LLM Rate Limiting

Implement robust rate limiting to prevent quota exhaustion and handle API limits gracefully.

When to Use

• Hitting API rate limits
• Managing concurrent requests
• Preventing quota exhaustion
• Implementing fair usage policies
• Handling burst traffic

API Rate Limits (2026)

Anthropic Claude

| Tier | Requests/min | Tokens/min | Tokens/day | |------|-------------|------------|------------| | Free | 5 | 20K | 300K | | Tier 1 | 50 | 40K | 1M | | Tier 2 | 1000 | 80K | 2.5M | | Tier 3 | 2000 | 160K | 5M | | Tier 4 | 4000 | 400K | 10M |

OpenAI

| Tier | RPM | TPM | |------|-----|-----| | Free | 3 | 40K | | Tier 1 | 500 | 200K | | Tier 2 | 5000 | 450K | | Tier 3 | 5000 | 800K | | Tier 4 | 10000 | 2M |

Rate Limiter Implementation

Token Bucket Algorithm

class TokenBucket {
  private tokens: number;
  private lastRefill: number;

  constructor(
    private capacity: number,      // Max tokens
    private refillRate: number,    // Tokens per ms
  ) {
    this.tokens = capacity;
    this.lastRefill = Date.now();
  }

  private refill(): void {
    const now = Date.now();
    const elapsed = now - this.lastRefill;
    const newTokens = elapsed * this.refillRate;

    this.tokens = Math.min(this.capacity, this.tokens + newTokens);
    this.lastRefill = now;
  }

  async acquire(tokens: number = 1): Promise<boolean> {
    this.refill();

    if (this.tokens >= tokens) {
      this.tokens -= tokens;
      return true;
    }

    return false;
  }

  async waitForTokens(tokens: number = 1): Promise<void> {
    while (!(await this.acquire(tokens))) {
      const waitTime = (tokens - this.tokens) / this.refillRate;
      await sleep(Math.min(waitTime, 1000)); // Max 1 second wait
    }
  }
}

// Usage
const limiter = new TokenBucket(
  1000,  // 1000 tokens capacity
  1000 / 60000  // 1000 tokens per minute = ~16.67 per second
);

async function makeRequest() {
  await limiter.waitForTokens(1);
  return callAPI();
}

Sliding Window Rate Limiter

class SlidingWindowLimiter {
  private timestamps: number[] = [];

  constructor(
    private maxRequests: number,
    private windowMs: number
  ) {}

  canProceed(): boolean {
    const now = Date.now();
    const windowStart = now - this.windowMs;

    // Remove old timestamps
    this.timestamps = this.timestamps.filter(t => t > windowStart);

    return this.timestamps.length < this.maxRequests;
  }

  recordRequest(): void {
    this.timestamps.push(Date.now());
  }

  getWaitTime(): number {
    if (this.canProceed()) return 0;

    const oldestInWindow = this.timestamps[0];
    return oldestInWindow + this.windowMs - Date.now();
  }
}

// 50 requests per minute
const limiter = new SlidingWindowLimiter(50, 60000);

async function rateLimitedRequest() {
  while (!limiter.canProceed()) {
    const waitTime = limiter.getWaitTime();
    console.log(`Rate limited, waiting ${waitTime}ms`);
    await sleep(waitTime);
  }

  limiter.recordRequest();
  return makeAPICall();
}

Concurrent Request Limiter

class ConcurrencyLimiter {
  private running = 0;
  private queue: (() => void)[] = [];

  constructor(private maxConcurrent: number) {}

  async acquire(): Promise<void> {
    if (this.running < this.maxConcurrent) {
      this.running++;
      return;
    }

    // Wait in queue
    return new Promise(resolve => {
      this.queue.push(resolve);
    });
  }

  release(): void {
    this.running--;

    if (this.queue.length > 0) {
      const next = this.queue.shift()!;
      this.running++;
      next();
    }
  }

  async run<T>(fn: () => Promise<T>): Promise<T> {
    await this.acquire();
    try {
      return await fn();
    } finally {
      this.release();
    }
  }
}

// Max 10 concurrent requests
const concurrencyLimiter = new ConcurrencyLimiter(10);

async function processMany(items: string[]): Promise<string[]> {
  return Promise.all(
    items.map(item =>
      concurrencyLimiter.run(() => processItem(item))
    )
  );
}

Exponential Backoff

interface BackoffConfig {
  initialDelayMs: number;
  maxDelayMs: number;
  multiplier: number;
  jitterFactor: number; // 0-1
}

class ExponentialBackoff {
  private attempt = 0;

  constructor(private config: BackoffConfig) {}

  getDelay(): number {
    const baseDelay = this.config.initialDelayMs *
      Math.pow(this.config.multiplier, this.attempt);

    const cappedDelay = Math.min(baseDelay, this.config.maxDelayMs);

    // Add jitter
    const jitter = cappedDelay * this.config.jitterFactor * Math.random();

    return cappedDelay + jitter;
  }

  increment(): void {
    this.attempt++;
  }

  reset(): void {
    this.attempt = 0;
  }
}

async function withBackoff<T>(
  fn: () => Promise<T>,
  maxAttempts: number = 5
): Promise<T> {
  const backoff = new ExponentialBackoff({
    initialDelayMs: 1000,
    maxDelayMs: 60000,
    multiplier: 2,
    jitterFactor: 0.1
  });

  let lastError: Error;

  for (let attempt = 0; attempt < maxAttempts; attempt++) {
    try {
      const result = await fn();
      backoff.reset();
      return result;
    } catch (error) {
      lastError = error as Error;

      if (!isRetryable(error)) {
        throw error;
      }

      const delay = backoff.getDelay();
      console.log(`Attempt ${attempt + 1} failed, retrying in ${delay}ms`);
      await sleep(delay);
      backoff.increment();
    }
  }

  throw lastError!;
}

function isRetryable(error: any): boolean {
  // Rate limit errors
  if (error.status === 429) return true;

  // Server errors
  if (error.status >= 500) return true;

  // Network errors
  if (error.code === 'ECONNRESET' || error.code === 'ETIMEDOUT') return true;

  return false;
}

Handling Rate Limit Responses

Anthropic Rate Limit Headers

async function handleAnthropicResponse(response: Response): Promise<void> {
  // Check rate limit headers
  const requestsRemaining = response.headers.get('anthropic-ratelimit-requests-remaining');
  const tokensRemaining = response.headers.get('anthropic-ratelimit-tokens-remaining');
  const requestsReset = response.headers.get('anthropic-ratelimit-requests-reset');
  const tokensReset = response.headers.get('anthropic-ratelimit-tokens-reset');

  console.log(`Requests remaining: ${requestsRemaining}`);
  console.log(`Tokens remaining: ${tokensRemaining}`);

  // Proactively slow down if approaching limits
  if (parseInt(requestsRemaining || '999') < 10) {
    const resetTime = new Date(requestsReset!).getTime();
    const waitMs = Math.max(0, resetTime - Date.now());
    console.log(`Approaching rate limit, waiting ${waitMs}ms`);
    await sleep(waitMs);
  }
}

OpenAI Rate Limit Headers

interface OpenAIRateLimitInfo {
  limitRequests: number;
  limitTokens: number;
  remainingRequests: number;
  remainingTokens: number;
  resetRequests: string;
  resetTokens: string;
}

function parseOpenAIHeaders(headers: Headers): OpenAIRateLimitInfo {
  return {
    limitRequests: parseInt(headers.get('x-ratelimit-limit-requests') || '0'),
    limitTokens: parseInt(headers.get('x-ratelimit-limit-tokens') || '0'),
    remainingRequests: parseInt(headers.get('x-ratelimit-remaining-requests') || '0'),
    remainingTokens: parseInt(headers.get('x-ratelimit-remaining-tokens') || '0'),
    resetRequests: headers.get('x-ratelimit-reset-requests') || '',
    resetTokens: headers.get('x-ratelimit-reset-tokens') || ''
  };
}

Request Queue

interface QueuedRequest {
  id: string;
  execute: () => Promise<any>;
  resolve: (value: any) => void;
  reject: (error: Error) => void;
  priority: number;
  addedAt: number;
}

class RequestQueue {
  private queue: QueuedRequest[] = [];
  private processing = false;

  constructor(
    private rateLimiter: SlidingWindowLimiter,
    private concurrencyLimiter: ConcurrencyLimiter
  ) {}

  async enqueue<T>(
    execute: () => Promise<T>,
    priority: number = 0
  ): Promise<T> {
    return new Promise((resolve, reject) => {
      this.queue.push({
        id: crypto.randomUUID(),
        execute,
        resolve,
        reject,
        priority,
        addedAt: Date.now()
      });

      // Sort by priority (higher first), then by age
      this.queue.sort((a, b) =>
        b.priority - a.priority || a.addedAt - b.addedAt
      );

      this.process();
    });
  }

  private async process(): Promise<void> {
    if (this.processing) return;
    this.processing = true;

    while (this.queue.length > 0) {
      // Wait for rate limit
      while (!this.rateLimiter.canProceed()) {
        await sleep(100);
      }

      const request = this.queue.shift()!;
      this.rateLimiter.recordRequest();

      // Run with concurrency limit
      this.concurrencyLimiter.run(async () => {
        try {
          const result = await request.execute();
          request.resolve(result);
        } catch (error) {
          request.reject(error as Error);
        }
      });
    }

    this.processing = false;
  }
}

Monitoring

interface RateLimitMetrics {
  totalRequests: number;
  rateLimitedRequests: number;
  avgWaitTimeMs: number;
  peakConcurrency: number;
  errorRate: number;
}

class RateLimitMonitor {
  private metrics: RateLimitMetrics = {
    totalRequests: 0,
    rateLimitedRequests: 0,
    avgWaitTimeMs: 0,
    peakConcurrency: 0,
    errorRate: 0
  };

  recordRequest(wasLimited: boolean, waitTimeMs: number): void {
    this.metrics.totalRequests++;
    if (wasLimited) {
      this.metrics.rateLimitedRequests++;
    }

    // Update average wait time
    this.metrics.avgWaitTimeMs =
      (this.metrics.avgWaitTimeMs * (this.metrics.totalRequests - 1) + waitTimeMs) /
      this.metrics.totalRequests;
  }

  getMetrics(): RateLimitMetrics {
    return { ...this.metrics };
  }
}

Best Practices

1. Implement client-side limiting - Don't rely only on API errors
2. Use exponential backoff - With jitter to avoid thundering herd
3. Monitor remaining quota - Proactively slow down
4. Queue requests - Don't fire and forget
5. Set appropriate timeouts - Don't wait forever
6. Log rate limit events - For capacity planning
7. Have fallback strategies - What happens when limit is hit?