Force Google Gemini to return JSON using javascript

An example using node.JS and @google-cloud/vertexai — Since Google’s documentation only shows Python examples.

If you’re developing an enterprise application, you want to be using Gemini through the @google-cloud/vertexai npm package

Import

const {
  FunctionDeclarationSchemaType,
  VertexAI,
} = require("@google-cloud/vertexai");

Instantiate VertexAI

  const vertexAI = new VertexAI({
    project: "PROJECT_ID",
    location: "us-central1", // whichever region you are using
  });

Prompt and JSON Schema — this is where the magic happens.

  let responseSchema = {
    type: FunctionDeclarationSchemaType.OBJECT,
    properties: {
      summary: {
        type: FunctionDeclarationSchemaType.STRING,
        description:
          "Body of the summary, do not include emojis or expressions like OMG, LOL, etc.",
        nullable: false,
      },
      title: {
        type: FunctionDeclarationSchemaType.STRING,
        description: "short title for the summary",
        nullable: false,
      },
      summarizationError: {
        type: FunctionDeclarationSchemaType.BOOLEAN,
        nullable: false,
        description:
          "whether the summary was generated successfully, true when the article only focuses on information about a captcha challenge",
      },
      category: {
        type: FunctionDeclarationSchemaType.STRING,
        enum: [
          "politics",
          "sports",
          "entertainment",
          "technology",
          "economy",
          "health",
          "other",
        ],
        description: "topic that best represents the article",
        nullable: false,
      },
      language: {
        type: FunctionDeclarationSchemaType.STRING,
        enum: ["en", "es", "fr", "it", "de"],
        description: "",
        nullable: false,
      },
    },
  };

Here’s what you can specify about each JSON key

export interface Schema {
    /**
     * Optional. The type of the property. {@link
     * SchemaType}.
     */
    type?: SchemaType;
    /** Optional. The format of the property. */
    format?: string;
    /** Optional. The description of the property. */
    description?: string;
    /** Optional. Whether the property is nullable. */
    nullable?: boolean;
    /** Optional. The items of the property. {@link Schema} */
    items?: Schema;
    /** Optional. The enum of the property. */
    enum?: string[];
    /** Optional. Map of {@link Schema}. */
    properties?: {
        [k: string]: Schema;
    };
    /** Optional. Array of required property. */
    required?: string[];
    /** Optional. The example of the property. */
    example?: unknown;
}

As you can tell from my example above, I only used type , description nullable enum (where applicable).

Sending the request to Gemini on VertexAI. Note the responseMimeType key under generationConfig

  const generativeModel = vertexAI.getGenerativeModel({
      model: "gemini-1.5-flash-002", // Fresh out of the oven 🍞😋
      generationConfig: {
        maxOutputTokens: 2048,
        responseSchema,
        responseMimeType: "application/json", // this line forces JSON response
      },
      systemInstruction: {
        role: "system",
        parts: [{ text: prompt }],
      },
    });

    const request = {
      contents: [{ role: "user", parts: [{ text: "veeeery long text to summarize"}] }],
    };
    const result = await generativeModel.generateContent(request);

print out the response

    console.log(JSON.stringify(result, null, 2));

gives me

{
  category: 'technology',
  language: 'en',
  summarizationError: false,
  summary: "🚨Home Assistant, the open-source smart home platform, is going mainstream!  Paulus Schoutsen, the founder, created the Open Home Foundation, a non-profit to ensure privacy and local control. They're launching new products like a Home Assistant hub sold on Amazon and a voice-control device to rival Google and Amazon.  Plus, they're working with Nvidia on local AI for a super-smart home experience!  This isn't just about convenience; it's about taking back control of your data and creating a sustainable smart home future. This move will shake up the smart home industry.",
  title: 'Home Assistant Goes Mainstream'
}