There's no GET endpoint. You can create, fully replace, or partially update - but you can't read a sheet's current config back. Combined with PATCH's semantics, where only provided fields are updated and omitted fields stay unchanged, that's field-level granularity, not array-element-level. So sending with two new columns very likely replaces the whole column array rather than appending to it. With no GET to read the existing state first and no way to verify afterward, a careless patch could silently wipe twenty columns.

So it is essential to have a GET Sheet API.