Loc Catalog

Library of Congress API Skill

Search and retrieve items from LOC's digitized collections — books, photos, maps, manuscripts, newspapers, audio, and more.

Critical: Things Claude Won't Know Without This Skill

This API covers digitized collections, NOT the full library catalog

The loc.gov JSON API searches LOC's digitized and online collections — not the full OPAC. By default, only digitized items are returned. Add all=true to include non-digitized catalog records.

JSON requires fo=json parameter, NOT a URL extension

Unlike LibraryCloud (.json extension), LOC uses a query parameter:

CORRECT: https://www.loc.gov/search/?q=hamlet&fo=json
WRONG:   https://www.loc.gov/search.json?q=hamlet

Facet filters use fa= with pipe separators

Single:   ?fa=subject:physics
Multiple: ?fa=subject:wildlife|location:yellowstone

Available facet fields: subject, contributor, location, language, original-format, online-format, partof, digitized, access-restricted.

Format-specific endpoints are shortcuts

/books/, /photos/, /maps/, /audio/, /newspapers/, /manuscripts/, /film-and-videos/, /notated-music/ all accept the same params as /search/.

Key Search Parameters

Item Lookup by LCCN/ID

https://www.loc.gov/item/{LCCN}/?fo=json

Returns full item detail including cite_this (APA/Chicago/MLA citations), resources (download links), other_formats (MARCXML, MODS, Dublin Core).

Response Structure

Search Results

{
  "results": [
    {
      "id": "http://www.loc.gov/item/95521789/",
      "title": "Item title",
      "date": "1796-01-01",
      "contributor": ["Name"],
      "subject": ["Subject term"],
      "language": ["English"],
      "original_format": ["book"],
      "description": ["..."],
      "image_url": ["//tile.loc.gov/..."],
      "digitized": true,
      "number_lccn": ["95521789"]
    }
  ],
  "pagination": {
    "of": 85604,
    "total": 17121,
    "perpage": 5,
    "current": 1
  },
  "facets": { ... }
}

Item Detail

The item object adds: call_number, created_published, rights, summary, stmt_of_responsibility, genre, and cite_this with pre-formatted citations.

Python Script

from scripts.loc_api import LocAPI
loc = LocAPI()

# Keyword search
results = loc.search("hamlet shakespeare", limit=10)

# Format-specific
photos = loc.search("civil war", format="photos", dates="1860/1865")

# Faceted search
results = loc.search("wildlife", facets={"subject": "birds", "location": "yellowstone"})

# Item lookup
item = loc.get_item("95521789")
citation = loc.get_citation(item, style="apa")

# Browse collections
collections = loc.list_collections()
items = loc.browse_collection("civil-war-maps", limit=20)

# Summarize
for r in results[:5]:
    print(loc.summarize(r))

Pagination

• Max 150 results per page (c=150)
• Hard limit: cannot paginate past 100,000th result
• Use facets/date ranges to narrow large result sets

API Etiquette

• No authentication required
• Rate limiting is enforced (exact limits unpublished) — add 0.5-1s between requests
• Include a User-Agent header

Related Skills

• harvard-library-catalog: Cross-reference Harvard holdings with LOC items
• hathitrust-catalog: Use LCCNs from LOC to check HathiTrust for digital full-text
• wikidata-search: LOC authority IDs (P244) link Wikidata entities to LOC records

Resources

• references/api_reference.md — Complete parameter and response field reference
• scripts/loc_api.py — Python client with search, item lookup, collection browsing, and citation extraction