/**
 * Deep-transform an object's own keys to snake_case. Values are left untouched
 * unless they are plain objects/arrays (recursed). Used by `createApi` when the
 * consumer opts into `requestCasing: 'snake'` so request bodies match a backend
 * expecting snake_case keys.
 *
 * Non-plain objects (URLSearchParams, FormData, Blob, Date, …) pass through.
 */
export declare function toSnakeCaseKeys<T>(value: T): T;
/** Convert a single camelCase / PascalCase key to snake_case. */
export declare function toSnake(key: string): string;
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