Minimal SvelteKit + query.live reproduction showing that the application/x-ndjson response from a live remote function silently breaks behind any reverse proxy that buffers responses by default (nginx being the obvious one) — and the one-line hooks.server.ts workaround that fixes it.

A live demo is up at: https://query-live-repro.server.ollema.xyz

query.live returns application/x-ndjson, an open response that yields newline-delimited JSON frames as the server-side async generator yields. nginx's default proxy_pass config buffers upstream responses, so the frames never reach the client; the connection just hangs. After nginx hits proxy_read_timeout (default 60s) it closes the upstream and flushes whatever (typically nothing) it had buffered, at which point Chrome reports net::ERR_HTTP2_PROTOCOL_ERROR 200 (OK) and SvelteKit flips live.connected to false.

The fix from the framework side is one header: X-Accel-Buffering: no, which nginx interprets as "do not buffer this response."

Two pages, sharing the same watchCounter + bump remote functions and the same in-memory counter:

• /broken — no proxy hint, hangs forever behind nginx.
• /fixed — X-Accel-Buffering: no set in hooks.server.ts for this route, works as expected.

Open both in two tabs side-by-side: /fixed ticks live; /broken doesn't, even when the bump comes from the other tab.

pnpm install
pnpm dev
# both /broken and /fixed work — vite dev does not buffer responses

The bug only shows up once nginx (or any other default-buffering reverse proxy) is in front of the Node server.

Setup for the live demo: @sveltejs/adapter-node running in Docker, exposed via nginx's default proxy_pass config. No special configuration applied at the proxy.

1. Page renders, but live.connected === false from the start.
2. Request to /_app/remote/<hash>/watchCounter sits pending for ~60s (matches nginx's default proxy_read_timeout).
3. Then it "completes" with the response below — and Chrome immediately logs net::ERR_HTTP2_PROTOCOL_ERROR 200 (OK).

Status Code:    200 OK
cache-control:  private, no-store
content-type:   application/x-ndjson
server:         nginx

Pressing "bump" or "retry" reproduces the hang for the next request.

1. live.connected === true immediately.
2. bump ticks the counter live.
3. Two /fixed tabs stay in sync.
4. With one /fixed and one /broken open, /fixed still ticks on every bump — the workaround is per-response, scoped to that page.

The /fixed response carries the same Cache-Control: private, no-store and Content-Type: application/x-ndjson as /broken, plus the hook adds X-Accel-Buffering: no. nginx honors the header (and strips it from the response sent to the client, so you won't see it in devtools — its absence on /fixed responses is the expected sign that nginx acted on it).

query.live correctly sets Cache-Control: private, no-store on the response, which Cloudflare and some other proxies respect for both caching and buffering decisions. nginx ignores Cache-Control for proxy buffering (docs). Its only opt-outs from the upstream side are:

• proxy_buffering off; in the nginx config, or
• an X-Accel-Buffering: no response header from the upstream.

The hook in this repo uses the latter — it requires no changes on the proxy host.

• @sveltejs/kit ^2.59.0
• @sveltejs/adapter-node ^5.5.4
• svelte ^5.55.5
• node 22
• nginx