fix: correct CompletionRequest messages type to match OpenAI API spec (#12980)

* fix: correct CompletionRequest messages type to match OpenAI API spec

- Changed messages field type from List[str] to List[ChatCompletionMessageParam]
- This ensures proper OpenAI API compatibility where messages should be objects with role and content fields
- Fixes type inconsistency in completion request handling

* feat(tests): Add comprehensive tests for CompletionRequest model

- Add test_completion.py for litellm.types.completion module
- Test ChatCompletionMessageParam type validation
- Test tool message format compatibility
- Test function message format (deprecated)
- Test multimodal content (text + image)
- Test default empty messages list
- Test all optional parameters
- Validate OpenAI ChatCompletion API message format compatibility

litellm/types/completion.py

@@ -164,7 +164,7 @@ ChatCompletionMessageParam = Union[
 
 class CompletionRequest(BaseModel):
     model: str
-    messages: List[str] = []
+    messages: List[ChatCompletionMessageParam] = []
     timeout: Optional[Union[float, int]] = None
     temperature: Optional[float] = None
     top_p: Optional[float] = None

tests/test_litellm/types/test_completion.py

@@ -0,0 +1,175 @@
+"""
+Tests for litellm.types.completion module
+
+This test suite validates the CompletionRequest model and its compatibility with 
+OpenAI ChatCompletion API message formats.
+
+Usage:
+    pytest tests/test_litellm/types/test_completion.py -v
+"""
+from typing import List
+
+from litellm.types.completion import (
+    CompletionRequest, 
+    ChatCompletionMessageParam
+)
+
+
+def test_completion_request_messages_type_validation():
+    """
+    Test that CompletionRequest.messages field accepts proper ChatCompletionMessageParam types.
+    """
+    # Valid message formats according to OpenAI API
+    valid_messages: List[ChatCompletionMessageParam] = [
+        {"role": "system", "content": "You are a helpful assistant"},
+        {"role": "user", "content": "Hello, how are you?"},
+        {"role": "assistant", "content": "I'm doing well, thank you!"},
+    ]
+    
+    request = CompletionRequest(
+        model="gpt-3.5-turbo",
+        messages=valid_messages
+    )
+    
+    assert request.model == "gpt-3.5-turbo"
+    assert len(request.messages) == 3
+
+
+def test_completion_request_tool_message():
+    """
+    Test CompletionRequest with tool message format.
+    """
+    messages: List[ChatCompletionMessageParam] = [
+        {"role": "user", "content": "Calculate 2+2"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_123",
+                    "type": "function", 
+                    "function": {
+                        "name": "calculate",
+                        "arguments": '{"expression": "2+2"}'
+                    }
+                }
+            ]
+        },
+        {
+            "role": "tool",
+            "content": "4",
+            "tool_call_id": "call_123"
+        }
+    ]
+    
+    request = CompletionRequest(
+        model="gpt-3.5-turbo",
+        messages=messages
+    )
+    
+    assert len(request.messages) == 3
+    assert request.messages[1]["role"] == "assistant"
+    assert request.messages[2]["role"] == "tool"
+
+
+def test_completion_request_function_message():
+    """
+    Test CompletionRequest with deprecated function message format.
+    """
+    messages: List[ChatCompletionMessageParam] = [
+        {"role": "user", "content": "What's the weather?"},
+        {
+            "role": "assistant",
+            "content": None,
+            "function_call": {
+                "name": "get_weather",
+                "arguments": '{"location": "NYC"}'
+            }
+        },
+        {
+            "role": "function",
+            "name": "get_weather",
+            "content": "Sunny, 75°F"
+        }
+    ]
+    
+    request = CompletionRequest(
+        model="gpt-3.5-turbo", 
+        messages=messages
+    )
+    
+    assert len(request.messages) == 3
+    assert request.messages[2]["role"] == "function"
+    assert request.messages[2]["name"] == "get_weather"
+
+
+def test_completion_request_multimodal_content():
+    """
+    Test CompletionRequest with multimodal content (text + image).
+    """
+    messages: List[ChatCompletionMessageParam] = [
+        {
+            "role": "user",
+            "content": [
+                {
+                    "type": "text",
+                    "text": "What's in this image?"
+                },
+                {
+                    "type": "image_url",
+                    "image_url": {
+                        "url": "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD..."
+                    }
+                }
+            ]
+        }
+    ]
+    
+    request = CompletionRequest(
+        model="gpt-4-vision-preview",
+        messages=messages
+    )
+    
+    assert len(request.messages) == 1
+    assert request.messages[0]["role"] == "user"
+
+
+def test_completion_request_empty_messages_default():
+    """
+    Test that CompletionRequest defaults to empty messages list.
+    """
+    request = CompletionRequest(model="gpt-3.5-turbo")
+    
+    assert request.messages == []
+    assert isinstance(request.messages, list)
+
+
+def test_completion_request_with_all_params():
+    """
+    Test CompletionRequest with various optional parameters.
+    """
+    messages: List[ChatCompletionMessageParam] = [
+        {"role": "user", "content": "Hello"}
+    ]
+    
+    request = CompletionRequest(
+        model="gpt-3.5-turbo",
+        messages=messages,
+        temperature=0.7,
+        max_tokens=100,
+        top_p=0.9,
+        frequency_penalty=0.0,
+        presence_penalty=0.0,
+        stop={"sequences": ["END"]},
+        stream=False,
+        n=1
+    )
+    
+    assert request.model == "gpt-3.5-turbo"
+    assert request.temperature == 0.7
+    assert request.max_tokens == 100
+    assert request.top_p == 0.9
+    assert request.frequency_penalty == 0.0
+    assert request.presence_penalty == 0.0
+    assert request.stream is False
+    assert request.n == 1