Record Opening Balance

Record the balance sheet an entity's books start from, when its history does not come with them: one balance per balance-sheet account (assets, liabilities, equity), in cents, debit or credit in the account's normal direction. It posts as one opening entry dated the day before the entity's first fiscal month, so every month's opening balances include it and no month's activity does: the cash flow and the equity statement start from it rather than counting it. Name the equity account that takes any difference between debits and credits (equity_element_id, usually Opening Balance Equity); unbalanced balances without one are refused, and an income or expense account is refused, since income before the books begin is in retained earnings. One per entity: recording it again replaces it, until the entity's first close, after which a correction is an adjusting entry in an open month. Set up the fiscal calendar first; moving its start (change-calendar-start) moves the opening with it.

Idempotency: supply an Idempotency-Key header to make safe retries; replays within 24 hours return the same envelope. Reusing the key with a different body returns HTTP 409 Conflict.