⚠️ 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 the applicable custom field values of a partner without changing any other partner properties. This action requires permission to view the partner and the Partners.update-custom-fields permission.

Send a JSON object containing only the required customFields property. Query parameters and additional body properties are rejected. The customFields object may be empty. Omitted identifiers remain unchanged, while an explicit null clears the corresponding value. Every submitted identifier is validated against the partner, the current custom field configuration and the acting administrator's access. Unknown, inapplicable or currently disabled identifiers return a 422 error keyed as customFields.<identifier>, and no submitted values are changed.