# X_IDEMPOTENCY_KEY_INVALID

## Means

an `Idempotency-Key` header was sent that cannot identify one request

## Typical cause

a blank value — `Headers.get()` answers `''` for `Idempotency-Key:` and never `null`, so a blank header used to become a live key every blank sender shared — or one past the 255 characters the OpenAPI operation publishes. A **400**, raised before the handler, so nothing was written

## Fix

set the header to a fresh `crypto.randomUUID()` on the client, one per request — or omit it entirely to run this call without idempotency

[Actions](/error-codes#actions)