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Fetch the complete documentation index at: https://documentation.uponai.com/llms.txt

Use this file to discover all available pages before exploring further.

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Set Up the SDK

1

Install the Web SDK

npm install retell-client-js-sdk
2

Initialize the SDK

import { RetellWebClient } from "retell-client-js-sdk";

const retellWebClient = new RetellWebClient();

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Get a Call Access Token

Your client code should call your server endpoint, which internally calls create-web-call to get an access token. This must happen server-side to protect your API key from being exposed in client code. The access token is then passed to your frontend to start the call.
If you do not start the call within 30 seconds of obtaining the access token, it will be invalidated and the call will be marked with an error.

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Start the Call

await retellWebClient.startCall({
  accessToken: createCallResponse.access_token,
});
Optional parameters: 
await retellWebClient.startCall({
  accessToken: createCallResponse.access_token,
  sampleRate: 24000,          // Sample rate of audio capture and playback
  captureDeviceId: "default", // Microphone device ID
  playbackDeviceId: "0ec1...", // Speaker device ID
  emitRawAudioSamples: false,  // Emit raw PCM audio bytes as Float32Array
});

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Stop the Call

retellWebClient.stopCall();

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Listen to Events

The SDK emits events for real-time call updates: 
retellWebClient.on("call_started", () => {
  console.log("call started");
});

retellWebClient.on("call_ended", () => {
  console.log("call ended");
  setIsCallActive(false);
});

// When agent starts talking (useful for animations)
retellWebClient.on("agent_start_talking", () => {
  console.log("agent_start_talking");
});

// When agent stops talking (useful for animations)
retellWebClient.on("agent_stop_talking", () => {
  console.log("agent_stop_talking");
});

// Raw PCM audio bytes (only when emitRawAudioSamples is true)
retellWebClient.on("audio", (audio) => {
  // audio is a Float32Array
});

// Real-time transcript updates (last 5 sentences)
retellWebClient.on("update", (update) => {
  // update.transcript contains the transcript
});

retellWebClient.on("metadata", (metadata) => {
  // handle metadata
});

retellWebClient.on("error", (error) => {
  console.error("An error occurred:", error);
  retellWebClient.stopCall();
});