Implementing SMS Authentication for Signups in Next.js in 5 Minutes (No Paperwork)

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When building a side project or an MVP (Minimum Viable Product), one of the most frustrating features developers have to implement during the signup phase is SMS Authentication (OTP). If you try to integrate traditional SMS APIs, you immediately hit a wall of bureaucracy.

"Please submit your Business License."
"We need proof of your registered sender ID."
"Review will take 3 to 5 business days."

When you barely have enough time to code and spin up your servers, being bogged down by paperwork is the last thing you need. In this tutorial, we will learn how to implement SMS verification in a Next.js (App Router) environment in just 5 minutes using EasyAuth—a developer-first SMS API that requires absolutely no paperwork to get started.

What You Will Learn

Setting up API Routes (Route Handlers) in Next.js 14/15 App Router
Sending (/send) and verifying (/verify) OTP codes using the EasyAuth API
Building a copy-paste ready, user-friendly client-side signup form

Step 1: Environment Setup

First, sign up for EasyAuth (you get 10 free credits immediately) and grab your API key. Save this key in your Next.js project's .env.local file.

EASYAUTH_API_KEY=your_api_key_here

Step 2: Implementing Server-side API Routes

We will use the Next.js App Router to create two API endpoints for sending and verifying the SMS code. You must route these requests through your server to prevent your secret API key from being exposed to the client browser.

1. Send SMS API (`app/api/send-sms/route.ts`)

This endpoint calls EasyAuth's /send API to text the verification code to the user.

import { NextResponse } from 'next/server';

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

    // Request EasyAuth API to send SMS
    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({ to: phone })
    });

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

    return NextResponse.json({ success: true, message: 'Verification code sent.' });
  } catch (error) {
    return NextResponse.json({ error: 'Internal server error.' }, { status: 500 });
  }
}

2. Verify SMS API (`app/api/verify-sms/route.ts`)

This endpoint checks if the code entered by the user matches by calling EasyAuth's /verify API.

import { NextResponse } from 'next/server';

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

    // Request EasyAuth API to verify code
    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({ to: phone, code })
    });

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

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

Step 3: Building the Client-side Signup Form

Now, let's create the user interface where users can input their phone number and the OTP code. (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(1);
  const [status, setStatus] = useState("");

  const handleSendSms = async () =&gt; {
    setStatus("Sending...");
    const res = await fetch("/api/send-sms", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({ phone }),
    });

    if (res.ok) {
      setStep(2);
      setStatus("Verification code sent. (Expires in 3 mins)");
    } else {
      setStatus("Failed to send. Please check the number.");
    }
  };

  const handleVerifySms = async () =&gt; {
    setStatus("Verifying...");
    const res = await fetch("/api/verify-sms", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({ phone, code }),
    });

    if (res.ok) {
      setStep(3);
      setStatus("✅ Phone verification completed!");
    } else {
      setStatus("❌ Invalid verification code.");
    }
  };

  return (
    <div>
      <h1>Verify Your Phone</h1>
      <div>
        <div>
          Phone Number
          <div>
             setPhone(e.target.value)}
              placeholder="Numbers only"
              className="border border-gray-300 p-2 flex-1 rounded focus:outline-none focus:ring-2 focus:ring-blue-500"
              disabled={step === 3}
            /&gt;
            
              {step === 1 ? "Send" : "Resend"}
            
          </div>
        </div>

        {step &gt;= 2 &amp;&amp; (
          <div>
            6-Digit Code
            <div>
               setCode(e.target.value)}
                placeholder="Enter OTP"
                className="border border-gray-300 p-2 flex-1 rounded focus:outline-none focus:ring-2 focus:ring-blue-500"
                disabled={step === 3}
              /&gt;
              
                Verify
              
            </div>
          </div>
        )}
        {status &amp;&amp; (
          <p>
            {status}
          </p>
        )}
      </div>
    </div>
  );
}

💡 Security Tips & Best Practices

Implement Rate Limiting: To prevent malicious bots from spamming your /send endpoint and racking up API charges, implement IP-based rate limiting (e.g., max 5 requests per hour) using Next.js Middleware or Upstash Redis.
Final Server-side Validation: Relying solely on the client state (step === 3) is not secure. When the user submits the final registration form, your server should cross-check the verified phone number with your database or session cache before finalizing the signup.

Conclusion

We have successfully built a fully functioning SMS verification flow in Next.js in just a few minutes.

Are you an indie developer, freelancer, or building a startup MVP? Stop wasting your valuable development time on tedious paperwork, business license reviews, and sender ID registrations required by legacy API providers. EasyAuth is the ultimate alternative designed strictly for developer productivity.

Zero Paperwork: Start instantly with just an email—no business license required.
Auto Sender ID: Skip the complex telecommunication sender registration process.
Highly Affordable: Costs roughly 15~25 KRW per text, effectively half the price of traditional 30~50 KRW competitors.

Sign up for EasyAuth today, claim your 10 free credits, and integrate SMS authentication into your project in under 5 minutes!