Resend

The @effect-kit/emails/resend subpath sends messages through Resend and provides the Email service.

InstallationCopy Anchor Link

npm install @effect-kit/emails effect resend@6.34.1

The SDK is a pinned optional peer. It is needed only when you import this subpath.

SetupCopy Anchor Link

Resend.layerConfig() reads these variables:

Example (Sending a Message With Resend)

import { ConfigProvider, Effect, Layer } from "effect";
import { Email } from "@effect-kit/emails";
import * as Resend from "@effect-kit/emails/resend";

const EmailLive = Resend.layerConfig().pipe(
  Layer.provide(ConfigProvider.layer(ConfigProvider.fromEnv())),
);

const program = Effect.gen(function* () {
  const email = yield* Email;
  return yield* email.send({
    from: "hello@example.com",
    to: "ada@example.com",
    subject: "Welcome",
    text: "Your account is ready.",
  });
});

Effect.runPromise(program.pipe(Effect.provide(EmailLive)));

Use a from address your Resend account is allowed to send from.

OptionsCopy Anchor Link

Resend.layer takes these options. layerConfig builds the same object from a Config.

CapabilitiesCopy Anchor Link

Resend reports this in email.capabilities:

Every message feature is supported, so no message fails the capability check.

IdempotencyCopy Anchor Link

Mode replay, with a retention of 24 hours. The package passes the key to Resend as the SDK's idempotencyKey option. A key longer than 256 characters fails with InvalidMessageError (path idempotencyKey) before sending.

When Resend reports invalid_idempotent_request or concurrent_idempotent_requests, the send fails with IdempotencyConflictError and acceptance: "unknown".

Example (Sending With an Idempotency Key)

import { Effect } from "effect";
import { Email } from "@effect-kit/emails";

const program = Effect.gen(function* () {
  const email = yield* Email;
  return yield* email.send(
    {
      from: "hello@example.com",
      to: "ada@example.com",
      subject: "Welcome",
      text: "Your account is ready.",
    },
    { idempotencyKey: "welcome-ada-2026-10-29" },
  );
});

Provider notesCopy Anchor Link

The receipt's requestId comes from the x-request-id response header when Resend returns one.

Using your own clientCopy Anchor Link

layerClient takes any object with emails.send(message, { signal, idempotencyKey }), which the Resend SDK client has. options is optional.

Example (Borrowing a Resend Client)

import { Resend as ResendSdk } from "resend";
import * as Resend from "@effect-kit/emails/resend";

const client = new ResendSdk("re_fake_key");

const EmailLive = Resend.layerClient(client);

You own a borrowed client. The Layer never closes it, and the client keeps its own retry, timeout and logging policy.

ErrorsCopy Anchor Link

Layer construction fails with ResendConfigurationError, whose field names the bad option. layerClient fails with it for the client options.

retryAfter is read from the retry-after header on rate limit, quota and unavailable failures. The Resend error name is kept as providerCode when it is one of the names above and the status as statusCode.