/**
 * L3 — real JSON Schema validation of tool arguments (Phase B).
 *
 * `Tool.parameters` was always declared to be a JSON Schema the kernel
 * validates; until Phase B it was only advertised, never enforced. ajv is
 * the core's single runtime dependency (ADR-0023): the alternative —
 * hand-rolling a draft-07 subset — is how schemas silently stop meaning
 * what they say. Validators are compiled once per schema and cached.
 */
/** Returns a human-readable failure reason, or null when the input is valid. */
export declare function validateArgs(schema: Readonly<Record<string, unknown>>, input: unknown): string | null;