SMS Consent Management with Next.js and Sinch: Complete TCPA Compliance Guide (2024-2025)

Build a production-ready SMS consent management system that complies with TCPA and GDPR regulations. This comprehensive guide shows you how to implement SMS opt-in and opt-out handling using Next.js API routes and the Sinch SMS API. Learn to process consent keywords (SUBSCRIBE, STOP, HELP), manage marketing groups, handle webhooks securely, and meet the new FCC opt-out requirements effective April 11, 2025. Includes complete code examples, 10DLC registration guidance, HMAC authentication setup, and best practices for SMS marketing compliance with Next.js 15 and Sinch Node SDK 1.2.1.

---

Building Production-Ready SMS Consent Management with Next.js and Sinch

Manage user consent for your SMS marketing campaigns to comply with regulations like TCPA in the US and GDPR in the EU. Failing to properly handle opt-ins and opt-outs can lead to significant legal and financial consequences, alongside damaging your brand reputation.

This guide walks you through building a robust SMS consent management system using Next.js and the Sinch SMS API. You will create a Next.js application with an API endpoint that listens for incoming SMS messages sent to your Sinch number. This endpoint will process standard keywords like SUBSCRIBE and STOP, manage user participation in a Sinch group, and send appropriate confirmation messages back to users.

Project Goal: Create a Next.js API endpoint that securely handles incoming Sinch SMS webhooks to manage user opt-ins (SUBSCRIBE) and opt-outs (STOP) for an SMS marketing group, leveraging the Sinch Node SDK for communication and group management.

Technologies Used:

• Next.js: A React framework enabling server-side rendering and API routes, perfect for handling webhooks and server-side logic within a single project. Latest version: 15.5 (as of December 2024), with React 19 support and stable Node.js middleware runtime.
• Sinch SMS API & Node SDK: Provides the necessary tools to send/receive SMS messages and manage contact groups programmatically. Sinch Node SDK version 1.2.1 (GA release, December 2024). Install via npm install @sinch/sdk-core @sinch/sms.
• Node.js: The runtime environment for Next.js and the Sinch SDK. Requires Node.js v18.17.0 or later (v20.x or v22.x LTS recommended for 2024-2025).

System Architecture:

(Note: The following visualization describes a flow where User SMS -> Sinch -> Next.js API -> Sinch API -> Sinch Group/User SMS.)

Prerequisites:

• Node.js (v18.17.0 or later recommended; v20.x or v22.x LTS preferred for 2024-2025) and npm/yarn installed.
• A Sinch account with access to the SMS API.
• A provisioned phone number within your Sinch account capable of sending and receiving SMS.
• Sinch API Credentials:
  • Project ID
  • API Key ID
  • API Key Secret
  • Service Plan ID associated with your SMS service.
• Familiarity with JavaScript/TypeScript and Next.js fundamentals.
• Crucially for US Traffic (A2P 10DLC): A registered Brand and an approved Campaign within the Sinch platform/TCR (The Campaign Registry). All unregistered traffic will be blocked after December 1, 2024. This guide focuses on implementing consent logic, but you must complete 10DLC registration separately to legally send A2P marketing messages in the US. Registration typically takes 3-5 business days. See the Sinch 10DLC Documentation for details.
• TCPA Compliance (US): Be aware that new FCC opt-out rules take effect April 11, 2025. Under these rules, you must honor opt-out requests within 10 business days (reduced from 30 days), and consumers can revoke consent using any reasonable method. Keywords like STOP, QUIT, END, REVOKE, OPT OUT, CANCEL, and UNSUBSCRIBE via reply text constitute per se reasonable opt-out methods.

By the end of this guide, you will have a deployable Next.js application capable of handling SMS consent keywords reliably and securely.

1. Setting up the Project

Let's start by creating a new Next.js project and installing the necessary dependencies.

1. Create a new Next.js App: Open your terminal and run:

   bash

   npx create-next-app@latest sinch-consent-manager
   cd sinch-consent-manager
   

   Follow the prompts (TypeScript recommended: Yes, ESLint: Yes, Tailwind CSS: No (or Yes if desired), src/ directory: Yes, App Router: No (we'll use Pages Router for simpler API routes in this example), import alias: default).

2. Install Sinch SDK: Install the core Sinch SDK and the SMS module:

   bash

   npm install @sinch/sdk-core @sinch/sms
   

3. Set up Environment Variables: Create a file named .env.local in the root of your project. Never commit this file to version control. Add your Sinch credentials and configuration:

   # Sinch API Credentials & Configuration
   SINCH_KEY_ID=YOUR_SINCH_KEY_ID
   SINCH_KEY_SECRET=YOUR_SINCH_KEY_SECRET
   SINCH_PROJECT_ID=YOUR_SINCH_PROJECT_ID
   SINCH_SERVICE_PLAN_ID=YOUR_SINCH_SERVICE_PLAN_ID
   SINCH_NUMBER=YOUR_PROVISIONED_SINCH_PHONE_NUMBER # e.g., +12025550181
   
   # Application Configuration
   SINCH_MARKETING_GROUP_NAME=""Marketing Subscribers"" # Name for the Sinch Group
   
   # Webhook Security (Basic Auth Example)
   # Generate strong random username/password
   WEBHOOK_USERNAME=your_secure_username
   WEBHOOK_PASSWORD=your_secure_password
   
   • SINCH_KEY_ID, SINCH_KEY_SECRET, SINCH_PROJECT_ID: Find these in your Sinch Customer Dashboard under your Account > API Credentials or Project settings.
   • SINCH_SERVICE_PLAN_ID: Locate this ID associated with your SMS API service plan in the Sinch Dashboard (often under SMS > APIs).
   • SINCH_NUMBER: The full E.164 formatted phone number you've provisioned in Sinch for this campaign.
   • SINCH_MARKETING_GROUP_NAME: A descriptive name for the group we'll create in Sinch to manage subscribers.
   • WEBHOOK_USERNAME, WEBHOOK_PASSWORD: Credentials for securing your webhook endpoint using Basic Authentication. Choose strong, unique values.

4. Project Structure: Your src/pages/api/ directory is where the core webhook logic will reside. The rest follows a standard Next.js structure.

2. Implementing the API Endpoint Handler

We'll create a single API route in Next.js to handle incoming SMS messages from Sinch.

1. Create the API Route File: Create a new file: src/pages/api/webhooks/sinch-sms.js

2. Implement the Webhook Handler: Add the following code to src/pages/api/webhooks/sinch-sms.js. This code initializes the Sinch client, handles Basic Auth, parses incoming messages, manages group membership, and sends replies.

   javascript

   import { SinchClient } from '@sinch/sdk-core';
   import { SmsRegion } from '@sinch/sms'; // Adjust region if needed (e.g., SmsRegion.EU)
   
   // --- Configuration ---
   const sinchClient = new SinchClient({
     projectId: process.env.SINCH_PROJECT_ID,
     keyId: process.env.SINCH_KEY_ID,
     keySecret: process.env.SINCH_KEY_SECRET,
     // Optional: Specify region if not US default
     // smsRegion: SmsRegion.US,
   });
   
   const servicePlanId = process.env.SINCH_SERVICE_PLAN_ID;
   const sinchNumber = process.env.SINCH_NUMBER;
   const groupName = process.env.SINCH_MARKETING_GROUP_NAME || 'Default Marketing Group';
   const webhookUser = process.env.WEBHOOK_USERNAME;
   const webhookPass = process.env.WEBHOOK_PASSWORD;
   
   // --- Helper Functions ---
   
   /**
    * Finds or creates a Sinch group by name.
    * Returns the Group ID.
    */
   async function findOrCreateGroup(name) {
     try {
       const { groups } = await sinchClient.sms.groups.list();
       let group = groups.find((g) => g.name === name);
   
       if (!group) {
         console.log(`Group ""${name}"" not found, creating...`);
         const newGroup = await sinchClient.sms.groups.create({
           createGroupRequestBody: { name: name },
         });
         console.log(`Group ""${name}"" created with ID: ${newGroup.id}`);
         return newGroup.id;
       } else {
         console.log(`Found group ""${name}"" with ID: ${group.id}`);
         return group.id;
       }
     } catch (error) {
       console.error('Error finding or creating Sinch group:', error?.response?.data || error.message);
       throw new Error('Could not find or create Sinch group.');
     }
   }
   
   /**
    * Sends an SMS message using the Sinch SDK.
    */
   async function sendSms(to, message) {
     try {
       console.log(`Sending SMS to ${to}: ""${message}""`);
       // Note: Verify the method name and parameters below against the current Sinch SDK documentation.
       await sinchClient.sms.batches.send({
         sendBatchSmsRequestBody: {
           to: [to],
           from: sinchNumber,
           body: message,
           delivery_report: 'none', // Adjust if delivery reports are needed
         },
         // Pass servicePlanId if not using default project settings
         // servicePlanId: servicePlanId
       });
       console.log(`SMS successfully sent to ${to}`);
     } catch (error) {
       console.error(`Error sending SMS to ${to}:`, error?.response?.data || error.message);
       // Implement retry logic here if needed (see Section 4 discussion)
     }
   }
   
   /**
    * Basic Authentication Middleware
    */
   function handleBasicAuth(req, res) {
     if (!webhookUser || !webhookPass) {
       console.warn('Webhook basic auth credentials not set. Endpoint is unsecured.');
       return true; // Skip auth if not configured (not recommended for production)
     }
   
     const authHeader = req.headers.authorization;
     if (!authHeader || !authHeader.startsWith('Basic ')) {
       console.warn('Missing or invalid Authorization header');
       res.setHeader('WWW-Authenticate', 'Basic realm=""Secure Area""');
       res.status(401).json({ error: 'Authorization Required' });
       return false;
     }
   
     const base64Credentials = authHeader.split(' ')[1];
     const credentials = Buffer.from(base64Credentials, 'base64').toString('ascii');
     const [username, password] = credentials.split(':');
   
     if (username === webhookUser && password === webhookPass) {
       return true; // Authenticated
     } else {
       console.warn('Invalid Basic Auth credentials provided');
       res.setHeader('WWW-Authenticate', 'Basic realm=""Secure Area""');
       res.status(401).json({ error: 'Invalid Credentials' });
       return false;
     }
   }
   
   
   // --- API Handler ---
   export default async function handler(req, res) {
     // 1. Only accept POST requests
     if (req.method !== 'POST') {
       console.log(`Received non-POST request: ${req.method}`);
       res.setHeader('Allow', ['POST']);
       return res.status(405).json({ error: `Method ${req.method} Not Allowed` });
     }
   
     // 2. Authenticate the request (Basic Auth)
     if (!handleBasicAuth(req, res)) {
       return; // Response already sent by handleBasicAuth
     }
   
     // 3. Parse the incoming webhook payload
     const incomingSms = req.body;
     console.log('Received Sinch Webhook:', JSON.stringify(incomingSms, null, 2));
   
     // Basic validation of expected payload structure
     if (!incomingSms || typeof incomingSms !== 'object' || !incomingSms.from || !incomingSms.body) {
         console.error('Invalid or missing webhook payload structure:', incomingSms);
         return res.status(400).json({ error: 'Invalid payload structure' });
     }
   
     const fromNumber = incomingSms.from; // Sender's phone number
     const messageBody = incomingSms.body.trim().toUpperCase(); // Normalize keyword
   
     try {
       // 4. Find or Create the Target Group
       const groupId = await findOrCreateGroup(groupName);
   
       // 5. Process Keywords
       if (messageBody === 'SUBSCRIBE' || messageBody === 'JOIN' || messageBody === 'START') {
         console.log(`Processing SUBSCRIBE request from ${fromNumber} for group ${groupId}`);
         // Add number to the group
         // Note: Verify the method name and parameters below against the current Sinch SDK documentation.
         await sinchClient.sms.groups.replaceMembers({
           groupId: groupId,
           addPhoneNumbersRequest: { add: [fromNumber] }, // Use 'add' to avoid removing others
         });
         console.log(`Added ${fromNumber} to group ${groupId}`);
   
         // Send confirmation message
         const reply = `Thanks for subscribing to ${groupName}! Msg frequency varies. Msg&Data rates may apply. Reply HELP for help, STOP to cancel.`;
         await sendSms(fromNumber, reply);
   
       } else if (messageBody === 'STOP' || messageBody === 'UNSUBSCRIBE' || messageBody === 'CANCEL' || messageBody === 'END' || messageBody === 'QUIT') {
         console.log(`Processing STOP request from ${fromNumber} for group ${groupId}`);
         // Remove number from the group
         // Note: Verify the method name and parameters below against the current Sinch SDK documentation.
         await sinchClient.sms.groups.removeMember({
           groupId: groupId,
           phoneNumber: fromNumber,
         });
          console.log(`Removed ${fromNumber} from group ${groupId}`);
   
         // Send confirmation message
         const reply = `You have unsubscribed from ${groupName} and will no longer receive messages. Reply SUBSCRIBE to rejoin.`;
         await sendSms(fromNumber, reply);
   
       } else if (messageBody === 'HELP') {
            console.log(`Processing HELP request from ${fromNumber}`);
            // Send help message
            const reply = `${groupName}: For help, contact support@example.com or reply STOP to unsubscribe. Msg&Data rates may apply.`; // Customize help info
            await sendSms(fromNumber, reply);
   
       } else {
         // Optional: Handle unknown keywords or general messages
         console.log(`Received unknown keyword ""${messageBody}"" from ${fromNumber}`);
         // Decide if you want to reply to unknown messages. Sometimes it's better not to.
         // Example reply:
         // const reply = `Sorry, I didn't understand ""${incomingSms.body}"". Reply SUBSCRIBE to join or STOP to unsubscribe.`;
         // await sendSms(fromNumber, reply);
       }
   
       // 6. Respond to Sinch Platform
       // Sinch expects a 2xx status code to acknowledge receipt of the webhook.
       // No specific response body is usually required unless defined by Sinch for specific features.
       return res.status(200).json({ status: 'ok', message: 'Webhook processed' });
   
     } catch (error) {
       console.error('Error processing webhook:', error);
       // Respond with an error status code to indicate failure.
       // Avoid sending detailed internal errors back in the response for security.
       return res.status(500).json({ error: 'Internal Server Error' });
     }
   }
   

   Code Explanation:

   • Initialization: Sets up the SinchClient using credentials from environment variables. Defines constants for configuration.
   • findOrCreateGroup: Checks if the group defined by SINCH_MARKETING_GROUP_NAME exists. If not, it creates it using sinchClient.sms.groups.create. It returns the group ID. Error Handling: Includes basic error logging. Production systems might require more sophisticated handling (e.g., specific error types, retries), but logging is sufficient for this guide.
   • sendSms: A utility to send an SMS using sinchClient.sms.batches.send. It takes the recipient number and message body. Error Handling: Logs errors during sending.
   • handleBasicAuth: Implements Basic Authentication checking based on WEBHOOK_USERNAME and WEBHOOK_PASSWORD. It sends appropriate 401 responses if auth fails or is missing. Returns true on success, false on failure (response sent).
   • handler (Main Logic):
     • Checks for POST method.
     • Calls handleBasicAuth to secure the endpoint.
     • Parses the req.body (Sinch sends JSON). Includes basic payload validation. While this basic check is useful, production applications might benefit from more rigorous validation using libraries like zod or joi (as discussed in the Security section) to strictly enforce the expected schema.
     • Extracts the sender's number (from) and the message body (body). Normalizes the body to uppercase for case-insensitive keyword matching.
     • Calls findOrCreateGroup to get the relevant group ID.
     • Uses if/else if blocks to check for SUBSCRIBE/JOIN/START, STOP/UNSUBSCRIBE/etc., and HELP keywords.
     • Subscribe Logic: Uses sinchClient.sms.groups.replaceMembers with the add property to add the number to the group without affecting other members. The Sinch Groups API handles duplicate adds gracefully (won't error if the number is already in the group). Sends a confirmation SMS. (Note: Verify the exact method name against current Sinch Node SDK v1.2.1 documentation – the method may be update with add array rather than replaceMembers.)
     • Stop Logic: Uses sinchClient.sms.groups.removeMember to remove the specific number. The API handles unknown removes gracefully (won't error if the number isn't in the group). Sends an opt-out confirmation SMS. (Note: Verify method name against current SDK documentation.)
     • Help Logic: Sends a pre-defined help message.
     • Unknown Keywords: Currently logs them. You can optionally add a reply here.
     • Response: Sends a 200 OK response back to Sinch to acknowledge successful processing. Sends 500 Internal Server Error if any part of the processing fails within the try...catch block.

3. Integrating with Sinch (Configuration)

Now that the code is ready, you need to configure your Sinch number to send incoming SMS messages to your deployed Next.js API endpoint.

1. Deploy Your Application: You need a publicly accessible URL for your API route. Deploy your Next.js application to a hosting provider like Vercel, Netlify, or AWS Amplify.

   • Using Vercel (Example):
     • Push your code to a Git repository (GitHub, GitLab, Bitbucket).
     • Connect your repository to Vercel.
     • Configure Environment Variables in the Vercel project settings (copy paste from your .env.local).
     • Deploy. Vercel will provide a production URL (e.g., https://your-app-name.vercel.app).
   • Your webhook URL will be: https://your-app-name.vercel.app/api/webhooks/sinch-sms

2. Configure Sinch Webhook:

   • Log in to your Sinch Customer Dashboard.
   • Navigate to SMS -> APIs.
   • Find your Service Plan ID (the one specified in .env.local) and click on it or its associated settings/edit icon.
   • Look for a section related to Callback URLs or Webhooks.
   • In the field for Incoming Messages (MO) or a similar label, enter the full URL of your deployed API route: https://your-app-name.vercel.app/api/webhooks/sinch-sms
   • Configure Authentication: Find the settings for webhook authentication. Select Basic Authentication.
     • Enter the Username defined in your .env.local (WEBHOOK_USERNAME).
     • Enter the Password defined in your .env.local (WEBHOOK_PASSWORD).
   • Save the configuration.

   (Dashboard navigation might vary slightly. Consult the official Sinch documentation for the most up-to-date instructions on configuring SMS webhooks.)

4. Error Handling, Logging, and Retries

• Error Handling: The provided API route includes try...catch blocks around the main processing logic and Sinch SDK calls. Errors are logged to the console (which Vercel or your hosting provider typically captures). A generic 500 Internal Server Error is sent back to Sinch upon failure, preventing leakage of internal details.
• Logging: We use console.log for informational messages and console.error for errors. For production, consider integrating a dedicated logging service (like Logtail, Datadog, Sentry) for better aggregation, searching, and alerting.
• Retries:
  • Sinch Webhook Retries: Sinch typically retries sending webhooks if it doesn't receive a 2xx response within a timeout period. The exact policy (number of retries, intervals) can vary, so it's recommended to consult the official Sinch documentation regarding their webhook retry behavior. Ensuring your endpoint responds quickly (even with an error code) is important.
  • Sinch API Call Retries: The current code doesn't implement explicit retries for sendSms or group operations. For critical operations, you could wrap Sinch SDK calls in a retry mechanism (e.g., using libraries like async-retry) with exponential backoff, especially for transient network errors or temporary Sinch API issues (like 5xx errors from Sinch). However, be cautious about retrying operations that modify state. Implementing idempotency keys or careful state checking is crucial when retrying write operations. For instance, retrying a groups.list (read operation) is generally safe, whereas retrying groups.addMember (write operation) could potentially lead to duplicate actions if the initial request succeeded but the response was lost.

5. Database Schema and Data Layer (Consideration)

While this guide uses Sinch Groups for simplicity, a production system often benefits from its own database to store subscriber information and consent status.

• Benefits: Persistence independent of Sinch, ability to store additional user data, detailed consent history (timestamps, source), easier querying and segmentation.
• Implementation (Example with Prisma):
  1. Install Prisma: npm install prisma @prisma/client --save-dev
  2. Initialize Prisma: npx prisma init --datasource-provider postgresql (or your preferred DB)
  3. Configure prisma/schema.prisma:
     sql

     datasource db {
       provider = ""postgresql""
       url      = env(""DATABASE_URL"")
     }
     
     generator client {
       provider = ""prisma-client-js""
     }
     
     model Subscriber {
       id            String   @id @default(cuid())
       phoneNumber   String   @unique // E.164 format
       isSubscribed  Boolean  @default(false)
       subscribedAt  DateTime?
       unsubscribedAt DateTime?
       createdAt     DateTime @default(now())
       updatedAt     DateTime @updatedAt
       // Add other fields as needed (e.g., source, name)
     }
     
  4. Set DATABASE_URL in .env.local.
  5. Run npx prisma db push to sync the schema.
  6. Modify the API route to import PrismaClient and update the database record alongside the Sinch Group operations.

Using a database adds complexity but provides greater control and data richness. For this guide's scope, we rely solely on Sinch Groups.

6. Security Features

• Webhook Authentication: Basic Authentication is implemented in this guide. This is crucial to ensure only Sinch can trigger your API route. Sinch also supports HMAC-SHA256 Signed Requests, which offer stronger security as credentials aren't sent directly with each request. HMAC authentication uses a hash-based message authentication code combining the request body, nonce, and timestamp, then hashed with your secret key. To configure HMAC authentication, contact your Sinch account manager – it's managed at the account/service plan level and can be used alongside Basic Auth or OAuth 2.0 for enhanced security. Consider upgrading to HMAC for production deployments with higher security requirements.
• Input Validation: Basic checks ensure the payload exists and has from and body. More robust validation could use libraries like zod or joi to strictly enforce the expected Sinch payload schema.
• HTTPS: Deploying via platforms like Vercel automatically enforces HTTPS, protecting data in transit.
• Rate Limiting: High-traffic webhooks could be rate-limited to prevent abuse or resource exhaustion. Vercel offers built-in rate limiting on certain plans. Alternatively, use middleware with libraries like express-rate-limit (if using a custom server) or similar concepts in Next.js API routes.
• Environment Variable Security: Keep .env.local secure and never commit it. Use your hosting provider's secret management features (like Vercel Environment Variables).
• Least Privilege (Sinch API Key): Ensure the Sinch API Key used has only the necessary permissions (SMS sending, group management) required for this application.

7. Handling Special Cases

• Keyword Case-Insensitivity: Handled by converting the incoming message body to uppercase (.toUpperCase()).
• Whitespace: Handled using .trim() on the message body.
• Multiple Keywords: The code handles common variations like SUBSCRIBE/JOIN/START and STOP/UNSUBSCRIBE/etc. You can easily add more synonyms.
• International Numbers: The code assumes E.164 format (+ followed by country code and number) as provided by Sinch.
• Character Encoding: Sinch webhooks typically use UTF-8 JSON, which Node.js handles correctly by default.
• Duplicate Messages: Sinch might occasionally send duplicate webhooks. While the group add/remove operations are somewhat idempotent (adding an existing member or removing a non-existent member might not cause errors, though check SDK specifics), consider adding logic to check the isSubscribed status in a database (if used) before performing the action.

8. Performance Optimizations

• Webhook Response Time: Respond to Sinch quickly (ideally under 2 seconds) to avoid timeouts and retries. The current logic is lightweight. If you add slow operations (complex DB queries, external API calls), consider deferring them using background jobs/queues (e.g., Vercel Serverless Functions triggered by your webhook, or dedicated queue services).
• Sinch Client Initialization: The SinchClient is initialized outside the handler function, so it's reused across requests within the same serverless function instance, improving performance slightly.
• Caching Group ID: The findOrCreateGroup call involves an API request. If performance is critical and the group rarely changes, you could cache the groupId in memory (with a short TTL) or a faster cache like Redis, but this adds complexity and is likely unnecessary for typical volumes.

9. Monitoring, Observability, and Analytics

• Health Checks: Create a simple health check endpoint (e.g., src/pages/api/health.js) that returns 200 OK to verify the deployment is live.
  javascript

  export default function handler(req, res) {
    res.status(200).json({ status: 'ok', timestamp: new Date().toISOString() });
  }
  
• Logging: Centralized logging (Vercel Logs, Datadog, etc.) is crucial for monitoring webhook activity, errors, and execution flow.
• Error Tracking: Integrate services like Sentry (npm install @sentry/nextjs, npx @sentry/wizard@latest -i nextjs) to capture, track, and get alerted on runtime errors in your API route.
• Metrics: Track key metrics:
  • Webhook request count & latency.
  • Number of SUBSCRIBE, STOP, HELP events processed.
  • Sinch API call latency and error rates.
  • (If using DB) Subscriber growth/churn rate.
  • Hosting platforms often provide basic metrics. For more detail, use monitoring services.
• Dashboards: Create dashboards (e.g., in Datadog, Grafana, or your logging provider) visualizing these metrics to understand traffic patterns and system health.
• Alerting: Set up alerts (e.g., in Sentry or your monitoring tool) for:
  • High error rates (>1% of requests).
  • Sustained high latency (>2 seconds).
  • Failures in critical operations (e.g., findOrCreateGroup).

10. Troubleshooting and Caveats

• Webhook Not Firing:
  • Verify the Callback URL in Sinch Dashboard is exactly correct and points to your deployed application URL.
  • Check your application logs (e.g., Vercel Logs) for any incoming requests or startup errors.
  • Ensure your deployment is healthy and accessible.
  • Check Sinch Dashboard for any delivery errors related to webhooks.
  • Ensure Basic Auth credentials match between .env.local/Vercel variables and Sinch configuration.
• Authentication Errors (401 Unauthorized):
  • Double-check WEBHOOK_USERNAME and WEBHOOK_PASSWORD match exactly between your environment variables and the Sinch webhook configuration.
  • Ensure the Authorization: Basic ... header is being sent correctly by Sinch (usually automatic if configured).
• Sinch API Errors (Logged in Console):
  • 401 Unauthorized: Check SINCH_KEY_ID, SINCH_KEY_SECRET, SINCH_PROJECT_ID. Ensure the key is active and has SMS permissions.
  • 400 Bad Request: Often indicates an issue with the request body sent to Sinch (e.g., invalid phone number format, missing required fields). Check the error details logged. Could also be an incorrect SINCH_SERVICE_PLAN_ID.
  • 404 Not Found: Could mean the groupId is incorrect or the resource doesn't exist.
  • 5xx Server Error: Transient issue on Sinch's side. Consider implementing retries (see Section 4).
  • Check SinchClient Configuration: Ensure the correct smsRegion (e.g., SmsRegion.EU, SmsRegion.US) is configured when initializing the SinchClient if your service plan is not based in the default US region.
• Consent Logic Not Working:
  • Check logs to see if keywords are being recognized correctly (case/trimming).
  • Verify the groupId being used is correct. Use the Sinch API (or dashboard, if available) to inspect the group members directly.
  • Ensure fromNumber is being parsed correctly from the webhook.
• Caveats:
  • 10DLC Compliance (US): This code implements consent handling but does not fulfill 10DLC registration requirements. You must register your Brand and Campaign with The Campaign Registry (TCR) via Sinch for A2P SMS marketing in the US. Critical deadline: All unregistered traffic will be blocked after December 1, 2024. The registration process involves two steps: (1) Brand registration with business information (legal name, EIN/Tax ID, address), and (2) Campaign registration describing your use case. Approval typically takes 3-5 business days. You cannot register directly with TCR – you must work through Sinch as your messaging service provider. Failure to register will result in blocked messages and potential fines ranging from $500-$1,500 per violation.
  • TCPA Opt-Out Requirements (US): New FCC rules effective April 11, 2025 require you to honor opt-out requests within 10 business days (reduced from up to 30 days). The keywords STOP, QUIT, END, REVOKE, OPT OUT, CANCEL, and UNSUBSCRIBE constitute per se reasonable opt-out methods. You may send one brief clarification message within 5 minutes of receiving an opt-out to confirm intent. Retain documentation of opt-out requests for at least 4 years (TCPA statute of limitations). This implementation processes opt-outs immediately, which exceeds the legal requirement.
  • Sinch Groups vs. Database: Sinch Groups are convenient but may have limitations in querying, scalability, or data richness compared to managing subscribers in your own database.
  • Idempotency: While basic group operations might be somewhat idempotent, ensure your logic handles potential duplicate webhook deliveries gracefully, especially if integrating with other systems or a database.
  • Rate Limits: Be aware of Sinch API rate limits. High-volume processing might require strategies to stay within limits.
  • Error Handling Granularity: The current error handling logs errors and returns 500. More granular error handling could provide better insights or trigger specific recovery actions.
  • SDK Verification: The Sinch SDK method names and parameters used in the code examples should be verified against the latest official Sinch Node SDK v1.2.1 documentation to ensure they are current and accurate.

11. Deployment and CI/CD

• Deployment (Vercel Example):
  1. Ensure .env.local is in your .gitignore.
  2. Push code to your Git provider.
  3. Create a new Project on Vercel, importing your Git repository.
  4. Framework Preset: Should be detected as Next.js.
  5. Environment Variables: Copy all key-value pairs from .env.local into the Vercel Project Settings > Environment Variables section. Ensure they are available for the ""Production"" environment (and Preview/Development if needed).
  6. Deploy. Vercel builds and deploys the app, providing the production URL.
• CI/CD:
  • Vercel automatically sets up CI/CD. Pushing to the main branch triggers a production deployment. Pushing to other branches or creating Pull Requests triggers preview deployments.
  • Automated Testing: Integrate testing (Unit, Integration) into your pipeline. Add an npm run test script to your package.json and configure Vercel (or your CI/CD tool like GitHub Actions) to run tests before deploying. Fail the build if tests fail.
  • Environment Configuration: Use Vercel's environment variable system to manage different configurations (e.g., separate Sinch credentials or group names for staging vs. production).
• Rollbacks: Vercel maintains previous deployments. If a deployment introduces issues, you can instantly roll back to a previous working deployment via the Vercel dashboard.

---

13. Frequently Asked Questions (FAQ)

What is SMS consent management and why is it required?

SMS consent management is the process of obtaining, storing, and honoring user preferences for receiving text messages. It's legally required under regulations like TCPA (Telephone Consumer Protection Act) in the US and GDPR in the EU. You must obtain explicit written consent before sending marketing SMS messages, and you must honor opt-out requests promptly. Failure to comply can result in fines ranging from $500 to $1,500 per violation, class-action lawsuits, and severe brand reputation damage.

What are the new TCPA opt-out requirements effective April 11, 2025?

The FCC's new opt-out rules, effective April 11, 2025, require businesses to honor opt-out requests within 10 business days (reduced from up to 30 days). Consumers can revoke consent using any reasonable method, and keywords like STOP, QUIT, END, REVOKE, OPT OUT, CANCEL, and UNSUBSCRIBE via reply text constitute per se reasonable opt-out methods. You may send one brief clarification message within 5 minutes of receiving an opt-out to confirm intent. You must retain documentation of opt-out requests for at least 4 years (the TCPA statute of limitations).

What is A2P 10DLC registration and when is the deadline?

A2P (Application-to-Person) 10DLC registration is required for sending marketing SMS messages in the US using standard 10-digit phone numbers. You must register your brand and campaign with The Campaign Registry (TCR) through your messaging provider (like Sinch). The critical deadline is December 1, 2024 – all unregistered traffic will be blocked after this date. Registration involves two steps: (1) Brand registration with business information (legal name, EIN/Tax ID, address), and (2) Campaign registration describing your use case. Approval typically takes 3–5 business days.

How do I implement SMS webhooks with Next.js and Sinch?

To implement SMS webhooks with Next.js and Sinch, create an API route (e.g., pages/api/webhooks/sinch-sms.js) that handles POST requests from Sinch. Initialize the Sinch Node SDK (version 1.2.1) with your API credentials, implement webhook authentication (Basic Auth or HMAC-SHA256), parse incoming SMS messages, process consent keywords (SUBSCRIBE, STOP, HELP), update Sinch Groups using the SDK, and send confirmation SMS messages. Deploy to Vercel or another hosting platform, then configure your Sinch Service Plan to send webhooks to your deployed URL.

What Node.js and Next.js versions do I need for Sinch integration?

You need Node.js v18.17.0 or later (v20.x or v22.x LTS recommended for 2024–2025) and Next.js 15.5 or later. Next.js 15 requires Node.js >= v18.17.0 minimum. The latest Sinch Node SDK (version 1.2.1, GA release December 2024) is compatible with these versions. Install the SDK using npm install @sinch/sdk-core @sinch/sms.

What's the difference between Basic Auth and HMAC for Sinch webhooks?

Basic Authentication sends credentials (username and password) with each webhook request in the Authorization header. HMAC-SHA256 (Hash-based Message Authentication Code) offers stronger security by creating a cryptographic signature using the request body, nonce, timestamp, and your secret key. HMAC doesn't send credentials directly with each request, making it more secure. To configure HMAC authentication with Sinch, contact your account manager – it's managed at the account/service plan level and can be used alongside Basic Auth or OAuth 2.0. Basic Auth is simpler to implement and suitable for getting started, while HMAC is recommended for production deployments with higher security requirements.

How do Sinch Groups work for managing SMS subscribers?

Sinch Groups are collections of phone numbers (in E.164 format) that you can use as targets when sending SMS messages. You can create, update, and delete groups using the Sinch Node SDK. The Groups API handles duplicate adds and unknown removes gracefully (won't error if you add an existing member or remove a non-existent one). Groups use add and remove arrays to control membership, and additions are processed before deletions. While convenient for simple use cases, production systems often benefit from maintaining their own database for additional features like consent history, timestamps, and advanced querying.

What keywords must I support for SMS opt-out compliance?

For TCPA compliance, you must support standard opt-out keywords including STOP, QUIT, END, UNSUBSCRIBE, CANCEL, REVOKE, and OPT OUT (per se reasonable methods under the new FCC rules effective April 11, 2025). Best practice is also to support opt-in keywords like SUBSCRIBE, JOIN, and START, plus a HELP keyword to provide customer support information. Process these keywords case-insensitively and handle whitespace using .trim() and .toUpperCase() methods.

Can I test Sinch webhooks locally before deployment?

Yes, but Sinch requires a publicly accessible URL. For local testing, use ngrok or similar tunneling services to expose your local Next.js development server. Run ngrok http 3000 (matching your dev server port) to get a public HTTPS URL, then configure this URL in your Sinch Service Plan webhook settings. Use the ngrok web inspector (http://127.0.0.1:4040) to view incoming webhook requests and debug. Always test webhook authentication, keyword processing, group operations, and SMS replies before deploying to production.

Do I need a database for SMS consent management?

While this guide uses Sinch Groups for simplicity, production systems typically benefit from their own database. A database provides persistence independent of Sinch, ability to store additional user data (name, source, preferences), detailed consent history with timestamps, easier querying and segmentation, and better compliance auditing. Use PostgreSQL with Prisma, MySQL, or MongoDB depending on your needs. Store at minimum: phone number, subscription status, subscribed/unsubscribed timestamps, and consent source. This data helps prove compliance during audits and supports TCPA's 4-year record retention requirement.

---

12. Verification and Testing

1. Manual Verification:
   • Deploy the application and configure the Sinch webhook correctly (Section 3).
   • Using a test phone number (not the Sinch number itself), send SMS messages to your provisioned SINCH_NUMBER:
     • Send SUBSCRIBE (or Join, Start).
     • Expected: Receive the opt-in confirmation SMS. Check application logs for successful processing. Check Sinch Group members (via API or dashboard if possible) to confirm the number was added.
     • Send STOP (or Unsubscribe, Quit).
     • Expected: Receive the opt-out confirmation SMS. Check logs. Check Sinch Group to confirm removal.
     • Send HELP.
     • Expected: Receive the help message SMS. Check logs.
     • Send a random message (e.g., Hello).
     • Expected: No reply (based on current code). Check logs for ""unknown keyword"" message.
     • Send STOP again after unsubscribing.
     • Expected: Receive the opt-out confirmation SMS again (or handle gracefully, e.g., log ""already unsubscribed""). Check logs. Verify the number is still not in the group.