Resend idempotency keys in Python: SendOptions with idempotency_key, one key per email

Idempotent sends: the SendOptions argument

import resend

params: resend.Emails.SendParams = {
    "from": "[SENDER]",
    "to": ["[RECIPIENT]"],
    "subject": "Welcome",
    "html": "<strong>it works!</strong>",
}

options: resend.Emails.SendOptions = {
    "idempotency_key": "welcome-user/[USER-ID]",
}

resend.Emails.send(params, options)

The rules, from the docs

• Include a key on any send you might retry, so the same request processed twice still sends once.

• The key must be unique per API request. Namespace it by the business event: welcome-user/[USER-ID], not a constant.

• Keys expire after 24 hours. A retry three days later is a new send.
• Maximum length is 256 characters.

Checklist

• The option name is idempotency_key (snake_case) in Python, idempotencyKey in Node. Mixing the cases silently drops the key in the Python SDK.

• It is the second positional argument to resend.Emails.send, not part of the params dict.

• On retry-after-timeout, reuse the SAME key. Generating a fresh key per attempt is the most common way to defeat idempotency.

Resend documents idempotency keys with the Python SendOptions form carrying idempotency_key, plus the unique-per-request rule, 24-hour expiry, and 256-character limit. Verified live at https://resend.com/docs/dashboard/emails/idempotency-keys and https://resend.com/docs/api-reference/emails/send-email.