/**
 * Per-IP / per-token rate limiter for the public MCP HTTP endpoint.
 *
 * Backend: Upstash Redis REST API. We use raw fetch (no SDK dep) because:
 *   - It runs cleanly on Vercel serverless without bundler tweaks.
 *   - Upstash's REST surface is small and stable.
 *
 * Algorithm: fixed window. INCR a counter keyed by `prefix:bucket:windowStart`
 * and set EXPIRE on first increment. Cheap, accurate within ±1 window, and
 * Upstash bills per command — INCR is a single command.
 *
 * Fail-open policy: if UPSTASH_REDIS_REST_URL or UPSTASH_REDIS_REST_TOKEN is
 * unset (local dev, tests, preview deploys without secrets), the limiter
 * returns { ok: true } without contacting any backend. This is intentional:
 * a misconfigured limiter must NOT block production traffic, only relax it.
 *
 * Configuration (all env, all optional):
 *   UPSTASH_REDIS_REST_URL    e.g. https://us1-foo-12345.upstash.io
 *   UPSTASH_REDIS_REST_TOKEN  bearer token
 *   RATE_LIMIT_ANON_PER_MIN   default 60
 *   RATE_LIMIT_BEARER_PER_MIN default 200
 *   RATE_LIMIT_STRICT_PER_MIN default 5    (credential endpoints)
 *
 * Vercel's "Upstash for Redis" Marketplace integration with a custom prefix
 * injects env vars in the form UPSTASH_REDIS_KV_REST_API_URL /
 * UPSTASH_REDIS_KV_REST_API_TOKEN. Those names are also accepted as a
 * fallback so the limiter works without manual env-var aliasing.
 */
export type RateKind = "anon" | "bearer" | "strict";
export interface RateLimitResult {
    ok: boolean;
    /** When ok=false, suggested seconds until the caller can retry. */
    retryAfterSec?: number;
    /** Configured limit for this kind. Undefined when limiter is disabled. */
    limit?: number;
    /** Remaining quota in the current window. Undefined when disabled. */
    remaining?: number;
}
export interface RateLimitDeps {
    /** Defaults to globalThis.fetch — injectable for tests. */
    fetch?: typeof fetch;
    /** Defaults to Date.now — injectable for tests. */
    now?: () => number;
    /** Defaults to process.env. */
    env?: NodeJS.ProcessEnv;
}
/**
 * Check (and increment) the rate limit for `kind`+`identifier`.
 * Returns { ok: true } if the request is allowed, { ok: false, retryAfterSec }
 * if it should be rejected with HTTP 429.
 */
export declare function checkRateLimit(kind: RateKind, identifier: string, deps?: RateLimitDeps): Promise<RateLimitResult>;
/**
 * Stable, low-PII identifier for an anonymous caller. Falls back to "unknown"
 * when no IP-bearing header is present (e.g. local stdin tests).
 *
 * Vercel sets x-forwarded-for; the first comma-separated value is the client.
 */
export declare function anonIdentifier(headers: Record<string, string | string[] | undefined>): string;
/**
 * Identifier derived from a Bearer token. We hash the token (FNV-1a 32-bit) so
 * the rate-limit key never contains the secret — Upstash logs and our own
 * structured logs would otherwise leak it.
 *
 * FNV-1a is non-cryptographic but is sufficient as a key-bucketing function:
 * an attacker cannot use a hash collision to bypass the limiter (collisions
 * land in the same bucket, increasing the count).
 */
export declare function bearerIdentifier(authorization: string | undefined): string;