custom_fields is a free-form map shaped by your organisation's own custom-column configuration — Mercura does not enforce a JSON schema on the contents. Keys and value formats can evolve on your side without an API version bump. Partners rendering custom fields should treat unknown keys gracefully (skip, or show verbatim); Mercura will not remove keys behind your back.
Values follow the column's type: text / number / date fields are returned as-is, while a multi-select field is returned as a list of its selected option labels (the same text you see in the Mercura UI), e.g. "Gewerk": ["DD - DACHDECKUNG DACHABDICHTUNG", "EL - ELEMENTE TÜREN"]. The internal option ids are never exposed on the read path.
Empty custom-field sets are omitted from the response body entirely (the field is not returned as null or {}). Compact wire, easier diffs against the Mercura admin UI.