Why does AzureOpenAIClient.CompleteChatAsync hang in .NET when sending image data?

Below is an explanation of the most likely causes and how you might resolve the issue.

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Background

Azure’s GPT‑4o (multimodal model) expects a very specific payload format for image input. In the Python example, the user message is constructed as a list containing both a text component (e.g. "What?") and a structured image component (with type "image_url"). That lets the service know that an image is being passed along with text.

In your .NET code you are merely concatenating the base64 string into a text message. The service may not be interpreting that as a valid image input, so it ends up waiting indefinitely for the correct structure or timing out on the backend.

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Likely Causes

1. Message Structure

   • Python client: The message for the user is a list that includes an explicit image component.
   • .NET SDK: The code uses a simple string (with the base64 data appended) inside a UserChatMessage. This is not the structured (multipart) message that GPT‑4o expects.

2. API Version Differences

   • The Python client explicitly uses the preview API version ("2024-05-01-preview"). In contrast, the .NET client might be defaulting to another API version or not exposing an option for GPT‑4o’s multimodal API features.

3. SDK Support Status

   • The Azure.OpenAI SDK for .NET may not yet support the multimodal input format or might require additional configuration to work with image inputs.

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Recommended Troubleshooting Steps

• Check API Version and SDK Update
  Confirm you are (if possible) using the exact same API version as the Python client. Look up in Microsoft’s documentation or SDK release notes if there’s a way to set the API version (for example via client options).

• Payload Structure Alignment
  Examine if there’s a way in the .NET SDK to send multipart messages (i.e. separate text and image parts) rather than a concatenated text string. If the SDK is not yet updated to support a multi-part structure for GPT‑4o, then you may need to either:

  • Use raw HTTP calls where you can properly structure the payload as follows:

    • A system message (instruction).
    • A user message whose content is an array of structured objects (one for a text part, one for the image part).

• Use an Alternative Approach
  If the .NET SDK does not support the required format, consider either:

  • Using a custom HTTP request to the Azure endpoint where you are able to mimic the Python client’s request payload exactly.
  • Or use the Python client (via an interop or microservice call) until the .NET SDK catches up with the newest multimodal features.

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Example – Custom HTTP Call (Conceptual)

Below is a conceptual example of how you might structure a raw HTTP request in .NET:

CSHARPCopied
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using Newtonsoft.Json; // For JSON serialization

public async Task<string> SendMultimodalChatRequestAsync(string base64Image)
{
    string endpoint = "https://xyz.openai.azure.com/openai/deployments/gpt-4o/chat/completions?api-version=2024-05-01-preview";
    using var httpClient = new HttpClient();
    httpClient.DefaultRequestHeaders.Add("api-key", "LFK");

    var payload = new {
        messages = new object[] {
            new { role = "system", content = "Analyze the uploaded image and return a single-word description of the main subject. The response should be only one word, representing the most general yet accurate category." },
            new {
                role = "user",
                content = new object[] {
                    new { type = "text", text = "What?" },
                    new { type = "image_url", image_url = new { url = $"data:image/png;base64,{base64Image}" } }
                }
            }
        }
    };

    var jsonContent = new StringContent(JsonConvert.SerializeObject(payload), Encoding.UTF8, "application/json");
    var response = await httpClient.PostAsync(endpoint, jsonContent);
    response.EnsureSuccessStatusCode();
    var responseBody = await response.Content.ReadAsStringAsync();
    return responseBody;
}

This example shows how to explicitly build the payload to match the Python request’s structure. You would then call EncodeImage(...) to create the base64 string and use this function to send the request.

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Conclusion

The .NET code isn’t responding because it is not sending a properly structured message for multimodal input. The model expects a separate image component alongside the text and the SDK (or API version in use) must support that. To resolve the issue, check for an updated SDK or use a custom HTTP request that replicates the Python payload format exactly.

Let me know if you have any further questions!