/**
 * The FDv1 fallback directive parsed from a connection's response headers.
 * Its presence (`fdv1Fallback === true`) means the server asked the SDK to
 * fall back to FDv1.
 *
 * `fdv1FallbackTtlMs` is how long to remain on FDv1 before retrying FDv2:
 * - `undefined`: the server gave no TTL header (caller uses a 1-hour default).
 * - `0`: indefinite fallback (no automatic recovery).
 * - `> 0`: milliseconds to wait before attempting FDv2 recovery.
 *
 * This is the single place that interprets `x-ld-fd-fallback` and
 * `x-ld-fd-fallback-ttl`.
 */
export interface FallbackDirective {
    fdv1Fallback: boolean;
    fdv1FallbackTtlMs?: number;
}
/**
 * Reads the FDv1 fallback directive from response headers. Returns
 * `{ fdv1Fallback: false }` when `x-ld-fd-fallback` is absent or not `"true"`.
 *
 * @param headers Header accessor. The `get` method must accept header names
 *   in any casing; callers normalize to lowercase before calling this
 *   function.
 */
export declare function readFallbackDirective(headers: {
    get(name: string): string | null;
}): FallbackDirective;
/**
 * Reads the FDv1 fallback directive from an FDv2 `goodbye` event's data.
 *
 * SDKs that cannot read streaming response headers (e.g. browsers using the
 * native `EventSource` API) receive the fallback directive in-band via the
 * goodbye message's `protocolFallbackTTL` field.
 * Presence of a finite numeric `protocolFallbackTTL` signals FDv1 fallback;
 * the value carries the same semantics as the `x-ld-fd-fallback-ttl` header
 * (`0` indicates indefinite fallback). A missing, non-numeric, or non-finite
 * value is not a fallback signal and yields `{ fdv1Fallback: false }`.
 *
 * @param data The raw, parsed goodbye event data (typed `unknown` because the
 *   caller has not narrowed it).
 */
export declare function readGoodbyeFallbackDirective(data: unknown): FallbackDirective;
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