Optional idempotency token for authenticated POST and PATCH requests. Reusing the same key and body returns the cached response for 24 hours, except credential operations that explicitly document a 409 because one-time secret material is never cached; reusing it with a different body returns 409 IdempotencyKeyConflict. Use 1-255 printable ASCII characters.

Select the Account for an Account-scoped operation. A direct ServiceAccount API key MUST supply this header, and the target must be within the ServiceAccount owner's or Authorization grant's Account boundary. A Person bearer may select an Account on which it has an active Stakeholder, and an Account session may select its bound Account (or a descendant only when the session explicitly includes descendants).