Building a Serverless SMS Auth API in 5 Minutes with Hono & Cloudflare Workers (Zero Paperwork)

Blocked by Paperwork Just to Send an SMS?

When building an MVP or a side project, you eventually need to verify your users. You think, "I'll just add a simple SMS OTP," but then reality hits. Most SMS gateways require business registration certificates, proof of phone number ownership, and days of approval time. This is a massive roadblock for solo developers, freelancers, and startups trying to move fast.

In this tutorial, we will build a lightning-fast, serverless SMS authentication API using Cloudflare Workers, the Hono framework, and EasyAuth—a developer-first SMS API that requires absolutely ZERO paperwork and sets up in 5 minutes.

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Solution Overview

In this post, you will learn how to:

• Set up Hono & Cloudflare Workers: Create an ultra-fast, serverless backend at the edge.
• Integrate EasyAuth: Add SMS verification using simple POST /send and POST /verify endpoints without needing sender ID pre-registration.
• Deploy a Production-Ready API: Build an API ready to be consumed by your Next.js or React frontend.

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Step-by-Step Implementation

1. Initialize the Hono Project

Hono is a lightweight, ultrafast web framework optimized for Edge computing platforms like Cloudflare Workers. It uses an Express-like syntax, making it incredibly easy to pick up.

npm create hono@latest my-sms-api
# Select template: cloudflare-workers
cd my-sms-api
npm install

2. Set Up Environment Variables (.dev.vars)

Get your API key by signing up for EasyAuth. (You get 10 free credits upon signup, so you can test right away!)
Create a .dev.vars file in your project root and add your key:

EASYAUTH_API_KEY=your_easyauth_api_key_here

3. Implement the Send API (POST /send)

Open src/index.ts and add the logic to trigger the SMS. With EasyAuth, you don't need to configure a registered sender ID—it handles the dispatch automatically.

app.post('/api/auth/send', async (c) => {
  const { phone } = await c.req.json();

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

  if (!response.ok) return c.json({ error: 'Failed to send SMS' }, 500);
  return c.json({ message: 'Verification code sent successfully' });
});

4. Implement the Verify API (POST /verify)

Next, add the endpoint to verify the OTP code submitted by the user.

app.post('/api/auth/verify', async (c) => {
  const { phone, code } = await c.req.json();

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

  if (!response.ok) return c.json({ error: 'Invalid verification code' }, 400);
  return c.json({ message: 'Verification successful' });
});

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Complete Code

Here is the complete src/index.ts file, including CORS configuration and error handling. You can copy, paste, and deploy this directly (npm run deploy).

import { Hono } from 'hono';
import { cors } from 'hono/cors';

// Define environment bindings
type Bindings = {
  EASYAUTH_API_KEY: string;
};

const app = new Hono<{ Bindings: Bindings }>();

// Enable CORS for frontend integration
app.use('/api/*', cors());

// 1. Send Verification Code
app.post('/api/auth/send', async (c) => {
  try {
    const { phone } = await c.req.json();
    
    const response = await fetch('https://api.easyauth.kr/send', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${c.env.EASYAUTH_API_KEY}`
      },
      body: JSON.stringify({ phone })
    });

    if (!response.ok) throw new Error('Send API Error');
    return c.json({ success: true, message: 'Verification code sent.' });
  } catch (error) {
    return c.json({ success: false, error: 'Internal server error.' }, 500);
  }
});

// 2. Verify Code
app.post('/api/auth/verify', async (c) => {
  try {
    const { phone, code } = await c.req.json();
    
    const response = await fetch('https://api.easyauth.kr/verify', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
        'Authorization': `Bearer ${c.env.EASYAUTH_API_KEY}`
      },
      body: JSON.stringify({ phone, code })
    });

    if (!response.ok) return c.json({ success: false, error: 'Invalid OTP code.' }, 400);
    return c.json({ success: true, message: 'Authentication completed.' });
  } catch (error) {
    return c.json({ success: false, error: 'Internal server error.' }, 500);
  }
});

export default app;

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Tips & Best Practices

1. Security & Rate Limiting
   When exposing public endpoints, protect them from abuse. Use Cloudflare's native Rate Limiting or @upstash/ratelimit to restrict the number of SMS requests per IP.
2. Cost Efficiency
   Traditional SMS APIs often charge between 30 to 50 KRW per message. EasyAuth offers a much more reasonable rate of 15~25 KRW per message, keeping your startup costs low while you scale.
3. Environment Variables
   Never hardcode your API keys. Always use Cloudflare's secrets management (npx wrangler secret put EASYAUTH_API_KEY) for production deployments.

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Conclusion: Focus on Building, Not Paperwork

We just built a robust, serverless SMS verification API using Hono and Cloudflare Workers. By leveraging modern edge computing and developer-friendly tools, you can skip the tedious infrastructure setup.

More importantly, you can skip the bureaucratic red tape. No business registration, no sender ID verifications. Sign up, integrate the API in 5 minutes, and launch your MVP faster.

👉 Start using EasyAuth today (Zero paperwork, 10 free credits upon signup!)