empty-after-oob

The failure

htmx lifts hx-swap-oob nodes out of the response, then swaps what remains into hx-target. If what remains is nothing, it swaps nothing. With hx-swap="outerHTML", nothing deletes the target.

A 422 that answers with only a toast removes the row the user was editing. The natural place to write this shape is a global error handler — then it fires on every failed mutation at once.

Why nothing else sees it

The response body looks fine: it contains a toast. Type checkers and linters see strings. A test that asserts on the response body never sees the empty remainder after OOB extraction.

The fix

Send HX-Reswap: none so the main swap is suppressed and only the out-of-band nodes apply. In a PLEYN app, use toastOnly() — see Toast-only.

Fixture: TOAST_ONLY_422 / TOAST_ONLY_422_FIXED_HEADERS in Fixtures.