How to Build a WhatsApp AI Assistant

// prompts/systemPrompt.js

const SYSTEM_PROMPT = `
You are a pre-operative patient assistant for the Reedy Orthopedic Institute.
Your role is to answer patient questions in the 48 hours before elective surgery.

KNOWLEDGE BASE:
- Fasting protocol: no solid food for at least 8 hours before surgery. 
  Clear liquids (water, plain tea, black coffee) are allowed up to 2 hours before.
  No chewing gum, hard candy, or milk-based drinks after midnight.
- Check-in location: Surgical Admissions Desk, Ground Floor, Main Campus.
  Arrive 90 minutes before scheduled surgery time.
  Required documents: government-issued ID, insurance card (if applicable), appointment confirmation.
  Admissions line: +34 931 234 567, option 2.
- Common emotional responses: anxiety before surgery is normal and expected.
  Acknowledge the patient's feelings before providing information.
  If anxiety is described as severe or debilitating, recommend discussing with the medical team.

BEHAVIOR RULES:
1. Answer only questions you can answer from the knowledge base above.
2. If a question is outside scope, say so clearly and provide the admissions line.
3. Never invent specific medical information not present in the knowledge base.
4. Keep responses concise. Patients read these on a phone screen.
5. If the patient gives different instructions from their surgeon, always defer to the surgeon.
6. Greet the patient by first name on the first message if their name is known from context.
7. Respond in the same language the patient uses.
`;

module.exports = SYSTEM_PROMPT;

// sessions/sessionStore.js

const sessions = new Map();

const MAX_HISTORY_TURNS = 10; // keep last 10 exchanges to manage context window

function getHistory(userId) {
  return sessions.get(userId) || [];
}

function addTurn(userId, role, content) {
  const history = getHistory(userId);
  history.push({ role, content });

  // trim to max turns (each turn = 2 messages: user + assistant)
  const maxMessages = MAX_HISTORY_TURNS * 2;
  if (history.length > maxMessages) {
    history.splice(0, history.length - maxMessages);
  }

  sessions.set(userId, history);
}

function clearHistory(userId) {
  sessions.delete(userId);
}

module.exports = { getHistory, addTurn, clearHistory };

// llm/claudeClient.js

const Anthropic = require('@anthropic-ai/sdk');
const SYSTEM_PROMPT = require('../prompts/systemPrompt');

const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY });

async function generateResponse(history, userMessage) {
  // append the new user message to history before calling the API
  const messages = [
    ...history,
    { role: 'user', content: userMessage }
  ];

  const response = await client.messages.create({
    model: 'claude-sonnet-4-20250514',
    max_tokens: 512,        // keep responses short for mobile reading
    system: SYSTEM_PROMPT,
    messages
  });

  return response.content[0].text;
}

module.exports = { generateResponse };

// server.js

require('dotenv').config();
const express = require('express');
const twilio = require('twilio');
const { getHistory, addTurn } = require('./sessions/sessionStore');
const { generateResponse } = require('./llm/claudeClient');

const app = express();
app.use(express.urlencoded({ extended: false }));

const twilioClient = twilio(
  process.env.TWILIO_ACCOUNT_SID,
  process.env.TWILIO_AUTH_TOKEN
);

app.post('/webhook', async (req, res) => {
  // Twilio sends form-encoded data
  const incomingMessage = req.body.Body?.trim();
  const fromNumber = req.body.From; // e.g. "whatsapp:+34612345678"

  if (!incomingMessage || !fromNumber) {
    return res.status(400).send('Missing required fields');
  }

  try {
    // 1. Retrieve conversation history for this user
    const history = getHistory(fromNumber);

    // 2. Generate response from LLM
    const reply = await generateResponse(history, incomingMessage);

    // 3. Persist the new turn
    addTurn(fromNumber, 'user', incomingMessage);
    addTurn(fromNumber, 'assistant', reply);

    // 4. Send reply via Twilio
    await twilioClient.messages.create({
      from: process.env.TWILIO_WHATSAPP_NUMBER,
      to: fromNumber,
      body: reply
    });

    // Twilio expects a 200 response even if you send the reply programmatically
    res.status(200).send('OK');

  } catch (error) {
    console.error('Webhook error:', error);
    // Send a safe fallback message rather than letting the conversation die
    await twilioClient.messages.create({
      from: process.env.TWILIO_WHATSAPP_NUMBER,
      to: fromNumber,
      body: 'We are experiencing a temporary issue. Please call +34 931 234 567 for immediate assistance.'
    });
    res.status(200).send('OK'); // still 200 to prevent Twilio retry loops
  }
});

app.listen(process.env.PORT, () => {
  console.log(`Webhook server running on port ${process.env.PORT}`);
});