'name': 'get_weather',
+        'type': 'function_call',
+        'id': 'ft_a90e65e81cd84553855dde1733e77749',
+        'status': 'completed'
+    }
+
+    # Create ResponsesRequest with function call dictionary
+    request_data = {'input': [function_call_dict], 'model': 'test-model'}
+
+    request = ResponsesRequest(**request_data)
+    parsed_function_call = request.input[0]
+
+    # Verify that the dictionary was parsed as ResponseFunctionToolCall
+    assert isinstance(parsed_function_call, ResponseFunctionToolCall), (
+        f"Expected ResponseFunctionToolCall, got {type(parsed_function_call)}")
+
+    # Verify all attributes are preserved
+    assert parsed_function_call.arguments == '{"location": "Tokyo"}'
+    assert (parsed_function_call.call_id ==
+            'call_a90e65e81cd84553855dde1733e77749')
+    assert parsed_function_call.name == 'get_weather'
+    assert parsed_function_call.type == 'function_call'
+    assert parsed_function_call.id == 'ft_a90e65e81cd84553855dde1733e77749'
+    assert parsed_function_call.status == 'completed'
+
+
+def test_direct_function_call_object_preservation():
+    """Test that direct ResponseFunctionToolCall objects are preserved."""
+    # Create ResponseFunctionToolCall object directly
+    function_call_obj = ResponseFunctionToolCall(
+        arguments='{"city": "New York"}',
+        call_id='call_12345',
+        name='get_temperature',
+        type='function_call',
+        id='ft_12345',
+        status='completed')
+
+    # Create ResponsesRequest with the object
+    request_data = {'input': [function_call_obj], 'model': 'test-model'}
+
+    request = ResponsesRequest(**request_data)
+    parsed_function_call = request.input[0]
+
+    # Verify that the object is preserved
+    assert isinstance(parsed_function_call, ResponseFunctionToolCall)
+    # Should be the same object
+    assert parsed_function_call is function_call_obj
+
+
+def test_mixed_input_types_with_function_calls():
+    """Test ResponsesRequest with mixed input types including function calls."""
+    # Mix of string input and function call
+    function_call_dict = {
+        'arguments': '{"query": "weather"}',
+        'call_id': 'call_mixed_test',
+        'name': 'search',
+        'type': 'function_call',
+        'id': 'ft_mixed_test',
+        'status': 'completed'
+    }
+
+    request_data = {
+        'input': [
+            {
+                'role': 'user',
+                'content': 'Hello, how can I help?'
+            },  # Regular dictionary input
+            function_call_dict  # Function call dict
+        ],
+        'model':
+        'test-model'
+    }
+
+    request = ResponsesRequest(**request_data)
+
+    # Verify mixed types are handled correctly
+    assert len(request.input) == 2
+    assert isinstance(request.input[0], dict)
+    assert request.input[0]['role'] == 'user'
+    assert request.input[0]['content'] == 'Hello, how can I help?'
+
+    assert isinstance(request.input[1], ResponseFunctionToolCall)
+    assert request.input[1].name == 'search'
+    assert request.input[1].arguments == '{"query": "weather"}'
+
+
+@pytest.mark.parametrize("arguments", [
+    '{"location": "Tokyo"}',
+    '{"location": {"city": "Tokyo", "country": "Japan"}, "units": "celsius"}',
+    '{"query": "weather", "filters": ["temperature", "humidity"], "count": 5}',
+    '{"complex": {"nested": {"data": true}}, "array": [1, 2, 3]}'
+])
+def test_function_call_with_complex_arguments(arguments):
+    """Test function call parsing with various argument complexities."""
+    complex_function_call = {
+        'arguments': arguments,
+        'call_id': 'call_complex',
+        'name': 'get_detailed_weather',
+        'type': 'function_call',
+        'id': 'ft_complex',
+        'status': 'completed'
+    }
+
+    request_data = {'input': [complex_function_call], 'model': 'test-model'}
+
+    request = ResponsesRequest(**request_data)
+    parsed_function_call = request.input[0]
+
+    assert isinstance(parsed_function_call, ResponseFunctionToolCall)
+    assert parsed_function_call.name == 'get_detailed_weather'
+    assert parsed_function_call.arguments == arguments
diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index c56c68cf7644..37203e84e86f 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -960,6 +960,30 @@ def check_cache_salt_support(cls, data):
                                  "non-empty string if provided.")
         return data
 
+    @model_validator(mode="before")
+    def function_call_parsing(cls, data):
+        """Function call parsing to ensure ResponseFunctionToolCall objects 
+        are created."""
+        input_data = data.get("input")
+        if isinstance(input_data, list):
+            fixed_input = []
+            for item in input_data:
+                if isinstance(item,
+                              dict) and item.get('type') == 'function_call':
+                    # Convert dict to ResponseFunctionToolCall object
+                    try:
+                        from openai.types.responses.response_function_tool_call import (  # noqa: E501
+                            ResponseFunctionToolCall)
+                        function_call_obj = ResponseFunctionToolCall(**item)
+                        fixed_input.append(function_call_obj)
+                    except Exception:
+                        # If conversion fails, keep the original dict
+                        fixed_input.append(item)
+                else:
+                    fixed_input.append(item)
+            data["input"] = fixed_input
+        return data
+
 
 class CompletionRequest(OpenAIBaseModel):
     # Ordered by official OpenAI API documentation