/** The SERVER→CLIENT half of the wire's keepalive (blueprint §4.1 item 5, §4.2).
 *
 * Not to be confused with `heartbeat.ts`, which points the other way:
 * `withTurnHeartbeat` is the CLIENT beating `POST /threads/:id/heartbeat` so the
 * server can idle-abort an abandoned turn. This file is transport framing —
 * bytes the server puts on an SSE response so the connection stays visibly
 * alive while nothing interesting is happening.
 *
 * Two problems, one policy:
 *
 * 1. **Time to first byte.** A turn's first real chunk waits on a provider call.
 *    Until then the response head may be sitting in a proxy buffer and the
 *    client has nothing at all.
 * 2. **Long silent gaps.** A slow tool call streams nothing for its whole
 *    duration; proxies and browsers drop connections that go quiet.
 *
 * The policy: emit a comment frame NOW, then one per `intervalMs` of silence.
 * SSE comments are ignored by every spec-compliant parser (the `eventsource-parser`
 * the ai-SDK's `DefaultChatTransport` uses treats a leading `:` as a comment and
 * emits nothing), so a keepalive is invisible to the client's message sequence.
 * It is deliberately NOT a `HarnessEvent` and NOT a `data-vendo-*` part: a
 * keepalive is not something that happened, it is the wire saying it is still there.
 */
/** The frame itself. An SSE comment — no event, no data, ~14 bytes. */
export declare const SSE_KEEPALIVE_FRAME = ": heartbeat\n\n";
/** Well inside the 30–60s idle windows proxies and load balancers default to. */
export declare const DEFAULT_SSE_KEEPALIVE_INTERVAL_MS = 15000;
export interface SseKeepaliveOptions {
    /** Cadence of silence-filling frames. Defaults to 15s. */
    intervalMs?: number;
}
/** Wrap an SSE `Response` so its first frame leaves at once and silence is
 *  punctuated.
 *
 *  Backpressure and ordering are preserved by construction: each pull races the
 *  in-flight source read against the keepalive deadline, and the read that loses
 *  is KEPT for the next pull — so a real chunk is never buffered, never delayed,
 *  and never dropped. No timer outlives the stream either: the deadline is a
 *  per-pull timeout, cleared on every path, and a closed or cancelled stream is
 *  never pulled again. */
export declare function withSseKeepalive(response: Response, options?: SseKeepaliveOptions): Response;