Object format does not support 'sparse' strategy because objects cannot have undefined values in a type-safe manner. Likewise, 'keep' and 'truncate' would drop keys or change row shape, so object output only allows 'fill' (default) or 'strict'.
'fill': Fill missing fields with empty string (default)'keep': Not allowed (throws). Use array output if you need ragged rows.'strict': Throw error if column count doesn't match header'truncate': Not allowed (throws). Use array output to drop extra fields.
Column count strategies allowed for object format.