/**
 * Pre-emptive usage warnings.
 *
 * Every successful metered call carries `x-misar-usage-*` headers describing
 * what the caller has left. Catching someone at 80% — while their task is still
 * working — converts far better than an error at 100%, and reads as helpful
 * rather than pushy.
 *
 * The last warning seen is stashed here and drained by the tool layer, so tools
 * do not each have to thread usage through their return path.
 */
/** Quota warning appended to a response when an allowance is nearly spent. */
export interface UsageWarning {
    feature: string;
    used: number;
    limit: number;
    remaining: number;
    ratio: number;
    upgradeUrl: string | null;
}
/**
 * Read the usage headers off a response; stash a warning if we're close.
 *
 * Never throws. This runs on the hot path of EVERY API call, so a response
 * without a standard `headers` object — a proxy, a test double, an older
 * runtime — must degrade to "no warning", never break the call itself.
 */
export declare function noteUsage(headers: Headers | null | undefined): void;
/** Take (and clear) the pending warning as a one-line footer, if any. */
export declare function takeUsageFooter(): string | null;
/** Append the pending warning to a tool's text result, when there is one. */
export declare function withUsageFooter(text: string): string;
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