Implementing SMS Authentication in Next.js: Complete in 5 Minutes Without Paperwork

The Hidden Struggle of Adding SMS Authentication to Side Projects

When building a login, registration, or password recovery feature, the most reliable method for user verification is SMS Authentication (OTP). However, just when you think, "I'll quickly integrate SMS verification and deploy my Next.js project!" you often hit a massive roadblock.

If you've looked into traditional SMS API providers, you've probably noticed their tedious onboarding processes:

• Business Registration Required: What if you're a solo indie developer or a student?
• Sender Number Pre-registration: You must submit telecom carrier certificates and wait days for approval.
• High Costs: Traditional APIs charge heavily per message, which quickly burdens startups testing their MVP.

These bureaucratic hurdles can completely kill your motivation, especially when you're in the MVP phase and need to validate your ideas quickly.

In this article, I will show you how to implement SMS authentication in your Next.js project in just 5 minutes—without submitting a single piece of paperwork.

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Why This Architecture?

Calling an SMS API directly from the client (browser) exposes your API keys, leading to severe security vulnerabilities. Therefore, we must use Next.js Route Handlers (App Router) to create proxy APIs that securely communicate with the external SMS service from the server side.

For this tutorial, we will use EasyAuth, a developer-friendly SMS API that requires zero paperwork, making it perfect for rapid integration.

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Step-by-Step Implementation Guide

Step 1. Environment Variable Setup

To keep your API key secure, store your EasyAuth API key in the .env.local file at the root of your project.

# .env.local
EASYAUTH_API_KEY="your_easyauth_api_key_here"

Step 2. Create Next.js API Routes (Server-Side Proxy)

We will create two endpoints using the Next.js App Router: /api/auth/send and /api/auth/verify.

1. Send OTP Code API (app/api/auth/send/route.ts)

import { NextResponse } from 'next/server';

export async function POST(request: Request) {
  try {
    const { phone } = await request.json();

    if (!phone) {
      return NextResponse.json({ error: 'Phone number is required.' }, { status: 400 });
    }

    // Call EasyAuth API to send the verification code
    const response = await fetch('https://api.easyauth.co.kr/send', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${process.env.EASYAUTH_API_KEY}`
      },
      body: JSON.stringify({ phone })
    });

    const data = await response.json();
    return NextResponse.json(data);

  } catch (error) {
    return NextResponse.json({ error: 'Failed to send SMS.' }, { status: 500 });
  }
}

2. Verify OTP Code API (app/api/auth/verify/route.ts)

import { NextResponse } from 'next/server';

export async function POST(request: Request) {
  try {
    const { phone, code } = await request.json();

    if (!phone || !code) {
      return NextResponse.json({ error: 'Phone number and code are required.' }, { status: 400 });
    }

    // Call EasyAuth API to verify the code
    const response = await fetch('https://api.easyauth.co.kr/verify', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${process.env.EASYAUTH_API_KEY}`
      },
      body: JSON.stringify({ phone, code })
    });

    const data = await response.json();
    return NextResponse.json(data);

  } catch (error) {
    return NextResponse.json({ error: 'Verification failed.' }, { status: 500 });
  }
}

Step 3. Create the Client UI Component

Now, let's build the frontend UI where users can input their phone number and the OTP code. We'll use TailwindCSS for quick and clean styling.

// app/page.tsx
'use client';

import { useState } from 'react';

export default function SmsVerification() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState<'INPUT_PHONE' | 'INPUT_CODE' | 'SUCCESS'>('INPUT_PHONE');
  const [loading, setLoading] = useState(false);

  // Function to send the verification code
  const handleSendCode = async () => {
    setLoading(true);
    try {
      const res = await fetch('/api/auth/send', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ phone })
      });
      
      if (res.ok) {
        alert('Verification code sent successfully.');
        setStep('INPUT_CODE');
      } else {
        alert('Failed to send verification code.');
      }
    } finally {
      setLoading(false);
    }
  };

  // Function to verify the entered code
  const handleVerifyCode = async () => {
    setLoading(true);
    try {
      const res = await fetch('/api/auth/verify', {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ phone, code })
      });
      
      const data = await res.json();
      if (data.success || res.ok) {
        alert('Verification successful!');
        setStep('SUCCESS');
      } else {
        alert('Invalid verification code.');
      }
    } finally {
      setLoading(false);
    }
  };

  return (
    <div>
      <h2>Phone Verification</h2>
      
      {step === 'SUCCESS' ? (
        <div>
          Authentication completed successfully! 🎉
        </div>
      ) : (
        <div>
          <div>
             setPhone(e.target.value)}
              disabled={step === 'INPUT_CODE'}
              className="w-full p-3 border rounded focus:outline-none focus:ring-2 focus:ring-blue-500"
            /&gt;
          </div>

          {step === 'INPUT_PHONE' &amp;&amp; (
            
              {loading ? 'Sending...' : 'Send OTP'}
            
          )}

          {step === 'INPUT_CODE' &amp;&amp; (
            &lt;&gt;
              <div>
                 setCode(e.target.value)}
                  className="w-full p-3 border rounded focus:outline-none focus:ring-2 focus:ring-blue-500"
                /&gt;
              </div>
              
                {loading ? 'Verifying...' : 'Verify Code'}
              
            &lt;/&gt;
          )}
        </div>
      )}
    </div>
  );
}

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Tips & Best Practices

1. Implement Rate Limiting
   To prevent malicious users from repeatedly calling the OTP endpoint and racking up SMS costs, you must implement rate limiting. It is highly recommended to use Next.js Middleware or tools like Upstash Redis to limit the number of API calls per IP address per day.

2. Validate Phone Number Formats
   Use regular expressions on both the client and server sides to validate that the input matches proper phone number formats before triggering the API. This prevents unnecessary API requests and saves you money.

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Conclusion: EasyAuth, the Easiest SMS API for Developers

As demonstrated in this tutorial, combining the Next.js App Router with a straightforward API structure allows you to implement SMS authentication in mere minutes. The code is ready, but are you still hesitating because of the tedious onboarding of traditional SMS providers?

EasyAuth is designed to solve all these headaches for you.

• Zero Paperwork: Absolutely no business registration or telecom certificates required.
• Instant Auto-Sender Number: A sender number is automatically assigned immediately upon signup—no need to register a representative number.
• Highly Cost-Effective: At just 15~25 KRW per message, it's roughly half the price of legacy services (which charge 30-50 KRW), significantly reducing the financial burden for startups and indie hackers.
• Start Instantly for Free: You receive 10 free SMS credits right upon signup to test your integration immediately.

Stop wasting your valuable time on administrative paperwork and manual approvals. Focus entirely on your product development and core business logic. Sign up for EasyAuth today and finish your SMS authentication integration in just 5 minutes!