Idempotency keys
A header that prevents duplicate sends on retry.
An idempotency key is a unique value the client sends in a header. The server caches the response and returns the same response for any request with the same key, even if the network retries. Critical for transactional sends where a duplicate password reset email is at minimum confusing and at worst a security issue.
reading this as teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to idempotency keys, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.