When building a side project or a startup MVP, you inevitably hit a frustrating roadblock: SMS phone authentication.

The Problem with Traditional SMS APIs

Integrating legacy telecom authentication or large messaging gateways usually requires jumping through massive administrative hoops:

• Submitting business registration certificates.
• Applying for pre-approved Sender IDs (which involves proof of telecom service).
• Waiting days or even weeks for manual review.
• High costs per SMS (around 30 to 50 KRW).

It makes you think, "I just wanted to add a simple login flow for my toy project..."

In this tutorial, we will learn how to implement SMS OTP (One-Time Password) verification in Next.js 14 (App Router) in just 5 minutes—without submitting a single piece of paperwork.

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1. Solution Overview (API Structure)

The flow for SMS authentication is incredibly simple. We only need two API endpoints:

1. POST /send: Sends a 6-digit OTP code to the user's phone number.
2. POST /verify: Verifies the code entered by the user.

We will set up server-side Route Handlers in Next.js to ensure our external API keys remain secure.

2. Setting Up Next.js Server APIs

Exposing your SMS provider's API key on the client is a severe security risk. Let's create our own backend routes under the app/api directory.

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

import { NextResponse } from 'next/server';

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

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

    if (!response.ok) {
      return NextResponse.json({ error: 'Failed to send SMS.' }, { status: 400 });
    }

    return NextResponse.json({ success: true, message: 'OTP sent successfully.' });
  } catch (error) {
    return NextResponse.json({ error: 'Internal Server Error.' }, { status: 500 });
  }
}

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

import { NextResponse } from 'next/server';

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

    // Call EasyAuth Verify API
    const response = await fetch('https://api.easyauth.io/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();

    if (!response.ok || !data.verified) {
      return NextResponse.json({ error: 'Invalid verification code.' }, { status: 400 });
    }

    return NextResponse.json({ success: true, message: 'Verification complete.' });
  } catch (error) {
    return NextResponse.json({ error: 'Internal Server Error.' }, { status: 500 });
  }
}

3. Client UI (React Component)

Now, let's create a client-side component where users can input their phone number, request an SMS, and verify the code.

app/page.tsx

'use client';

import { useState } from 'react';

export default function PhoneAuth() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState(1); // 1: Input Phone, 2: Input Code

  const handleSend = async () => {
    const res = await fetch('/api/auth/send', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ phone })
    });
    
    if (res.ok) {
      alert('OTP has been sent.');
      setStep(2);
    } else {
      alert('Failed to send. Please try again.');
    }
  };

  const handleVerify = async () => {
    const res = await fetch('/api/auth/verify', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ phone, code })
    });

    if (res.ok) {
      alert('Verification successful!');
      // TODO: Proceed with login or routing
    } else {
      alert('Invalid code.');
    }
  };

  return (
    <div>
      <h2>Phone Authentication</h2>
      
      {step === 1 ? (
        <div>
           setPhone(e.target.value)}
            className="border p-2 rounded"
          /&gt;
          
            Send Code
          
        </div>
      ) : (
        <div>
           setCode(e.target.value)}
            className="border p-2 rounded"
          /&gt;
          
            Verify Code
          
        </div>
      )}
    </div>
  );
}

4. Tips & Best Practices

1. Rate Limiting: Use tools like Upstash Redis to limit the number of SMS requests per IP address to prevent SMS pumping attacks and abuse.
2. Countdown Timer: OTP codes usually expire in 3 minutes (180 seconds). Implementing a visual countdown timer drastically improves the user experience.
3. Input Validation: Use regex on the client side to ensure the phone number string contains only numbers before sending it to the API.

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Conclusion: EasyAuth, the Easiest Way to Send SMS

We’ve walked through implementing SMS phone authentication in Next.js. While the code is straightforward, actually getting permission from telecom operators to send messages usually takes over a week of bureaucratic pain.

If you just want to focus on development, try [EasyAuth (이지어스)].

• 🚫 No Paperwork: Zero requirement for business registration certificates or service proof.
• 🚀 Instant Start: Get an automatic sender ID and complete integration in 5 minutes after signup.
• 💰 Highly Affordable: Costs as low as 15~25 KRW per message (compared to the usual 30~50 KRW).
• 🎁 Free Trial: Receive 10 free credits upon signup to test your implementation instantly.

Perfect for indie hackers, freelance developers, and startups building MVPs. Ditch the authentication stress and spend your valuable time building your core features with EasyAuth!