How to Test OTP Locally Without SMS Costs

Testing OTP integrations end-to-end without burning the wallet is easy with the right pattern. This guide covers the three main approaches.

Options Overview

• Sandbox key — provider simulates delivery.
• Mock provider — local HTTP service that mimics the API.
• Fixed test phone — real OTPs sent to one staging number.

Provider Sandbox Mode

Most providers ship a sandbox key. With StartMessaging, sandbox mode accepts any phone number and verifies any OTP code marked as the test code.

Mock Provider

// tests/mock-sm.ts
import { rest } from 'msw';
export const handlers = [
  rest.post('https://api.startmessaging.com/otp/send', (req, res, ctx) =>
    res(ctx.json({ data: { requestId: 'mock_123', expiresAt: 'X', attemptsLeft: 3 }}))
  ),
  rest.post('https://api.startmessaging.com/otp/verify', (req, res, ctx) =>
    res(ctx.json({ data: { verified: true }}))
  ),
];

Fixed Test Phones

Designate one phone you control as the staging recipient. Real OTPs delivered, manual entry. Fine for human tests, painful in CI.

CI Strategy

• Use mock provider in unit / integration tests.
• Use sandbox key in nightly e2e.
• Real provider only in pre-prod staging environment.

FAQ

For the broader staging story see OTP testing in staging guide .