In apps/webapp/app/routes/realtime.v1.sessions.$session.$io.append.ts on main (commit 3cacfa4), the action handler checks if (!result.ok && result.kind === "unsupported_backend") after calling ensureRunThenAppend, but does not handle the result.kind === "rejected" or result.kind === "failed" branches. When a stream append is rejected with a 422 validation status or fails in the backend, execution falls through and responds with 200 { ok: true, seq: undefined }. Happy to put up a PR to return the rejected status and 500 error responses if that looks right.