Mocking OpenAI Chat Client

Question

How can the OpenAIClient be mocked using MOQ to return a Completions response? I have a json response and would like to deserialize that to Response<Completions>.

      private OpenAIClient client = // initialize;
    
      private async Task<Response<Completions>> Complete(string model, string prompt) =>
          await client.GetCompletionsAsync(model, prompt);

The below attempt to return a response failed as the Completions class does not contain a constructor that can be used to deserialize the json response -

private readonly Mock<OpenAIClient> client= new();
          client
              .Setup(x => x.GetCompletionsAsync(It.IsAny<string>(), It.IsAny<string>(), It.IsAny<CancellationToken>()))
              .ReturnsAsync(Response.FromValue(JsonConvert.DeserializeObject<Completions>(completionsJson), Mock.Of<Response>()));

The Completions class constructor is marked as internal and so are all the objects within the Completions class.

Answer 1

I know this is an old question, but I had the same issue and solved it by creating a Frankenstein object composed of a mix of Substitutes/Mocks (I am using NSubstitute) alongside real objects.

const string userPrompt = "test";
const string responsePrompt = "response";

// real completion
var completion = OpenAIChatModelFactory.ChatCompletion(
    role: ChatMessageRole.User,
    content: new ChatMessageContent(ChatMessageContentPart.CreateTextPart(responsePrompt)));

// mocked PipelineResponse
var pipelineResponse = Substitute.For<PipelineResponse>();
// real ClientResult
var completionResponse = ClientResult<ChatCompletion>.FromValue(completion, pipelineResponse);

// mocking my final result
_chatClientMock.CompleteChatAsync(Arg.Any<List<ChatMessage>>(), Arg.Any<ChatCompletionOptions>(), Arg.Any<CancellationToken>())
    .Returns(completionResponse);
// call your test service here that invokes _chatClientMock.CompleteChatAsync 
// assertions...