name: Library of Congress API Rate Limits
description: The Library of Congress enforces rate limits per service to ensure equitable access. Exceeding limits triggers a 1-hour block for loc.gov JSON API endpoints. The Congress.gov API returns HTTP 429 when the hourly limit is reached.
rateLimits:
  - api: loc.gov JSON API
    endpoint: /search/, /collections/, /item/, /{format}/
    limit: 20 requests per minute
    window: 1 minute
    enforcement: 1-hour block on exceeding the limit
    notes: >
      Rate limits may decrease during periods of heavy traffic. Users may also
      encounter 429 status codes or CAPTCHA HTML pages. Implement exponential
      backoff and pause when a 429 or non-JSON response is received.
      The /storage-services/ media content endpoint has a separate limit.
  - api: loc.gov Text Services
    endpoint: /resource/ (text)
    limit: 150 requests per minute
    window: 1 minute
    enforcement: 1-hour block on exceeding the limit
  - api: loc.gov Image Services (IIIF)
    endpoint: /iiif/
    limit: 150 requests per minute
    window: 1 minute
    enforcement: 1-hour block on exceeding the limit
    notes: IIIF-compliant image API for accessing and manipulating collection images.
  - api: loc.gov Streaming Services
    endpoint: /av/ (audio/video)
    limit: 60 requests per minute
    window: 1 minute
    enforcement: 1-hour block on exceeding the limit
  - api: loc.gov Storage Services
    endpoint: /storage-services/
    limit: 150 requests per minute
    window: 1 minute
    enforcement: 1-hour block on exceeding the limit
  - api: Congress.gov API
    endpoint: /v3/*
    limit: 5000 requests per hour
    window: 1 hour
    enforcement: HTTP 429 Too Many Requests
    notes: >
      Default response page size is 20 results. Maximum page size is 250 results.
      Use offset parameter for pagination. API key required for all requests.
      Requests without a valid API key are rejected.
bestPractices:
  - Add handling in your code to pause and retry when a 429 or CAPTCHA response is received
  - Use the fo=json parameter explicitly on all loc.gov requests to ensure JSON responses
  - For large-scale harvesting of loc.gov, consider using the sitemap endpoints instead of the search API
  - For Congress.gov bulk data needs, consider downloading bulk data files from github.com/LibraryOfCongress/api.congress.gov rather than making many API calls
  - Implement exponential backoff with jitter when retrying after rate limit errors