Why is Adding SMS Authentication to a Side Project So Hard?

If you've ever built a side project or a startup MVP, you've likely hit this wall: SMS Phone Verification (OTP). The feature itself seems trivial, but the moment you start looking for an API provider, you are bombarded with requests for business registration certificates, proof of carrier subscriptions, and sender ID pre-registrations.

"I just want to test my app. Why do I need to submit a pile of legal paperwork?"

To solve this exact frustration, today we will walk through how to implement an SMS authentication system in a Next.js App Router environment in just 5 minutes, with absolutely zero paperwork. To achieve this, we will use [EasyAuth], a developer-first, ultra-simple SMS API.

What You Will Learn

1. Setting up serverless API proxy routes using Next.js App Router's Route Handlers.
2. Building an interactive OTP verification UI with React Client Components.
3. Practical tips for security and UX (number formatting, rate limiting).

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Step 1: Setting up the API Routes (Backend)

In Next.js App Router, you can easily create backend APIs using the app/api/.../route.ts convention. Because calling third-party APIs directly from the client browser exposes your secret API keys, we will use the Next.js server as a proxy. EasyAuth's architecture requires only two endpoints (/send and /verify), making this exceptionally simple.

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();

    // Call EasyAuth Send API
    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, { status: response.status });
  } catch (error) {
    return NextResponse.json({ message: '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();

    // Call EasyAuth Verify API
    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, { status: response.status });
  } catch (error) {
    return NextResponse.json({ message: 'Verification failed' }, { status: 500 });
  }
}

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Step 2: Building the Client UI (Frontend)

Now, let's create a functional UI where users can input their phone numbers, request a code, and submit the 6-digit OTP.

Complete Working Code (app/components/SmsVerification.tsx)

'use client';

import { useState } from 'react';

export default function SmsVerification() {
  const [phone, setPhone] = useState('');
  const [code, setCode] = useState('');
  const [step, setStep] = useState(1); // 1: Input Phone, 2: Input OTP Code
  const [loading, setLoading] = useState(false);
  const [message, setMessage] = useState('');

  const handleSend = async () => {
    setLoading(true);
    setMessage('');
    try {
      const res = await fetch('/api/auth/send', {
        method: 'POST',
        body: JSON.stringify({ phone: phone.replace(/[^0-9]/g, '') }), // Extract digits only
      });

      if (res.ok) {
        setStep(2);
        setMessage('Verification code sent! Please check your messages.');
      } else {
        setMessage('Failed to send the code. Please check your number.');
      }
    } finally {
      setLoading(false);
    }
  };

  const handleVerify = async () => {
    setLoading(true);
    setMessage('');
    try {
      const res = await fetch('/api/auth/verify', {
        method: 'POST',
        body: JSON.stringify({ phone: phone.replace(/[^0-9]/g, ''), code }),
      });

      if (res.ok) {
        setMessage('✅ Verification successful!');
        // TODO: Proceed with user registration or issue a JWT token
      } else {
        setMessage('❌ Invalid verification code.');
      }
    } finally {
      setLoading(false);
    }
  };

  return (
    <div>
      <h2>Phone Verification</h2>

      {step === 1 &amp;&amp; (
        <div>
           setPhone(e.target.value)}
            className="p-3 border rounded-md focus:ring-2 focus:ring-blue-500 focus:outline-none"
            maxLength={15}
          /&gt;
          
            {loading ? 'Sending...' : 'Get Verification Code'}
          
        </div>
      )}

      {step === 2 &amp;&amp; (
        <div>
           setCode(e.target.value)}
            className="p-3 border rounded-md focus:ring-2 focus:ring-blue-500 focus:outline-none tracking-widest text-center text-lg"
            maxLength={6}
          /&gt;
          
            {loading ? 'Verifying...' : 'Verify Code'}
          
           setStep(1)}
            className="text-sm text-gray-500 underline mt-2 text-center"
          &gt;
            Re-enter phone number
          
        </div>
      )}

      {message &amp;&amp; (
        <p>
          {message}
        </p>
      )}
    </div>
  );
}

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Tips & Best Practices for Production

To ensure a secure and user-friendly SMS authentication flow, keep these practices in mind:

1. Phone Number Formatting: Users will input numbers in unpredictable ways (e.g., +1 234-567-8900, 12345678900). It is highly recommended to strip everything except digits using a regex like .replace(/[^0-9]/g, '') before sending the payload to your server.
2. Environment Variable Security: Your EASYAUTH_API_KEY must strictly live inside .env.local and never be prefixed with NEXT_PUBLIC_. Keep the secret hidden within your Next.js Route Handlers.
3. Preventing Spam Clicks (Rate Limiting): Take advantage of the loading state to disable the send button while an API call is in flight. This prevents impatient users from double-clicking and racking up duplicate SMS charges.

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Conclusion: Skip the Paperwork, Start Coding with EasyAuth

We just built a fully functional SMS phone authentication flow using Next.js App Router in under 5 minutes. The logic itself is straightforward, but the biggest bottleneck for developers has historically been the administrative red tape.

Whether you are a solo developer working on a toy project, a freelancer, or a startup needing to validate an MVP fast, you should be focusing on code, not carrier documents. That is exactly why EasyAuth was created.

• Zero Paperwork: No business registration or carrier documents required.
• Instant Setup: API integration is ready the moment you sign up, with an auto-configured sender ID.
• Affordable Pricing: At 15~25 KRW per message, it easily undercuts the traditional 30~50 KRW rates.
• Incredibly Simple API: No clunky SDKs. Just two endpoints: POST /send and POST /verify.

When you sign up today, you automatically receive 10 free test credits. If your side project or startup needs SMS authentication, skip the hassle and plug in EasyAuth right now!