Controlled generation JSON output with predefined schema

This code sample demonstrates how to use the `response_mime_type` and `response_schema` parameters to get a response that follows the JSON format and schema that you've defined.

Code sample

C#

Before trying this sample, follow the C# setup instructions in the Vertex AI quickstart using client libraries. For more information, see the Vertex AI C# API reference documentation.

To authenticate to Vertex AI, set up Application Default Credentials. For more information, see Set up authentication for a local development environment. 

public async Task<string> GenerateContentWithResponseSchema2(
    string projectId = "your-project-id",
    string location = "us-central1",
    string publisher = "google",
    string model = "gemini-2.0-flash-001")
{

    var predictionServiceClient = new PredictionServiceClientBuilder
    {
        Endpoint = $"{location}-aiplatform.googleapis.com"
    }.Build();

    var responseSchema = new OpenApiSchema
    {
        Type = Type.Array,
        Items = new()
        {
            Type = Type.Object,
            Properties =
            {
                ["rating"] = new() { Type = Type.Integer },
                ["flavor"] = new() { Type = Type.String }
            },
            Required = { "rating", "flavor" }
        }
    };

    string prompt = @"
        Reviews from our social media:

        - ""Absolutely loved it! Best ice cream I've ever had."" Rating: 4, Flavor: Strawberry Cheesecake
        - ""Quite good, but a bit too sweet for my taste."" Rating: 1, Flavor: Mango Tango";

    var generateContentRequest = new GenerateContentRequest
    {
        Model = $"projects/{projectId}/locations/{location}/publishers/{publisher}/models/{model}",
        Contents =
        {
            new Content
            {
                Role = "USER",
                Parts =
                {
                    new Part { Text = prompt }
                }
            }
        },
        GenerationConfig = new GenerationConfig
        {
            ResponseMimeType = "application/json",
            ResponseSchema = responseSchema
        },
    };

    GenerateContentResponse response = await predictionServiceClient.GenerateContentAsync(generateContentRequest);

    string responseText = response.Candidates[0].Content.Parts[0].Text;
    Console.WriteLine(responseText);

    return responseText;
}

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