/** account-service could not verify the ID token (HTTP 502). Not a credential error; retry later. */
export declare class CustomTokenRetryableError extends Error {
    readonly retryable = true;
    constructor(message: string);
}
/** The ID token was rejected again after a forced refresh (HTTP 401). The session is treated as signed out. */
export declare class CustomTokenSessionExpiredError extends Error {
    constructor(message?: string);
}
/**
 * Exchanges a Firebase ID token for a Firebase custom token via account-service v2.
 *
 * - 401: force-refresh the ID token and retry once; a second 401 signs the session out.
 * - 502: throws {@link CustomTokenRetryableError} and keeps the session.
 */
export declare function getFirebaseCustomToken(idToken: string): Promise<string>;