This is Part 5 of the series — and the centerpiece. Part 3 gave the robot ears and Part 4 gave it eyes. This post is the loop that ties them — and everything still to come — together: "Hey Reachy" → transcribe → a fresh per-wake Strands Agent that picks a tool → speak one short sentence → tear down.

This is Part 3 of the series. In Part 2 I proved the hardware works — the head moves, the mic hears, the camera sees. Now the robot can start listening. This post covers the lowest layer of the voice stack: the "Hey Reachy" wake word, running fully offline with no LLM and no network.

These days I am often creating small generic re-usable building blocks that I can pontentially use across new or existing projects, in this blog I talk about the architecture for a LLM based voice chatbot in a web browser built entirely as a serverless based solution.

The key component of this solution is using Amazon Nova 2 Sonic, a speech-to-speech foundation model that can understand spoken audio directly and generate voice responses - all through a single bidirectional stream from the browser directly to Amazon Bedrock, with no backend servers required - no EC2 instances and no Containers.

Goals

• Enable real-time voice-to-voice conversations with AI using Amazon Nova 2 Sonic
• Direct browser-to-Bedrock communication using bidirectional streaming - no Lambda functions or API Gateway required
• Use AWS Amplify Gen 2 for infrastructure-as-code backend definition in TypeScript
• Implement secure authentication using Cognito User Pool and Identity Pool for temporary AWS credentials
• Handle real-time audio capture, processing, and playback entirely in the browser
• Must be a completely serverless solution with automatic scaling
• Support click-to-talk interaction model for intuitive user experience
• Display live transcripts of both user speech and AI responses

Architecture

End-to-End Voice Chat Flow​

This diagram illustrates the complete flow from a user speaking into their microphone to hearing the AI assistant's voice response.

Flow Steps:

1. User Speaks - User clicks the microphone button and speaks naturally
2. Audio Capture - Browser captures audio via Web Audio API at 48kHz
3. Authentication - React app authenticates with Cognito User Pool
4. Token Exchange - JWT tokens exchanged for Identity Pool credentials
5. AWS Credentials - Temporary AWS credentials (access key, secret, session token) returned
6. Bidirectional Stream - Audio streamed to Bedrock via InvokeModelWithBidirectionalStream
7. Voice Response - Nova Sonic processes speech and returns synthesized voice response
8. Audio Playback - Response audio decoded and played through speakers
9. User Hears - User hears the AI assistant's natural voice response

Interactive Sequence Diagram​

React Hooks Architecture​

This diagram details the internal architecture of the React application, showing how custom hooks orchestrate audio capture, Bedrock communication, and playback.

Components:

• VoiceChat.tsx - Main UI component that coordinates all hooks and renders the interface
• useNovaSonic - Core hook managing Bedrock bidirectional stream, authentication, and event protocol
• useAudioRecorder - Captures microphone input using AudioWorklet in a separate thread
• useAudioPlayer - Manages audio playback queue and Web Audio API buffer sources
• audioUtils.ts - Low-level utilities for PCM conversion, resampling, and Base64 encoding

Data Flow:

1. Microphone audio captured by useAudioRecorder via MediaStream
2. AudioWorklet processes samples in real-time (separate thread)
3. Audio resampled from 48kHz to 16kHz, converted to PCM16, then Base64
4. useNovaSonic streams audio chunks to Bedrock
5. Response audio received as Base64, decoded to PCM, converted to Float32
6. useAudioPlayer queues AudioBuffers and plays through AudioContext

Authentication Flow​

This diagram shows the multi-layer authentication flow that enables secure browser-to-Bedrock communication without exposing long-term credentials.

Authentication Layers:

• Cognito User Pool - Handles user registration and login with email/password
• Cognito Identity Pool - Exchanges JWT tokens for temporary AWS credentials
• IAM Role - Defines permissions for authenticated users (Bedrock invoke access)
• SigV4 Signing - AWS SDK automatically signs all Bedrock requests

Key Security Features:

• No AWS credentials stored in browser - only temporary session credentials
• Credentials automatically refreshed by Amplify SDK before expiration
• IAM policy scoped to specific Bedrock model (amazon.nova-2-sonic-v1:0)
• All communication over HTTPS with TLS 1.2+

Audio Processing Pipeline​

This diagram shows the real-time audio processing that converts browser audio to Bedrock's required format and vice versa.

Input Processing (Recording):

1. Microphone - Browser captures audio at native sample rate (typically 48kHz)
2. AudioWorklet - Processes audio in separate thread, accumulates 2048 samples
3. Resample - Linear interpolation converts 48kHz → 16kHz (Nova Sonic requirement)
4. Float32 → PCM16 - Converts floating point [-1,1] to 16-bit signed integers
5. Base64 Encode - Binary PCM encoded for JSON transmission

Output Processing (Playback):

1. Base64 Decode - Received audio converted from Base64 to binary
2. PCM16 → Float32 - 16-bit integers converted to floating point
3. AudioBuffer - Web Audio API buffer created at 24kHz (Nova Sonic output rate)
4. Queue & Play - Buffers queued and played sequentially through speakers

Interactive Sequence Diagram​

Bidirectional Streaming Protocol​

This diagram illustrates how the useNovaSonic hook manages the complex bidirectional streaming protocol with Amazon Bedrock.

Event Protocol: Nova Sonic uses an event-based protocol where each interaction consists of named sessions, prompts, and content blocks.

Input Events (sent to Bedrock):

• sessionStart - Initializes session with inference parameters (maxTokens: 1024, topP: 0.9, temperature: 0.7)
• promptStart - Defines output audio format (24kHz, LPCM, voice "matthew")
• contentStart - Marks beginning of content blocks (TEXT for system prompt, AUDIO for user speech)
• textInput - Sends system prompt text content
• audioInput - Streams user audio chunks as Base64-encoded 16kHz PCM
• contentEnd - Marks end of content block
• promptEnd / sessionEnd - Terminates prompt and session

Output Events (received from Bedrock):

• contentStart - Marks role transitions (USER for ASR, ASSISTANT for response)
• textOutput - Returns transcribed user speech and generated AI response text
• audioOutput - Returns synthesized voice response as Base64-encoded 24kHz PCM
• contentEnd - Marks end of response content

Async Generator Pattern: The SDK requires input as AsyncIterable<Uint8Array>. The hook implements this using:

• Event Queue - Pre-queued initialization events before stream starts
• Promise Resolver - Backpressure control for yielding events on demand
• pushEvent() - Adds new events during conversation (audio chunks)

Serverless Architecture Overview​

This diagram provides a comprehensive view of all components - the entire solution is serverless with no EC2 instances or containers to manage.

Frontend Stack:

• React - Component-based UI framework
• Vite - Build tool and dev server
• TypeScript - Type-safe development
• AWS Amplify Hosting - Static web hosting with global CDN

Backend Stack (Amplify Gen 2):

• amplify/backend.ts - Infrastructure defined in TypeScript
• Cognito User Pool - Email-based authentication
• Cognito Identity Pool - AWS credential vending
• IAM Policy - Grants bedrock:InvokeModel permission for bidirectional streaming

AI Service:

• Amazon Bedrock - Managed foundation model inference
• Nova 2 Sonic - Speech-to-speech model (us-east-1)
• Bidirectional Streaming - Real-time duplex communication

Technical Challenges & Solutions

Challenge 1: AudioWorklet CORS Issues​

Problem: Loading AudioWorklet from external file fails with CORS errors on some deployments.

Solution: Inline the AudioWorklet code as a Blob URL:

const blob = new Blob([audioWorkletCode], { type: 'application/javascript' });
const workletUrl = URL.createObjectURL(blob);
await audioContext.audioWorklet.addModule(workletUrl);
URL.revokeObjectURL(workletUrl);

Challenge 2: Sample Rate Mismatch​

Problem: Browsers capture audio at 48kHz, but Nova Sonic requires 16kHz input.

Solution: Linear interpolation resampling in real-time:

const resampleAudio = (audioData: Float32Array, sourceSampleRate: number, targetSampleRate: number) => {
  const ratio = sourceSampleRate / targetSampleRate;
  const newLength = Math.floor(audioData.length / ratio);
  const result = new Float32Array(newLength);
  for (let i = 0; i < newLength; i++) {
    const srcIndex = i * ratio;
    const floor = Math.floor(srcIndex);
    const ceil = Math.min(floor + 1, audioData.length - 1);
    const t = srcIndex - floor;
    result[i] = audioData[floor] * (1 - t) + audioData[ceil] * t;
  }
  return result;
};

Challenge 3: SDK Bidirectional Stream Input​

Problem: AWS SDK requires input as AsyncIterable<Uint8Array>, but events need to be pushed dynamically during the conversation.

Solution: Async generator with event queue and promise-based backpressure:

async function* createInputStream() {
  while (isActiveRef.current && !ctrl.closed) {
    while (ctrl.eventQueue.length > 0) {
      yield ctrl.eventQueue.shift();
    }
    const nextEvent = await new Promise(resolve => {
      ctrl.resolver = resolve;
    });
    if (nextEvent === null) break;
    yield nextEvent;
  }
}

Getting Started

GitHub Repository: https://github.com/chiwaichan/amplify-react-amazon-nova-2-sonic-voice-chat

Prerequisites​

• Node.js 18+
• AWS Account with Bedrock access enabled
• AWS CLI configured with credentials

Deployment Steps​

1. Enable Nova 2 Sonic in Bedrock Console (us-east-1 region)

2. Clone and Install:

git clone https://github.com/chiwaichan/amplify-react-amazon-nova-2-sonic-voice-chat.git
cd amplify-react-amazon-nova-2-sonic-voice-chat
npm install

3. Start Amplify Sandbox:

npx ampx sandbox

4. Run Development Server:

npm run dev

5. Open Application: Navigate to http://localhost:5173, create an account, and start talking!

Summary

This architecture provides a reusable building block for voice-enabled AI applications:

• Zero backend servers - Direct browser-to-Bedrock communication
• Real-time streaming - HTTP/2 bidirectional streaming for low latency
• Secure authentication - Cognito User Pool + Identity Pool + IAM policies
• Audio processing pipeline - Web Audio API, AudioWorklet, PCM conversion
• Infrastructure as code - AWS Amplify Gen 2 with TypeScript backend definition

The entire interaction happens in real-time: speak naturally, and hear the AI respond within seconds.