export declare const USER_AGENT_POOL: readonly string[];
/**
 * Pick a user agent for the next request. When `previous` is supplied the
 * result is guaranteed to differ from it, so a retry always presents a fresh
 * fingerprint; successive calls cycle deterministically through the pool. With
 * no previous UA a random pool member is returned so cold requests spread
 * across fingerprints.
 */
export declare function nextUserAgent(previous?: string): string;
/**
 * True when an engine error looks like a block the client can retry against
 * with a fresh fingerprint — an upstream 403 (forbidden / reputation) or 429
 * (rate limit). Keyed on error class, never on a specific engine.
 */
export declare function isBlockedError(err: unknown): boolean;
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