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Idempotency is a property of an operation that ensures it can be safely retried without producing duplicate side effects. In distributed systems (calling external APIs), it is not always possible to ensure an operation has only been performed once just by seeing if it succeeds. Consider a payment API that charges the user $10, but due to network failures, the confirmation response is lost. When the step retries (because the previous attempt was considered a failure), it will charge the user again. To prevent this, many external APIs support idempotency keys. An idempotency key is a unique identifier for an operation that can be used to deduplicate requests.

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The core pattern: use the step ID as your idempotency key

Every step invocation has a stable stepId that stays the same across retries. Use it as the idempotency key when calling third-party APIs. 
import { getStepMetadata } from "workflow";

async function chargeUser(userId: string, amount: number) {
  "use step";

  const { stepId } = getStepMetadata();

  // Example: Stripe-style idempotency key
  // This guarantees only one charge is created even if the step retries
  await stripe.charges.create(
    {
      amount,
      currency: "usd",
      customer: userId,
    },
    {
      idempotencyKey: stepId,
    }
  );
}
Why this works:
  • Stable across retries: stepId does not change between attempts.
  • Globally unique per step: Fulfills the uniqueness requirement for an idempotency key.

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Best practices

  • Always provide idempotency keys to external side effects that are not idempotent inside steps (payments, emails, SMS, queues).
  • Prefer stepId as your key; it is stable across retries and unique per step.
  • Keep keys deterministic; avoid including timestamps or attempt counters.
  • Handle 409/conflict responses gracefully; treat them as success if the prior attempt completed.

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