Configuring AutoSend transactional templates for Better Auth OTPs and verification links

API & SMTP Integration
1

When integrating Better Auth with AutoSend for sending OTPs and email verification links, how do we configure template mapping and prevent serverless execution timeouts or timing attacks on Vercel/Cloudflare Workers?

2

Better Auth provides native hooks (emailVerification, sendResetPassword, emailOTP plugin) where you call AutoSend’s POST /v1/mails/send endpoint.

Complete Better Auth Integration (lib/auth.ts):

import { betterAuth } from "better-auth";
import { emailOTP } from "better-auth/plugins";
import { waitUntil } from "@vercel/functions";

async function sendAutoSendTemplate(to: string, templateId: string, dynamicData: Record<string, any>) {
  await fetch("https://api.autosend.com/v1/mails/send", {
    method: "POST",
    headers: {
      "Authorization": `Bearer ${process.env.AUTOSEND_API_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      from: { email: "auth@yourdomain.com", name: "Security" },
      to: { email: to },
      templateId,
      dynamicData,
    }),
  });
}

export const auth = betterAuth({
  emailVerification: {
    sendOnSignUp: true,
    sendVerificationEmail: async ({ user, url }) => {
      // Use waitUntil on Vercel/Cloudflare so the function doesn't terminate prematurely
      waitUntil(
        sendAutoSendTemplate(user.email, "tmpl_verify_email", {
          userName: user.name || "there",
          verificationUrl: url,
        })
      );
    },
  },
  plugins: [
    emailOTP({
      sendVerificationOTP: async ({ email, otp, type }) => {
        const templates: Record<string, string> = {
          "sign-in": "tmpl_otp_signin",
          "email-verification": "tmpl_otp_verify",
          "forget-password": "tmpl_otp_reset",
        };
        waitUntil(
          sendAutoSendTemplate(email, templates[type], { otp })
        );
      },
    }),
  ],
});

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