⚠️ Experimental endpoint — not yet a stable contract. This action ships as experimental / beta. While experimental, its schema and behaviour may change without a version bump — including breaking changes within v1.0 itself. Anyone consuming it does so for evaluation only, or by coordinating adoption with their account manager so changes can be sequenced with them. It graduates to a stable, version-locked contract (where the normal breaking-change rules apply) only when this experimental notice is removed.

Set applicable transaction custom field values without changing the transaction's status, financial amounts, references or timestamps. This action requires permission to view the transaction and the Transactions.update-custom-fields permission. It works for finalized transactions even when ordinary transaction updates are unavailable.

Send a JSON object containing only the required customFields object; other body properties and any query parameters return 422 validation errors. Omitted identifiers remain unchanged, while null clears the corresponding value. When custom fields are enabled for transactions, an empty object succeeds as a no-op. Invalid or unavailable identifiers, invalid values, and disabled custom fields return 422 validation errors, and no submitted values are changed. A 404 response means the transaction was not found or is inaccessible. The response is the bare transaction read object, without a data wrapper.