export function classifyProviderFallbackFailure(
  stopReason: unknown,
  errorMessage: unknown,
): string | null {
  if (stopReason !== "error" || typeof errorMessage !== "string") return null;
  const diagnostic = errorMessage.trim();
  if (diagnostic.length === 0) return null;
  const normalized = diagnostic.toLowerCase();
  const category = /auth|credential|api[ _-]?key|unauthori[sz]ed|forbidden/.test(normalized)
    ? "provider-auth"
    : /quota|rate[ _-]?limit|resource exhausted|billing limit/.test(normalized)
      ? "provider-quota"
      : /capacity|overloaded|temporarily unavailable|no available.*server/.test(normalized)
        ? "provider-capacity"
        : null;
  return category ? `${category}: ${diagnostic}` : null;
}