/**
 * Methods are bound arrow fields: upstream Pi's `createEventBus()` returns
 * closures, so extensions (e.g. `pi-gemini-acp`) detach `pi.events.on` and
 * `pi.events.emit` and call them without a receiver.
 */
export declare class EventBus {
    #private;
    emit: (channel: string, data: unknown) => void;
    on: (channel: string, handler: (data: unknown) => void) => (() => void);
    /** Whether `channel` has at least one listener. */
    hasListeners: (channel: string) => boolean;
    /**
     * Calls `onChange` whenever `channel` gains its first listener (`true`) or loses its last
     * (`false`), so a forwarding emitter on another thread can drop frames nobody reads.
     * Returns the unsubscribe.
     */
    watchListeners: (channel: string, onChange: (present: boolean) => void) => (() => void);
    clear: () => void;
}
/**
 * Publishes a subagent frame on the session bus and the observability bus.
 * SDK embedders may pass the same EventBus for both slots; the identity check
 * skips the aliased re-emit so listeners never see duplicate frames.
 */
export declare function emitSubagentFrame(eventBus: EventBus | undefined, subagentEventBus: EventBus | undefined, channel: string, payload: unknown): void;