It seems that the issue might not be directly related to your code, as you've verified that everything is in line with the Azure.AI.OpenAI 2.1.0 documentation. Since you've already tried regenerating API keys, recreating the Azure Search components, and using Fiddler/Wireshark to inspect the traffic, let's consider the following specific possibilities for the 400 Bad Request error:

MaxOutputTokenCount: While you’ve set it to 4096, it's possible that this is too high for the current payload, especially if you're dealing with large requests. Lowering this number to something smaller (e.g., 1024 or 2048) might resolve the issue.

Endpoint Configuration: Double-check the endpoints for both OpenAI and Azure Search. Even though you've confirmed the correct values, sometimes there can be issues with region-specific endpoints or certain configuration settings that can cause the request to be malformed.

DataSource Authentication: Ensure that the API key used for the Azure Search service is correct, and verify that the Authentication method is properly handling it. This part sometimes causes issues if the key doesn't have the right permissions.

Payload Format: There might be an issue with how the payload is structured when you're sending the request. Ensure that the ChatCompletion object and the messages being sent are formatted correctly. It might be helpful to add some logging before the request is sent to verify the message structure.

Review Server Logs: The 500 Internal Server Error might also provide more context in the server logs. Since you’ve included a try-catch block, you can log the exception details more thoroughly to get a better idea of what went wrong.

If none of these suggestions solve the problem, it may be worth revisiting the API version you are using and seeing if there's a newer release or patch for Azure.AI.OpenAI that addresses this issue. Additionally, checking the Azure portal for any service disruptions or issues with the OpenAI integration could provide more insights.

Let me know if you'd like further assistance!

This response focuses more on providing specific diagnostic steps based on your provided context. Let me know if this works!