import type { PipelineEvent } from '@openpipeline/core';
/**
 * Format a single SSE frame: `event: <kind>\ndata: <json>\n\n`.
 *
 * Guards `JSON.stringify` (#E3): a node's `output` is `unknown` and can carry
 * a circular reference or a BigInt straight from a TOOL handler, before any
 * `safeJson` pass runs on the persistence path. Throwing here would tear down
 * the whole SSE response mid-stream over a single bad frame. Instead we
 * degrade to a minimal frame that still names the event kind and explicitly
 * signals the failure — never a silently dropped frame.
 */
export declare function sseFrame(event: PipelineEvent): string;
/** Standard SSE response headers. */
export declare const SSE_HEADERS: Record<string, string>;
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