Build multi-turn conversations (chat) with the Gemini API

Using the Gemini API, you can build freeform conversations across multiple turns. The Vertex AI in Firebase SDK simplifies the process by managing the state of the conversation, so unlike with generateContent() (or generateContentStream()), you don't have to store the conversation history yourself.

Before you begin

If you haven't already, complete the getting started guide, which describes how to set up your Firebase project, connect your app to Firebase, add the SDK, initialize the Vertex AI service, and create a GenerativeModel instance.

Send a chat prompt request

To build a multi-turn conversation (like chat), start off by initializing the chat by calling startChat(). Then use sendMessage() to send a new user message, which will also append the message and the response to the chat history.

There are two possible options for role associated with the content in a conversation:

  • user: the role which provides the prompts. This value is the default for calls to sendMessage(), and the function throws an exception if a different role is passed.

  • model: the role which provides the responses. This role can be used when calling startChat() with existing history.

Swift KotlinJavaWebDart

You can call startChat() and sendMessage() to send a new user message:

import FirebaseVertexAI

// Initialize the Vertex AI service
let vertex = VertexAI.vertexAI()

// Create a `GenerativeModel` instance with a model that supports your use case
let model = vertex.generativeModel(modelName: "gemini-2.0-flash")

// Optionally specify existing chat history
let history = [
  ModelContent(role: "user", parts: "Hello, I have 2 dogs in my house."),
  ModelContent(role: "model", parts: "Great to meet you. What would you like to know?"),
]

// Initialize the chat with optional chat history
let chat = model.startChat(history: history)

// To generate text output, call sendMessage and pass in the message
let response = try await chat.sendMessage("How many paws are in my house?")
print(response.text ?? "No text in response.")

You can call startChat() and sendMessage() to send a new user message:

For Kotlin, the methods in this SDK are suspend functions and need to be called from a Coroutine scope. 
// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
val generativeModel = Firebase.vertexAI.generativeModel("gemini-2.0-flash")

// Initialize the chat
val chat = generativeModel.startChat(
  history = listOf(
    content(role = "user") { text("Hello, I have 2 dogs in my house.") },
    content(role = "model") { text("Great to meet you. What would you like to know?") }
  )
)

val response = chat.sendMessage("How many paws are in my house?")
print(response.text)

You can call startChat() and sendMessage() to send a new user message:

For Java, the methods in this SDK return a ListenableFuture. 
// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
GenerativeModel gm = FirebaseVertexAI.getInstance()
        .generativeModel("gemini-2.0-flash");
GenerativeModelFutures model = GenerativeModelFutures.from(gm);

// (optional) Create previous chat history for context
Content.Builder userContentBuilder = new Content.Builder();
userContentBuilder.setRole("user");
userContentBuilder.addText("Hello, I have 2 dogs in my house.");
Content userContent = userContentBuilder.build();

Content.Builder modelContentBuilder = new Content.Builder();
modelContentBuilder.setRole("model");
modelContentBuilder.addText("Great to meet you. What would you like to know?");
Content modelContent = userContentBuilder.build();

List<Content> history = Arrays.asList(userContent, modelContent);

// Initialize the chat
ChatFutures chat = model.startChat(history);

// Create a new user message
Content.Builder messageBuilder = new Content.Builder();
messageBuilder.setRole("user");
messageBuilder.addText("How many paws are in my house?");

Content message = messageBuilder.build();

// Send the message
ListenableFuture<GenerateContentResponse> response = chat.sendMessage(message);
Futures.addCallback(response, new FutureCallback<GenerateContentResponse>() {
    @Override
    public void onSuccess(GenerateContentResponse result) {
        String resultText = result.getText();
        System.out.println(resultText);
    }

    @Override
    public void onFailure(Throwable t) {
        t.printStackTrace();
    }
}, executor);

You can call startChat() and sendMessage() to send a new user message:

import { initializeApp } from "firebase/app";
import { getVertexAI, getGenerativeModel } from "firebase/vertexai";

// TODO(developer) Replace the following with your app's Firebase configuration
// See: https://firebase.google.com/docs/web/learn-more#config-object
const firebaseConfig = {
  // ...
};

// Initialize FirebaseApp
const firebaseApp = initializeApp(firebaseConfig);

// Initialize the Vertex AI service
const vertexAI = getVertexAI(firebaseApp);

// Create a `GenerativeModel` instance with a model that supports your use case
const model = getGenerativeModel(vertexAI, { model: "gemini-2.0-flash" });

async function run() {
  const chat = model.startChat({
    history: [
      {
        role: "user",
        parts: [{ text: "Hello, I have 2 dogs in my house." }],
      },
      {
        role: "model",
        parts: [{ text: "Great to meet you. What would you like to know?" }],
      },
    ],
    generationConfig: {
      maxOutputTokens: 100,
    },
  });

  const msg = "How many paws are in my house?";

  const result = await chat.sendMessage(msg);

  const response = await result.response;
  const text = response.text();
  console.log(text);
}

run();

You can call startChat() and sendMessage() to send a new user message:

import 'package:firebase_vertexai/firebase_vertexai.dart';
import 'package:firebase_core/firebase_core.dart';
import 'firebase_options.dart';

await Firebase.initializeApp(
  options: DefaultFirebaseOptions.currentPlatform,
);

// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
final model =
      FirebaseVertexAI.instance.generativeModel(model: 'gemini-2.0-flash');

final chat = model.startChat();
// Provide a prompt that contains text
final prompt = [Content.text('Write a story about a magic backpack.')];

final response = await chat.sendMessage(prompt);
print(response.text);

Learn how to choose a model and optionally a location appropriate for your use case and app.

Stream the response

Make sure that you've completed the Before you begin section of this guide before trying this sample.

You can achieve faster interactions by not waiting for the entire result from the model generation, and instead use streaming to handle partial results. To stream the response, call sendMessageStream().

Swift KotlinJavaWebDart

You can call startChat() and sendMessageStream() to stream responses from the model:

import FirebaseVertexAI

// Initialize the Vertex AI service
let vertex = VertexAI.vertexAI()

// Create a `GenerativeModel` instance with a model that supports your use case
let model = vertex.generativeModel(modelName: "gemini-2.0-flash")

// Optionally specify existing chat history
let history = [
  ModelContent(role: "user", parts: "Hello, I have 2 dogs in my house."),
  ModelContent(role: "model", parts: "Great to meet you. What would you like to know?"),
]

// Initialize the chat with optional chat history
let chat = model.startChat(history: history)

// To stream generated text output, call sendMessageStream and pass in the message
let contentStream = try chat.sendMessageStream("How many paws are in my house?")
for try await chunk in contentStream {
  if let text = chunk.text {
    print(text)
  }
}

You can call startChat() and sendMessageStream() to stream responses from the model:

For Kotlin, the methods in this SDK are suspend functions and need to be called from a Coroutine scope. 
// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
val generativeModel = Firebase.vertexAI.generativeModel("gemini-2.0-flash")

// Initialize the chat
val chat = generativeModel.startChat(
  history = listOf(
    content(role = "user") { text("Hello, I have 2 dogs in my house.") },
    content(role = "model") { text("Great to meet you. What would you like to know?") }
  )
)

chat.sendMessageStream("How many paws are in my house?").collect { chunk ->
  print(chunk.text)
}

You can call startChat() and sendMessageStream() to stream responses from the model:

For Java, the streaming methods in this SDK return a Publisher type from the Reactive Streams library. 
// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
GenerativeModel gm = FirebaseVertexAI.getInstance()
        .generativeModel("gemini-2.0-flash");
GenerativeModelFutures model = GenerativeModelFutures.from(gm);

// (optional) Create previous chat history for context
Content.Builder userContentBuilder = new Content.Builder();
userContentBuilder.setRole("user");
userContentBuilder.addText("Hello, I have 2 dogs in my house.");
Content userContent = userContentBuilder.build();

Content.Builder modelContentBuilder = new Content.Builder();
modelContentBuilder.setRole("model");
modelContentBuilder.addText("Great to meet you. What would you like to know?");
Content modelContent = userContentBuilder.build();

List<Content> history = Arrays.asList(userContent, modelContent);

// Initialize the chat
ChatFutures chat = model.startChat(history);

// Create a new user message
Content.Builder messageBuilder = new Content.Builder();
messageBuilder.setRole("user");
messageBuilder.addText("How many paws are in my house?");

Content message = messageBuilder.build();

// Send the message
Publisher<GenerateContentResponse> streamingResponse =
        chat.sendMessageStream(message);

final String[] fullResponse = {""};

streamingResponse.subscribe(new Subscriber<GenerateContentResponse>() {
    @Override
    public void onNext(GenerateContentResponse generateContentResponse) {
        String chunk = generateContentResponse.getText();
            fullResponse[0] += chunk;
    }

    @Override
    public void onComplete() {
        System.out.println(fullResponse[0]);
    }

    // ... other methods omitted for brevity
});

You can call startChat() and sendMessageStream() to stream responses from the model:

import { initializeApp } from "firebase/app";
import { getVertexAI, getGenerativeModel } from "firebase/vertexai";

// TODO(developer) Replace the following with your app's Firebase configuration
// See: https://firebase.google.com/docs/web/learn-more#config-object
const firebaseConfig = {
  // ...
};

// Initialize FirebaseApp
const firebaseApp = initializeApp(firebaseConfig);

// Initialize the Vertex AI service
const vertexAI = getVertexAI(firebaseApp);

// Create a `GenerativeModel` instance with a model that supports your use case
const model = getGenerativeModel(vertexAI, { model: "gemini-2.0-flash" });

async function run() {
  const chat = model.startChat({
    history: [
      {
        role: "user",
        parts: [{ text: "Hello, I have 2 dogs in my house." }],
      },
      {
        role: "model",
        parts: [{ text: "Great to meet you. What would you like to know?" }],
      },
    ],
    generationConfig: {
      maxOutputTokens: 100,
    },
  });

  const msg = "How many paws are in my house?";
  const result = await chat.sendMessageStream(msg);

  let text = '';
  for await (const chunk of result.stream) {
    const chunkText = chunk.text();
    console.log(chunkText);
    text += chunkText;
  }
}

run();

You can call startChat() and sendMessageStream() to stream responses from the model:

import 'package:firebase_vertexai/firebase_vertexai.dart';
import 'package:firebase_core/firebase_core.dart';
import 'firebase_options.dart';

await Firebase.initializeApp(
  options: DefaultFirebaseOptions.currentPlatform,
);

// Initialize the Vertex AI service and create a `GenerativeModel` instance
// Specify a model that supports your use case
final model =
      FirebaseVertexAI.instance.generativeModel(model: 'gemini-2.0-flash');

final chat = model.startChat();
// Provide a prompt that contains text
final prompt = [Content.text('Write a story about a magic backpack.')];

final response = await chat.sendMessageStream(prompt);
await for (final chunk in response) {
  print(chunk.text);
}


What else can you do?

  • Learn how to count tokens before sending long prompts to the model.
  • Set up Cloud Storage for Firebase so that you can include large files in your multimodal requests and have a more managed solution for providing files in prompts. Files can include images, PDFs, video, and audio.
  • Start thinking about preparing for production, including setting up Firebase App Check to protect the Gemini API from abuse by unauthorized clients. Also, make sure to review the production checklist.

Try out other capabilities

Learn how to control content generation

You can also experiment with prompts and model configurations using Vertex AI Studio.

Learn more about the supported models

Learn about the models available for various use cases and their quotas and pricing.


Give feedback about your experience with Vertex AI in Firebase