OpenAI chat completion with Json output format

I can't recall how many times I've tried to convince an LLM to return JSON so that I could perform API calls based on natural language inputs from users. Recently, I discovered that this functionality is natively supported by the Semantic Kernel and Microsoft AI Extension Library. It is officially documented by the OpenAI API here. Note that this feature is only available in the latest large language models from GPT-4o/o1 and later. If you are using Azure OpenAI, ensure you have the supported versions when deploying models.

Chat completion

Semantic Kernel supports JSON output formatting in the ResponseFormat property from PromptExecutionSettings, as shown in the code below:

// Configure Azure/OpenAI and semantic kernel first.

var chatCompletionService = kernel.Services.GetRequiredService<IChatCompletionService>();

var history = new ChatHistory();
history.AddSystemMessage("Extract the event information.");
history.AddUserMessage("Alice and Bob are going to a science fair on Friday.");

var jsonSerializerOptions = new JsonSerializerOptions(JsonSerializerOptions.Default)
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    UnmappedMemberHandling = JsonUnmappedMemberHandling.Disallow,
};
var responseFormat = CalendarEvent.JsonResponseSchema(jsonSerializerOptions);

var response = await chatCompletionService.GetChatMessageContentAsync(
    history, 
    new AzureOpenAIPromptExecutionSettings
    {
        ResponseFormat = responseFormat // Json schema
    });
// Json result    
var result = JsonSerializer.Deserialize<CalendarEvent>(response.ToString(), jsonSerializerOptions);

Generate Json schema from types

JSON schema can be automatically generated using Microsoft.Extensions.AI.AIJsonUtilities, which is referenced from Semantic Kernel.

public sealed class CalendarEvent
{
    [Description("Name of the event")]
    public required string Name { get; init; }

    [Description("Day of the event")]
    public required string Day { get; init; }

    [Description("List of participants of the event")]
    public required string[] Participants { get; init; }

    public static ChatResponseFormat JsonResponseSchema(JsonSerializerOptions? jsonSerializerOptions = default)
    {
        var inferenceOptions = new AIJsonSchemaCreateOptions
        {
            IncludeSchemaKeyword = false,
            DisallowAdditionalProperties = true,
        };

        // Json schema from types with descriptions on properties
        var jsonElement = AIJsonUtilities.CreateJsonSchema(
            typeof(CalendarEvent),
            description: "Calendar event result",
            serializerOptions: jsonSerializerOptions,
            inferenceOptions: inferenceOptions);

        var kernelJsonSchema = KernelJsonSchema.Parse(jsonElement.GetRawText());
        var jsonSchemaData = BinaryData.FromObjectAsJson(kernelJsonSchema, jsonSerializerOptions);

        return ChatResponseFormat.CreateJsonSchemaFormat(
            nameof(CalendarEvent).ToLowerInvariant(),
            jsonSchemaData,
            jsonSchemaIsStrict: true);
    }
}

Sample code here

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