Update custom field values on an account or project space. Send a values map keyed by custom field ID (UUID) — fields you omit are left untouched, and a null value clears the field. A Link (url) field takes the URL as a string, or an object {"url": "...", "display_text": "OPTY0012345"} to show that text as the link. A plain string sets the URL with no display text.

The response returns the full field collection after the update plus changed_field_ids, listing only the fields whose value actually changed. An audit event (account.updated / project.updated) is recorded when at least one field changed.

Errors: 422 when a key is not a valid UUID, names a field the space's template does not define, or the value fails the field's validation (for example clearing a required field); 404 when the space does not exist in your organization; 409 when the space has no custom fields to update; 502 when the custom fields service is unavailable.