😫 Why SMS Authentication Usually Slows You Down

When building a side project or startup MVP, you inevitably reach a point where you need SMS authentication (OTP) for user signups to prevent spam and verify real users.

However, trying to integrate traditional SMS APIs quickly becomes a nightmare:

• Mountains of Paperwork: You need to submit business registration certificates and telecom service proofs.
• Long Approval Times: Registering a verified sender ID can take days.
• High Costs: Traditional providers charge around 30~50 KRW per message, which adds up for early-stage projects.

For solo developers, freelancers, and startups looking to quickly test a hypothesis, this friction is a massive bottleneck.

💡 The Solution: Next.js SMS Auth in 5 Minutes

In this tutorial, we will learn how to implement SMS authentication in a Next.js (App Router) environment in just 5 minutes—without submitting a single document.

By leveraging EasyAuth, a developer-friendly SMS API, you can complete your entire authentication logic using just two endpoints: Send and Verify.

🛠️ Step-by-Step Implementation Guide

We will build a secure server-side API Route Handler in Next.js 14+ App Router and connect it to a client-side UI.

Step 1. Environment Variables

First, set up your API key in the .env.local file at the root of your project.

EASYAUTH_API_KEY=your_easyauth_api_key_here

Step 2. Send OTP API Route (/api/send)

Create a file at app/api/send/route.ts. This server-side endpoint will receive the phone number from the client and request the EasyAuth API to send the SMS.

import { NextResponse } from 'next/server';

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

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

    if (!response.ok) throw new Error('Failed to send');
    
    return NextResponse.json({ success: true, message: 'Verification code sent.' });
  } catch (error) {
    return NextResponse.json({ success: false, error: 'Server error occurred.' }, { status: 500 });
  }
}

Step 3. Verify OTP API Route (/api/verify)

Create a file at app/api/verify/route.ts. This endpoint checks if the 6-digit code (OTP) entered by the user is correct.

import { NextResponse } from 'next/server';

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

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

    if (!response.ok) throw new Error('Verification failed');

    return NextResponse.json({ success: true, message: 'Successfully verified.' });
  } catch (error) {
    return NextResponse.json({ success: false, error: 'Invalid verification code.' }, { status: 400 });
  }
}

💻 Complete Code: Frontend Integration

Now, let's create the actual client component that interacts with the APIs we just built. Add the following code to app/signup/page.tsx.

'use client';

import { useState } from 'react';

export default function SignupPage() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState<'IDLE' | 'SENT' | 'VERIFIED'>('IDLE');

  const handleSendSMS = async () => {
    const res = await fetch('/api/send', {
      method: 'POST',
      body: JSON.stringify({ phone })
    });
    if (res.ok) {
      setStep('SENT');
      alert('A 6-digit verification code has been sent.');
    }
  };

  const handleVerifyCode = async () => {
    const res = await fetch('/api/verify', {
      method: 'POST',
      body: JSON.stringify({ phone, code })
    });
    if (res.ok) {
      setStep('VERIFIED');
      alert('Verification successful! 🎉');
    } else {
      alert('Invalid verification code.');
    }
  };

  return (
    <div>
      <h2>Phone Verification</h2>
      
      <div>
         setPhone(e.target.value)}
          disabled={step === 'VERIFIED'}
          className="border p-2 flex-1 rounded"
        /&gt;
        
          {step === 'SENT' ? 'Resend' : 'Send Code'}
        
      </div>

      {step !== 'IDLE' &amp;&amp; (
        <div>
           setCode(e.target.value)}
            disabled={step === 'VERIFIED'}
            className="border p-2 flex-1 rounded"
          /&gt;
          
            Verify
          
        </div>
      )}
    </div>
  );
}

🛡️ Tips & Best Practices for Production

1. 3-Minute Timeout Limit: SMS codes usually expire in 3 minutes. Implementing a visual countdown timer on the client side using setInterval greatly improves UX.
2. Rate Limiting: Protect your /api/send endpoint from malicious abuse to avoid unexpected billing spikes. Use Next.js Middleware or an Upstash Redis integration to limit requests to 3 times per minute per IP.
3. Benefits of Auto Sender IDs: By using EasyAuth, you don't have to manually register your sender ID. The API utilizes a pre-configured platform sender number, meaning you can start sending SMS immediately.

🎉 Conclusion: The Fastest Way to Validate Your MVP

We've covered how to implement SMS authentication in Next.js. What would normally take days of paperwork and waiting has been accomplished in just 5 minutes of coding.

For startup MVPs and solo developers where execution speed is everything, EasyAuth is the optimal choice.

• Zero Paperwork / Auto Sender ID included
• Highly affordable at just 15~25 KRW per message (cheaper than the standard 30~50 KRW)
• 10 Free Credits immediately upon sign-up

Stop wasting time building authentication infrastructure. Focus on developing your core business logic, and get your SMS verification done today with EasyAuth!