Frequently Asked Questions
Use the Vonage Verify API with Fastify and Node.js. Create a Fastify API endpoint that takes the user's phone number and makes a request to the Vonage Verify API to initiate the OTP process. This sends a unique OTP to the user's phone number via SMS. The response includes a request ID for verification.
The Vonage Verify API handles generating, sending, and verifying one-time passwords (OTPs) for two-factor authentication (2FA). It simplifies the process by providing pre-built logic and infrastructure for OTP delivery via SMS or voice calls. This offloads the complexity of building and maintaining an OTP system yourself.
Fastify is a high-performance Node.js web framework known for its speed and extensibility. Its built-in features like schema validation, logging, and a plugin-friendly architecture make it a strong choice for building robust and maintainable APIs. The low overhead also adds to improved efficiency.
Implement SMS OTP/2FA for actions requiring enhanced security, such as user registration, login, password resets, and sensitive transactions. This provides a strong second layer of authentication. The Vonage Verify API makes implementing 2FA easier.
While the default is 4 digits, you can configure Vonage to send 6-digit OTPs or define custom workflows. Uncomment and adjust the code_length or workflow_id options in the vonage.verify.start() call in the Node.js code for customizations.
You'll need Node.js v16 or later with npm/yarn, a Vonage API account with an API key and secret, and a basic understanding of APIs and asynchronous JavaScript. Installing the Vonage CLI is optional but helpful. These steps allow you to get up and running quickly.
Your Fastify API should have a separate endpoint that receives the requestId (from the OTP request) and the code entered by the user. Call vonage.verify.check() to verify the OTP against Vonage's records and return a verification success or failure status to the client app based on the response.
The dotenv module loads environment variables from a .env file into process.env.  This is crucial for keeping sensitive data like your Vonage API credentials secure. The file is put in the .gitignore file and not committed to the repository.
Fastify uses JSON Schema to validate request bodies. Define the expected request structure in route options. Fastify automatically checks incoming requests and returns a 400 Bad Request error if they don't match, enhancing security and providing clear error messaging to clients.
A status of '0' in the Vonage Verify API response means the request was successfully initiated. This typically appears after calling vonage.verify.start() indicating Vonage has accepted the request to send the OTP. It does not guarantee delivery.
Adding prefixes like /api/v1 to your API routes is a best practice for versioning. This allows you to introduce future versions (e.g., /api/v2) with breaking changes without impacting existing integrations that rely on the older version.
Check the status code in the Vonage Verify API response. Non-zero status codes indicate an error and should be handled appropriately. The error_text provides further details.  Use a centralized error handler in Fastify and custom messages for specific error codes.
Use the @fastify/rate-limit plugin to protect your OTP endpoints from abuse.  Configure global limits and/or per-route limits to control how often clients can request or verify OTPs.  Use the keyGenerator option to customize which factor (IP, phone number, requestId) the rate limiting should apply to.
Key security measures include: always using HTTPS, rate limiting OTP requests, validating and sanitizing input, storing API keys securely (environment variables, secrets management), using Helmet for security headers, and writing thorough automated tests to ensure the reliability of your OTP system.
SMS OTP two-factor authentication (2FA) adds critical security by sending One-Time Passwords to users' mobile devices during login or sensitive operations. This comprehensive guide teaches you how to implement production-ready SMS OTP verification using Node.js, the high-performance Fastify framework, and the Vonage Verify API v2. You'll learn security best practices, implement rate limiting, handle errors gracefully, and write automated tests for reliable authentication.
Build a complete backend API with two secure endpoints: one to request an OTP code sent via SMS to a user's phone number in E.164 format, and another to verify the OTP code entered by the user.
What Are You Building with Vonage and Fastify?
What You're Building:
A backend API service built with Node.js and Fastify that:
Problem Solved:
This implementation addresses the need for secure, out-of-band verification commonly used during:
Technologies Used:
dotenv: Module to load environment variables from a.envfile intoprocess.env.@vonage/server-sdk: Official Vonage Node.js SDK for interacting with Vonage APIs.System Architecture:
Your system follows this flow:
/request-otp/verify-otpPrerequisites:
npm install -g @vonage/cli.Technology Versions Used in This Guide:
Note on Vonage Verify API: This guide uses the Vonage Verify v2 API, which supports multiple authentication channels including SMS, Voice, Email, WhatsApp, and Silent Authentication. The API automatically handles OTP generation, delivery, expiration, and verification logic.
Important Security Considerations:
+14155552671for US numbers,+442071234567for UK numbers). The Vonage API expects this international format and may reject improperly formatted numbers. Common format errors: missing+prefix, including spaces or dashes, omitting country code.Sources: Fastify Official Site, Vonage Server SDK npm, Vonage Verify API Documentation, Vonage Verify v2 Fraud Protection, accessed October 2025.
Expected Outcome:
A functional Fastify API service capable of sending and verifying SMS OTPs via Vonage, ready for integration into a larger application. The service will include basic security, error handling, and logging.
How Does SMS OTP 2FA Work?
OTP Generation and Security:
The Vonage Verify API generates cryptographically secure random codes (typically 4–6 digits) that are valid for a limited time window. This approach provides:
Verification Flow:
request_idrequest_idand user-entered code to Vonage0) if the code matches and is still valid, or an error status if it doesn't match, expired, or was already usedSecurity Principles:
How Do You Set Up a Fastify Project for OTP Verification?
Initialize your Node.js project and install Fastify along with necessary dependencies.
1. Create Project Directory:
Open your terminal and create a new directory for the project, then navigate into it.
2. Initialize npm Project:
This creates a
package.jsonfile with default settings.3. Install Dependencies:
Install Fastify, the Vonage SDK,
dotenvfor environment variables, andpino-prettyfor development logging.fastify: The core web framework.@vonage/server-sdk: The official Vonage Node.js library.dotenv: Loads environment variables from a.envfile.pino-pretty: Makes Fastify's default JSON logs human-readable during development.4. Configure
package.jsonfor Development:Add a
devscript topackage.jsonto run the server with readable logs usingpino-pretty.(Note: Replace
^...with actual installed versions if needed, butnpm installhandles this.)5. Create Project Structure:
Organize the project for clarity:
src/: Contains the main application code.src/routes/: Will hold our API route definitions.src/server.js: The main entry point for the Fastify application..env: Stores sensitive information like API keys (DO NOT commit this file)..gitignore: Specifies intentionally untracked files that Git should ignore.6. Configure
.gitignore:Add
node_modulesand.envto your.gitignorefile to prevent committing them.7. Set Up Environment Variables:
Open the
.envfile and add your Vonage API credentials and a brand name for the SMS message.Replace
YOUR_API_KEYandYOUR_API_SECRETwith the actual credentials from your Vonage API Dashboard.8. Basic Fastify Server Setup:
Create the initial server configuration in
src/server.js.Explanation:
require('dotenv').config();: Loads variables from.envintoprocess.env. Crucially done before other modules might need them.fastify({ logger: true }): Initializes Fastify with logging enabled. Pino is used by default.fastify.listen(): Starts the server. We listen on0.0.0.0to make it accessible within Docker containers or VMs if needed later.Run
npm run devin your terminal. You should see log output indicating the server is running, likely on port 3000. Stop the server withCtrl+C.How Do You Integrate the Vonage Verify API?
Initialize the Vonage SDK client and make it available within your Fastify application.
1. Initialize Vonage Client:
Update
src/server.jsto create and configure the Vonage client using the environment variables. We'll use Fastify'sdecorateutility to make the client accessible within route handlers viarequest.vonageorfastify.vonage.Explanation:
Vonageclass from the SDK.Vonageusing the API key and secret loaded from.env.fastify.decorate('vonage', vonage)adds thevonageinstance to Fastify's application context, making it easily accessible in routes and plugins.How Do You Implement the OTP Request and Verification Flow?
Create two API endpoints within a dedicated route file:
POST /request-otp: Initiates the OTP process.POST /verify-otp: Verifies the submitted OTP.1. Create Route File:
Create a new file
src/routes/otp.js.2. Define Routes:
Add the route logic to
src/routes/otp.js. Use Fastify's schema validation for request bodies.Explanation:
otpRoutes(fastify, options): Standard Fastify plugin structure.requestOtpSchema,verifyOtpSchema): Define the expected structure and types for the request bodies. Fastify automatically validates incoming requests against these schemas and returns a 400 Bad Request error if validation fails.additionalProperties: falseprevents unexpected fields./request-otp:phoneNumberfrom the validatedrequest.body.vonageclient.vonage.verify.start()with the phone number and brand name.response.status. Astatusof'0'means Vonage accepted the request and will attempt to send the SMS. Other statuses mean an error occurred before sending (e.g., invalid number format, throttling).requestIdon success, which the client needs for the verification step.try...catchfor network/SDK errors./verify-otp:requestIdandcodefrom the validatedrequest.body.vonage.verify.check()with therequestIdand the user-providedcode.response.status:'0'means the code was correct.try...catchfor network/SDK errors during the check.For a complete list of Vonage Verify API error status codes, see the Vonage Verify API Error Codes Reference.
3. Register Routes in Server:
Register these routes in
src/server.js.Explanation:
otpRoutesfunction.fastify.register(otpRoutes, { prefix: '/api/v1' }): Registers all routes defined inotp.jsunder the/api/v1path prefix (e.g.,/api/v1/request-otp). Using a prefix is good practice for API versioning.At this point, you have the core API functionality. Test it using
curlor a tool like Postman after starting the server (npm run dev).Testing with
curl:Replace
+1XXXXXXXXXXwith a real phone number you can receive SMS on (use E.164 format).Request OTP:
Expected Response (Success):
You should receive an SMS with a 4-digit code (by default).
Verify OTP: Replace
YOUR_UNIQUE_REQUEST_IDwith the ID from the previous step andYOUR_OTP_CODEwith the code from the SMS.Expected Response (Success):
Expected Response (Incorrect Code):
(Note: The HTTP status code will be 400 for incorrect code)
Common Testing Issues:
.envfile has correct credentialsHow Do You Add Error Handling and Logging?
Fastify's built-in logger (Pino) is already active. Add a centralized error handler to catch unhandled exceptions and format error responses consistently.
1. Add Custom Error Handler:
Update
src/server.jsto includesetErrorHandler.Explanation:
fastify.setErrorHandler: Registers a function to handle errors that occur during request processing after the initial routing but before a reply is sent, or errors explicitly passed toreply.send(error).request.log.error(error).error.validation) and returns a structured 400 response.statusCodeandmessageif available (falling back to 500).NODE_ENVis not 'production' to avoid leaking sensitive information.Logging Best Practices:
request.log.info,request.log.error,request.log.warn) to provide context about the OTP flow.npm run dev),pino-prettyformats these JSON logs nicely. In production, pipe the raw JSON logs to a log management system (e.g., Datadog, ELK stack, Splunk) for analysis and alerting.Key Log Queries for Monitoring:
status:"failed" AND requestId:*– Identify suspicious patternsmessage:"Rate limit exceeded"– Adjust limits if legitimate users are blockederror_text:* AND status:!0– Track API reliabilityphoneNumber:"+9*"– Monitor international SMS costsWhat Security Features Does Your OTP System Need?
Security is paramount for an authentication mechanism.
1. Rate Limiting:
Protect against brute-force attacks on both requesting and verifying OTPs. Learn more about implementing 2FA security best practices for SMS authentication.
Install Rate Limiter Plugin:
Register and Configure: Add this to the "Plugin Registration" section in
src/server.js.Recommended Rate Limit Configuration:
/request-otp/verify-otpApply Specific Limits (Optional but Recommended): You can override the global settings within specific route options in
src/routes/otp.js. It's wise to have stricter limits on OTP requests than general API usage.Explanation:
@fastify/rate-limit: Provides robust rate limiting based on IP address by default.max,timeWindow: Control how many requests are allowed within a specific timeframe.errorResponseBuilder: Customizes the response when the limit is hit.keyGenerator: Allows using factors other than IP (like phone number,requestId, or authenticated user ID) for more granular limiting. Use with caution, as poorly chosen keys can block legitimate users. IP-based is often the simplest starting point.config.rateLimitoverrides the global settings for finer control.2. Input Validation and Sanitization:
google-libphonenumberon the backend before sending to Vonage provides earlier, more specific feedback to the user about formatting issues.Phone Number Validation Example:
3. Secure Handling of Secrets:
.envand loaded viadotenv..gitignore: The.envfile is in.gitignoreto prevent accidental commits..envfiles directly to production servers.4. Other Considerations:
@fastify/helmetto set various security-related HTTP headers (likeX-Frame-Options,Strict-Transport-Security).src/server.jswithin the "Plugin Registration" section:How Do You Test Your SMS OTP API?
Write automated tests to ensure reliability. Use
tap, Fastify's default test runner.1. Install
tapas a Dev Dependency:2. Update
testscript inpackage.json:(Note: Added
@fastify/helmet,@fastify/rate-limitto dependencies andtap,pino-prettyto devDependencies based on previous steps. Ensure versions match your installation.)3. Create Test File Structure:
4. Write Comprehensive Tests:
Here's an example testing multiple scenarios for both endpoints, mocking the Vonage API call.
5. Create Test Helper (Conceptual):
A file like
test/helper.jswould typically contain a function to build and configure the Fastify app instance for testing, allowing injection of mocks and handling teardown. Creating a full helper is beyond the scope of this guide, but the test file structure assumes its existence.Run tests using
npm test. Add comprehensive tests covering success paths, error paths (Vonage errors, validation errors), and security features like rate limiting.Integration Testing with Vonage:
For integration tests with real Vonage test numbers:
+15005550) that don't send real SMS+15005550006– Vonage treats this as valid but doesn't deliver SMStest/integration/) to avoid incurring costs during developmentWhat Should You Consider When Deploying to Production?
Follow these critical best practices when deploying your SMS OTP service to production:
Environment Variables and Secrets Management:
.envfiles to production servers. Use your hosting platform's environment variable management (AWS Systems Manager, Heroku Config Vars, Google Secret Manager, Azure Key Vault).HTTPS and Transport Security:
@fastify/helmetplugin (covered in Section 5) sets important security headers includingStrict-Transport-Security(HSTS).Logging and Monitoring:
In production, pipe Fastify's JSON logs to a log aggregation service (Datadog, ELK Stack, Splunk, Papertrail) for analysis and alerting.
Key Metrics to Monitor:
Alert Patterns:
Rate Limiting Tuning:
Testing Before Production:
Disaster Recovery and Failover:
/healthendpoints that verify Vonage API connectivityCompliance and Privacy:
Ensure your OTP implementation complies with telecommunications regulations:
Implementation Checklist:
Cost Management:
Sources: Fastify Production Best Practices, OWASP Authentication Cheat Sheet, Vonage API Security Best Practices, accessed October 2025.
Frequently Asked Questions About SMS OTP 2FA
How do I implement OTP verification in Node.js?
Implement OTP verification in Node.js by using the Vonage Verify API with Fastify. Install the
@vonage/server-sdk, create routes for sending OTP codes via/request-otp, and verify user-submitted tokens via/verify-otp. Usevonage.verify.start()to generate and send OTP codes, thenvonage.verify.check()to validate user input against the codes stored by Vonage.What is the Vonage Verify API?
The Vonage Verify API handles OTP generation, SMS delivery, and token verification for two-factor authentication. It automatically generates cryptographically secure random codes (4–6 digits), sends them via SMS, voice, email, or WhatsApp, and validates user-submitted tokens within configurable timeout windows (5–10 minutes by default).
Why use Fastify for SMS authentication?
Use Fastify for SMS authentication because it provides 6x faster performance than Express with built-in schema validation via JSON Schema and a robust plugin ecosystem. Fastify v5's native async/await support and low overhead make it ideal for high-throughput authentication services handling thousands of concurrent OTP requests.
What Node.js version is required for this tutorial?
This tutorial requires Node.js v20 or later. Fastify v5 specifically requires Node.js v20+ for native async hooks and performance optimizations. The
@vonage/server-sdkv3.24+ works with Node.js v14+ but performs best on v20+.How long do Vonage OTP codes remain valid?
Vonage OTP codes remain valid for 5–10 minutes by default, depending on the workflow configuration. You can configure expiration by setting the workflow_id parameter when calling
verify.start(). Expired codes cannot be verified and require users to request a new OTP.How do I secure my Vonage API credentials?
Store Vonage API keys in environment variables via
.envfiles (never commit to version control). Use live keys (starting withlive_) for production and test keys for development. Consider IP whitelisting in the Vonage dashboard if your server uses static IPs, and monitor usage for anomalies via the dashboard.What is E.164 phone number format?
E.164 is the international phone number format used by telecommunications systems. It starts with a
+followed by the country code and national number with no spaces or special characters (e.g.,+14155552671for US,+442071234567for UK). The Vonage API requires E.164 format and may reject improperly formatted numbers.How do I implement rate limiting for OTP endpoints?
Implement rate limiting using
@fastify/rate-limitplugin. Configure stricter limits for OTP request endpoints (3–5 requests per hour per IP) compared to verification endpoints (5–10 per 15 minutes). This prevents SMS flooding, reduces costs, and protects against brute-force attacks while allowing legitimate user typos.Can I use Vonage OTP for login authentication?
Yes, Vonage OTP is ideal for login authentication as a second factor beyond passwords. After users enter their username and password, send an OTP to their registered phone number via
/request-otp, then verify the code they enter via/verify-otp. This significantly enhances account security against credential theft. For more authentication patterns, see our guide on implementing two-factor authentication in Node.js.How much does each Vonage OTP request cost?
Each Vonage OTP request costs approximately $0.06–$0.10 USD depending on the destination country. Implement proper rate limiting, phone number validation, and consider adding CAPTCHA before OTP requests to prevent malicious actors from generating excessive verification requests that increase costs.
Conclusion
You've built a production-ready SMS OTP verification system using Fastify and the Vonage Verify API. This implementation includes:
Next Steps:
Integrate this API into your application with these specific patterns:
Frontend Integration:
User Account Association:
requestIdto user sessions during registration/login flowsMulti-Channel Support:
Scaling Strategies:
Additional Resources:
This guide provides a solid foundation for implementing secure SMS-based two-factor authentication in your Node.js applications.