Resend on Cloudflare Workers: needs a bundling setup, and the key comes from env, not source

TL;DR

Prerequisites from the guide: a Resend API key, a verified domain, and a Cloudflare worker with a bundling setup (the guide recommends bootstrapping with npm create cloudflare). ``ts import { Resend } from 'resend'; export default { async fetch(request: Request, env: Env): Promise[Response] { const resend = new Resend(env.RESEND\API\KEY); const { data, error } = await resend.emails.send({ from: '[SENDER]', to: ['[RECIPIENT]'], subject: 'Hello World', html: '<strong>It works!</strong>', }); if (error) { return Response.json({ error }, { status: 400 }); } return Response.json({ id:

import { Resend } from 'resend';

export default {
  async fetch(request: Request, env: Env): Promise[Response] {
    const resend = new Resend(env.RESEND_API_KEY);
    const { data, error } = await resend.emails.send({
      from: '[SENDER]',
      to: ['[RECIPIENT]'],
      subject: 'Hello World',
      html: '<strong>It works!</strong>',
    });
    if (error) {
      return Response.json({ error }, { status: 400 });
    }
    return Response.json({ id: data.id });
  },
};

Steps

1. Prerequisites from the guide: a Resend API key, a verified domain, and a

Cloudflare worker with a bundling setup (the guide recommends bootstrapping with npm create cloudflare).

import { Resend } from 'resend';

export default {
 async fetch(request: Request, env: Env): Promise[Response] {
 const resend = new Resend(env.RESEND_API_KEY);
 const { data, error } = await resend.emails.send({
 from: '[SENDER]',
 to: ['[RECIPIENT]'],
 subject: 'Hello World',
 html: '<strong>It works!</strong>',
 });
 if (error) {
 return Response.json({ error }, { status: 400 });
 }
 return Response.json({ id: data.id });
 },
};

2. • The guide writes new Resend('re_xxxxxxxxx'). That is a placeholder. Bind the

   real key as a worker secret and read it from env. A literal in source ends up in the bundle and in version control.

• Construct the client inside fetch where env is available. Module scope has no access to the worker environment.

• Keep the bundling setup the guide asks for. The SDK is an npm package; a worker without a bundler cannot import it.

When to use

You are seeing this: Bind the real key as a worker secret and read it from env. Use this skill when you run into "Resend on Cloudflare Workers: needs a bundling setup, and the key comes from env, not source".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Resend's Cloudflare Workers guide lists the bundling setup prerequisite and builds the send inside the fetch handler. Verified live at https://resend.com/docs/send-with-cloudflare-workers.