Errors › 400 Invalid request

`idempotency_key_required`

A side-effect-bearing POST was missing the `Idempotency-Key` header.

type invalid_request_error status 400

What happened

Required on `POST /v1/sends`, `POST /v1/threads/{id}/replies`, `POST /v1/contact_imports`, and `POST /v1/suppression_imports`. Optional but honored on plain CRUD POSTs.

How to recover

Generate a stable UUID per logical operation and pass `Idempotency-Key: <uuid>`. The SDK can mint one automatically — pass `idempotencyKey: () => crypto.randomUUID()` to the client constructor.

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