/**
 * A request configuration object.
 *
 * **Note:** This is a simplified version of the `RequestConfig` type from the
 * `fetch` API. It contains only the properties that are relevant for the
 * fal client. It also works around the fact that the `fetch` API `Request`
 * does not support mutability, its clone method has critical limitations
 * to our use case.
 */
export type RequestConfig = {
    url: string;
    method: string;
    headers?: Record<string, string | string[]>;
};
export type RequestMiddleware = (request: RequestConfig) => Promise<RequestConfig>;
/**
 * Setup a execution chain of middleware functions.
 *
 * @param middlewares one or more middleware functions.
 * @returns a middleware function that executes the given middlewares in order.
 */
export declare function withMiddleware(...middlewares: RequestMiddleware[]): RequestMiddleware;
/**
 * Runtime information passed to a user-provided {@link ProxyRuntimeGate}
 * predicate so it can decide whether the proxy middleware should apply.
 */
export type ProxyRuntimeEnv = {
    isBrowser: boolean;
};
/**
 * Controls when the proxy middleware applies:
 * - `"browser"` — only when a DOM `window` is available (default).
 * - `"always"` — in every runtime (Node, Electron, Bun, workers, etc.).
 * - A predicate receiving {@link ProxyRuntimeEnv} — full control per call.
 */
export type ProxyRuntimeGate = "browser" | "always" | ((env: ProxyRuntimeEnv) => boolean);
export type RequestProxyConfig = {
    targetUrl: string;
    when?: ProxyRuntimeGate;
};
export declare const TARGET_URL_HEADER = "x-fal-target-url";
export declare function withProxy(config: RequestProxyConfig): RequestMiddleware;