/**
 * Typed error surface for the protocol-version handshake.
 *
 * Raised by first-party clients + servers when the handshake observes
 * a version mismatch:
 *
 *   - Server side: `SubscribePayload.supportedVersions` is present AND
 *     does not contain the server's `PROTOCOL_SCHEMA_VERSION`. The
 *     server emits an `{type: 'error'}` frame with `code:
 *     UPGRADE_REQUIRED`; first-party servers also surface this class
 *     to their own observability path so operators see the
 *     unsatisfied subscribe locally.
 *   - Client side: on ack receipt, `AckPayload.serverVersion` is
 *     present AND NOT in `CLIENT_SUPPORTED_VERSIONS`. The client
 *     instantiates this class and surfaces it to the caller via the
 *     render's `onError` hook.
 *
 * The class pins `.name === 'UpgradeRequiredError'` and `.code ===
 * UPGRADE_REQUIRED` so consumers can pattern-match without
 * string-sniffing the message — `UPGRADE_REQUIRED` is reachable via
 * a typed protocol-error surface, not just via raw string compares.
 *
 * Wire-shape coupling: the class carries the observed version(s) +
 * the receiver's known-accepted set so operators can diagnose without
 * additional context. The `code` property matches the wire value
 * emitted in `ErrorPayload.code` — symmetric by design.
 */
import { UPGRADE_REQUIRED } from '../version.js';
export interface UpgradeRequiredErrorOptions {
    /**
     * The version the peer declared on the wire. Server-side: the
     * client's `supportedVersions` entry or the full list. Client-side:
     * `AckPayload.serverVersion`.
     */
    readonly observedVersion?: string | readonly string[];
    /**
     * The receiver's accepted-versions set. Server-side: a singleton
     * `[PROTOCOL_SCHEMA_VERSION]`. Client-side: `CLIENT_SUPPORTED_VERSIONS`.
     */
    readonly acceptedVersions: readonly string[];
    /**
     * Which side observed the mismatch. Server-side emission sets
     * `'server'`; client-side surface sets `'client'`.
     */
    readonly observedBy: 'server' | 'client';
}
export declare class UpgradeRequiredError extends Error {
    /** Canonical `ErrorPayload.code` value. Symmetric with wire shape. */
    readonly code: typeof UPGRADE_REQUIRED;
    readonly observedVersion?: string | readonly string[];
    readonly acceptedVersions: readonly string[];
    readonly observedBy: 'server' | 'client';
    constructor(opts: UpgradeRequiredErrorOptions);
}
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