Frequently Asked Questions
Build a Next.js API route to handle Sinch SMS webhooks. This route processes keywords like 'SUBSCRIBE' and 'STOP', manages user membership in Sinch groups, and sends confirmation messages, ensuring compliance with regulations like TCPA and GDPR.
The Sinch SMS API and Node SDK are used to send and receive SMS messages programmatically, and to manage contact groups within the Sinch platform. This allows for automated handling of subscriptions and opt-outs.
US A2P 10DLC registration is required for legal SMS marketing campaigns. While this guide focuses on consent logic implementation, separate 10DLC registration through Sinch's TCR is mandatory for sending marketing messages in the US.
Set up Basic Authentication credentials for your Sinch webhook during the initial project setup. Define WEBHOOK_USERNAME and WEBHOOK_PASSWORD in your .env.local file and configure these in the Sinch dashboard to secure your endpoint.
Yes, specify the appropriate region by setting smsRegion (e.g., SmsRegion.EU) when you initiate the SinchClient if your service plan isn't in the default US region.
Create a file named src/pages/api/webhooks/sinch-sms.js. This file will contain the code to handle incoming SMS messages, process keywords, manage Sinch groups, and send SMS replies using the Sinch SDK.
The findOrCreateGroup function checks if a Sinch group with the specified name exists. If not, it creates the group. It returns the group ID, which is essential for managing group membership.
The provided API route code normalizes the incoming message body to uppercase and checks for keywords like 'SUBSCRIBE', 'JOIN', 'START', 'STOP', 'UNSUBSCRIBE', 'CANCEL', etc. Based on the keyword, it adds or removes the user from the Sinch marketing group.
Basic Authentication ensures that only Sinch can trigger your API route, preventing unauthorized access and potential abuse. Configure this in your Sinch dashboard and .env.local file.
A dedicated database offers persistent storage independent of Sinch, detailed consent history, easier querying and segmentation, and the ability to store additional user data beyond what Sinch Groups provide.
Implement webhook authentication (Basic or HMAC), input validation using libraries like zod or joi, HTTPS, rate limiting, secure environment variables, and least privilege API keys for enhanced security.
Verify the Callback URL in the Sinch Dashboard, check application logs, ensure deployment health, check Sinch for delivery errors, and confirm Basic Auth credentials match between your application and Sinch configuration.
Use libraries like async-retry with exponential backoff to handle transient network errors or temporary Sinch API issues. Be mindful of idempotency when retrying write operations to prevent unintended duplicate actions.
If your webhook processing involves time-consuming operations like database interactions or external API calls, offload these to background tasks or queues to ensure fast responses to Sinch and prevent webhooks from timing out.
Implement health checks, centralized logging, error tracking (e.g., Sentry), metrics tracking for key operations, and dashboards visualizing these metrics to gain insights into system health and performance.
SMS Consent Management with Next.js and Sinch: Complete TCPA Compliance Guide (2024-2025)
Build a TCPA-compliant SMS consent management system using Next.js and Sinch. This step-by-step tutorial shows you how to implement SMS opt-in and opt-out handling, process consent keywords (SUBSCRIBE, STOP, HELP), configure secure webhooks with Basic Auth, and meet the new FCC opt-out requirements effective April 11, 2025. Perfect for developers building SMS marketing campaigns with Next.js 15 and Sinch Node SDK 1.2.1. Includes complete API route code examples, 10DLC registration guidance, and automated consent tracking.
Why SMS Consent Management Matters for TCPA Compliance
Proper SMS consent management is legally required under TCPA (US) and GDPR (EU) regulations. Failing to handle opt-ins and opt-outs correctly results in fines of $500–$1,500 per text message, potential class-action lawsuits, and severe brand reputation damage. In 2020 alone, companies paid over $120 million in TCPA settlements for SMS marketing violations.
This guide walks you through building a robust SMS consent management system using Next.js and the Sinch SMS API. Create a Next.js application with an API endpoint that listens for incoming SMS messages sent to your Sinch number. This endpoint processes standard keywords like
SUBSCRIBEandSTOP, manages user participation in a Sinch group, and sends 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:
npm install @sinch/sdk-core @sinch/sms.System Architecture:
User sends SMS → Sinch receives → Webhook to Next.js API → Process keyword → Update Sinch Group → Send confirmation SMS → User receives reply
Prerequisites:
By the end of this guide, you'll have a deployable Next.js application capable of handling SMS consent keywords reliably and securely.
Step 1: Setting Up Your Next.js Project for SMS Webhooks
Create a new Next.js project and install the Sinch SDK to handle SMS webhooks and consent management.
Create a new Next.js App: Open your terminal and run:
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).Install Sinch SDK: Install the core Sinch SDK and the SMS module:
Set up Environment Variables: Create a file named
.env.localin the root of your project. Never commit this file to version control. Add your Sinch credentials and configuration. These variables store sensitive API credentials (to authenticate with Sinch), your phone number (the sender identity), group name (to organize subscribers), and webhook security credentials (to verify incoming requests from Sinch).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.Project Structure: Your
src/pages/api/directory is where the core webhook logic will reside. The rest follows a standard Next.js structure.Step 2: Implementing the SMS Webhook API Route
Create a Next.js API route at
/pages/api/webhooks/sinch-sms.jsto receive and process incoming SMS consent keywords from Sinch webhooks.Create the API Route File: Create a new file:
src/pages/api/webhooks/sinch-sms.jsImplement 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.Code Explanation:
SinchClientusing credentials from environment variables. Defines constants for configuration.findOrCreateGroup: Checks if the group defined bySINCH_MARKETING_GROUP_NAMEexists. If not, creates it usingsinchClient.sms.groups.create. Returns the group ID. Includes error logging for troubleshooting.sendSms: Sends an SMS usingsinchClient.sms.batches.send. Takes the recipient number and message body. Logs errors during sending.handleBasicAuth: Implements Basic Authentication checking based onWEBHOOK_USERNAMEandWEBHOOK_PASSWORD. Sends appropriate 401 responses if auth fails or is missing. Returnstrueon success,falseon failure.handler(Main Logic):POSTmethod.handleBasicAuthto secure the endpoint.req.body(Sinch sends JSON). Includes basic payload validation. Production applications should use more rigorous validation with libraries likezodorjoito strictly enforce the expected schema.from) and message body (body). Normalizes the body to uppercase for case-insensitive keyword matching.findOrCreateGroupto get the relevant group ID.if/else ifblocks to check forSUBSCRIBE/JOIN/START,STOP/UNSUBSCRIBE/etc., andHELPkeywords.sinchClient.sms.groups.replaceMemberswith theaddproperty to add the number to the group without affecting other members. The Sinch Groups API handles duplicate adds gracefully. Sends a confirmation SMS.sinchClient.sms.groups.removeMemberto remove the specific number. The API handles unknown removes gracefully. Sends an opt-out confirmation SMS.200 OKresponse back to Sinch to acknowledge successful processing. Sends500 Internal Server Errorif processing fails within thetry…catchblock.Step 3: Configuring Sinch Webhooks for Inbound SMS
Configure your Sinch Service Plan to send SMS webhooks to your deployed Next.js API endpoint with Basic Authentication.
Deploy Your Application: Deploy your Next.js application to get a publicly accessible URL. Use hosting providers like Vercel, Netlify, or AWS Amplify.
.env.local).https://your-app-name.vercel.app).https://your-app-name.vercel.app/api/webhooks/sinch-smsConfigure Sinch Webhook:
.env.local) and click on it or its associated settings/edit icon.https://your-app-name.vercel.app/api/webhooks/sinch-sms.env.local(WEBHOOK_USERNAME)..env.local(WEBHOOK_PASSWORD).Dashboard navigation might vary slightly. Consult the official Sinch documentation for the most up-to-date instructions on configuring SMS webhooks.
Step 4: Implementing Error Handling and Webhook Retries
try…catchblocks around the main processing logic and Sinch SDK calls. Errors are logged to the console (which Vercel or your hosting provider typically captures). A generic500 Internal Server Erroris sent back to Sinch upon failure, preventing leakage of internal details.console.logfor informational messages andconsole.errorfor errors. For production, integrate a dedicated logging service (like Logtail, Datadog, Sentry) for better aggregation, searching, and alerting.2xxresponse within a timeout period. Consult the official Sinch documentation for their webhook retry behavior. Ensure your endpoint responds quickly (even with an error code).sendSmsor group operations. For critical operations, wrap Sinch SDK calls in a retry mechanism (e.g., using libraries likeasync-retry) with exponential backoff, especially for transient network errors or temporary Sinch API issues (like 5xx errors). Be cautious about retrying operations that modify state. Implement idempotency keys or careful state checking when retrying write operations. Retryinggroups.list(read operation) is generally safe, whereas retryinggroups.addMember(write operation) could potentially lead to duplicate actions if the initial request succeeded but the response was lost.Step 5: Adding Database Storage for Consent Records (Optional)
While this guide uses Sinch Groups for simplicity, production systems often benefit from maintaining their own database to store subscriber information and consent status.
Install Prisma:
npm install prisma @prisma/client --save-devInitialize Prisma:
npx prisma init --datasource-provider postgresql(or your preferred DB)Configure
prisma/schema.prisma:Set
DATABASE_URLin.env.local.Run
npx prisma db pushto sync the schema.Modify the API route to import
PrismaClientand 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.
Step 6: Securing Your SMS Webhooks with Authentication
Webhook Authentication: Basic Authentication is implemented in this guide. This ensures 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, 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. Upgrade to HMAC for production deployments with higher security requirements.
HMAC Implementation Example:
Input Validation: Basic checks ensure the payload exists and has
fromandbody. More robust validation could use libraries likezodorjoito 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.localsecure 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 has only the necessary permissions (SMS sending, group management) required for this application.
Step 7: Handling Edge Cases in SMS Consent Processing
.toUpperCase())..trim()on the message body.SUBSCRIBE/JOIN/STARTandSTOP/UNSUBSCRIBE/etc. Add more synonyms as needed.+followed by country code and number) as provided by Sinch.isSubscribedstatus in a database (if used) before performing the action.Step 8: Optimizing Webhook Performance
SinchClientis initialized outside the handler function, so it's reused across requests within the same serverless function instance, improving performance.findOrCreateGroupcall involves an API request. If performance is critical and the group rarely changes, cache thegroupIdin memory (with a short TTL) or a faster cache like Redis, but this adds complexity and is likely unnecessary for typical volumes.Step 9: Monitoring SMS Consent Management
Health Checks: Create a simple health check endpoint (e.g.,
src/pages/api/health.js) that returns200 OKto verify the deployment is live.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:
SUBSCRIBE,STOP,HELPevents processedDashboards: 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:
findOrCreateGroup)Step 10: Troubleshooting Common SMS Webhook Issues
.env.local/Vercel variables and Sinch configuration.WEBHOOK_USERNAMEandWEBHOOK_PASSWORDmatch exactly between your environment variables and the Sinch webhook configuration.Authorization: Basic …header is being sent correctly by Sinch (usually automatic if configured).401 Unauthorized: CheckSINCH_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 incorrectSINCH_SERVICE_PLAN_ID.404 Not Found: Could mean thegroupIdis incorrect or the resource doesn't exist.5xx Server Error: Transient issue on Sinch's side. Consider implementing retries (see Section 4).SinchClientConfiguration: Ensure the correctsmsRegion(e.g.,SmsRegion.EU,SmsRegion.US) is configured when initializing theSinchClientif your service plan is not based in the default US region.groupIdbeing used is correct. Use the Sinch API (or dashboard, if available) to inspect the group members directly.fromNumberis being parsed correctly from the webhook.Step 11: Deploying Your SMS Consent System to Production
.env.localis in your.gitignore..env.localinto the Vercel Project Settings > Environment Variables section. Ensure they are available for the "Production" environment (and Preview/Development if needed).npm run testscript to yourpackage.jsonand configure Vercel (or your CI/CD tool like GitHub Actions) to run tests before deploying. Fail the build if tests fail.Step 12: Testing SMS Opt-In and Opt-Out Flows
Manual Verification:
SINCH_NUMBER:SUBSCRIBE(orJoin,Start).STOP(orUnsubscribe,Quit).HELP.Hello).STOPagain after unsubscribing.Automated Testing Examples:
Unit Test (Jest):
Integration Test (with mock Sinch client):
Frequently Asked Questions About SMS Consent Management
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. Obtain explicit written consent before sending marketing SMS messages, and honor opt-out requests promptly. Failure to comply results 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. 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. 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?
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. 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, 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. Test webhook authentication, keyword processing, group operations, and SMS replies before deploying to production.Local Testing Steps:
npm run devngrok http 3000https://abc123.ngrok.io)https://abc123.ngrok.io/api/webhooks/sinch-smsDo 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.