/** A value that survives `JSON.parse(JSON.stringify(v))` unchanged. */
export type JsonValue = string | number | boolean | null | JsonValue[] | {
    [key: string]: JsonValue;
};
/**
 * One file's parsed frontmatter.
 *
 * **Values are JSON-serializable, always.** Frontmatter crosses the sandbox↔host
 * postMessage boundary and is written to the `mdx-metadata.json` sidecar, so a
 * `Date`, a function, or a `undefined`-valued key either throws on the wire or
 * silently disappears through JSON. The YAML parser is what enforces this in
 * practice (it yields plain JSON values); the type states the contract so a producer
 * that builds frontmatter programmatically cannot claim otherwise.
 *
 * **Emitter identity semantics.** The metadata store hands out the SAME object
 * reference for a given file until that file's frontmatter changes, and emits a NEW
 * object when it does. Consumers may therefore use reference equality as a
 * change signal (React deps, memo keys) — but must treat the object as **frozen in
 * spirit**: mutating it edits every other consumer's copy, and the next emission
 * replaces it wholesale rather than merging.
 *
 * An **empty** frontmatter is not stored at all. Both the runtime scan and the CLI
 * emitter drop a file whose parsed frontmatter has zero keys, so `{}` never appears
 * as a value — absence and emptiness are the same state, and a consumer that
 * distinguishes them is relying on something neither side provides.
 */
export type Frontmatter = Record<string, JsonValue>;
/**
 * True when `value` is a plain object usable as a {@link Frontmatter} envelope:
 * a non-null, non-array object. Deliberately shallow — this checks the ENVELOPE,
 * not the values, because the values are open by design. Use
 * {@link isJsonSerializable} when a producer needs the deeper guarantee.
 */
export declare function isFrontmatterEnvelope(value: unknown): value is Frontmatter;
/**
 * True when `value` round-trips through JSON unchanged in shape — no functions, no
 * `undefined`, no `Date`, no cycles, no non-finite numbers.
 *
 * For a PRODUCER (the CLI emitter, an authoring tool) to check what it is about to
 * write. The consume side does not re-run this: it reads JSON, so anything it parsed
 * is serializable by construction.
 */
export declare function isJsonSerializable(value: unknown, seen?: Set<unknown>): boolean;