Libofcongress Mcp Server

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Overview

Library of Congress digital collections, Chronicling America newspaper archives, and LC Subject Headings (LCSH) authority data. Search items and newspaper pages, retrieve full item metadata and OCR text, resolve LCSH subject terms, and browse curated collections from any MCP client. Runs as a stdio process, a local Streamable HTTP server, or the public hosted endpoint above.

Tools

Tool	Description
libofcongress_search_items	Search LOC digital collections by keyword with format, date range, subject, location, and collection filters.
libofcongress_get_item	Retrieve full metadata for a specific LOC digital item — imprint, contributors, subjects, rights, formats, its digital resources, and a paged list of its pages.
libofcongress_search_newspapers	Search historical newspaper pages in the Chronicling America corpus, with an excerpt of each page's opening OCR text.
libofcongress_get_newspaper_page	Retrieve the full text and metadata of one page — a newspaper page, or a book or manuscript page from libofcongress_get_item — or only the passages around a search term.
libofcongress_search_subjects	Search Library of Congress Subject Headings (LCSH) by keyword.
libofcongress_browse_collections	List and browse LOC curated digital collections, optionally filtered by keyword.

Resources

Resource	Description
libofcongress://item/{+item_id}	LOC digital item metadata by ID — stable URI for injecting item context into agent conversations.

All resource data is also reachable via libofcongress_get_item. Use libofcongress_search_items to discover item IDs first.

Capability reference

libofcongress_search_items _tool

• Filters: eight material formats (photo, map, newspaper, manuscript, audio, film, book, notated-music), inclusive year range (date_start/date_end), subject (an LCSH heading from libofcongress_search_subjects or a facet term, split at -- into lowercase terms with dates dropped and ANDed — United States--History--Civil War, 1861-1865 filters on united states + history + civil war), location, and collection_slug (from libofcongress_browse_collections, mutually exclusive with format); up to 100 results per page — a larger limit (up to 1000) is served as a 100-result page with effectiveLimit and a notice
• Each result's date is the date as LOC catalogs it (c1915 Feb. 19.), not the padded YYYY-01-01 sort key LOC uses for photos; where the catalog holds only a coded value (18950000, 185u, uuuu), the row's readable date (1895, 185?) stands in, or date is absent
• Each result carries is_item — true for catalog items whose id resolves via libofcongress_get_item, records keyed by an lccn.loc.gov permalink included (their id is the bare LCCN), false for collections, exhibit/guide pages, newspaper pages, catalog.loc.gov records, and hdl.loc.gov finding aids, whose url should be opened instead
• include_facets: true adds subject_facets and location_facets — LOC's top 10 subject and location terms across all matches, each with its item count, the values the subject and location filters accept. Off by default, since the facet block adds LOC response time to every search
• A zero-result notice names the subject terms and location it applied

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libofcongress_get_item _tool

• Takes a search result's id, simple or multi-segment (newspaper issues: sn95047246/1935-09-05/ed-1), or a loc.gov item link (https://www.loc.gov/item/2017760320/, http:// or without www., item/2017760320), read as its bare ID, or an LCCN permalink (https://lccn.loc.gov/2005567251, http:// or without a scheme), read as its LCCN in normalized form (lccn.loc.gov/2005-568294 as 2005568294, sn 83030214 as sn83030214); any other URL — a /resource/ page URL, another lccn.loc.gov path, another host — is rejected as invalid_item_id before any request. Returns the imprint (place, publisher, and date as cataloged: New York : Werner School Book Company, c1900.), contributors with their roles, LCSH subject_headings as cataloged (Transportation--Oklahoma--Maps), subject_terms (the lowercase facet terms the search subject filter accepts), notes, summary, languages, locations, rights, physical description, call number, former IDs, formats, and access_restricted
• date is the date as LOC catalogs it (1936 Apr., 1909-1911.), not the padded sort key LOC keeps for photos, and is absent when the record holds only a coded value (uuuu, ||||, 0w0 d1804 d1806)
• rights_information is plain text: where LOC sends its rights statement as HTML, the tags are dropped and each link's target is kept as an absolute URL (Copyright and other Restrictions (https://www.loc.gov/legal/)). Catalog text from www.loc.gov arrives with its HTML character references (’, …) decoded, on every tool; page text from libofcongress_get_newspaper_page is returned as the text service sends it, since that text is not HTML-escaped
• resources lists each digital resource with its page count, representative image, pdf, and audio (MP3); pages lists a window of the item's pages — each a page_url and has_text — 50 at a time by default (page_offset, page_limit, page_total, next_page_offset; at most 100 per call — a larger page_limit, up to 1000, is served as 100 with effectivePageLimit and a notice). A page with text reads through libofcongress_get_newspaper_page; a blank page can still show has_text: true (LOC lists a placeholder text file for it), and reads as empty text with a notice. include_files: true adds each listed page's download URLs (JPEG sizes, JP2, TIFF, text)
• related_items carries related record IDs or URLs in full; fields absent upstream are omitted rather than filled

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libofcongress_search_newspapers _tool

• Filters: keyword, inclusive date range down to the day (date_start/date_end take an integer year or a YYYY, YYYY-MM, or YYYY-MM-DD string), US state (full name or postal code, matched against the state LOC indexes each title under), and newspaper title (the exact newspaper_title of a search result); up to 100 results per page — a larger limit (up to 1000) is served as a 100-result page with effectiveLimit and a notice
• Each result carries the opening ~500 characters of the page's OCR text (in a fenced code block in the text output), states (every state LOC indexes the title under), and the url that libofcongress_get_newspaper_page takes verbatim
• The excerpt is the page's first lines, not the text around the match; pass the same query as match to libofcongress_get_newspaper_page for the passages around it

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libofcongress_get_newspaper_page _tool

• Takes the url from a libofcongress_search_newspapers result, or a pages[].page_url from libofcongress_get_item — newspaper, book, and manuscript pages alike. Also reads the same /resource/ URL over http:// or without www., a newspaper page path ending /ed-N/seq-M/, and a Chronicling America page link (chroniclingamerica.loc.gov/lccn/<lccn>/<date>/ed-N/seq-M/ or www.loc.gov/chroniclingamerica/lccn/…), each as its https://www.loc.gov/resource/…/?sp=M page; the output page_url is that URL, with only the sp query parameter kept. Rejected as invalid_page_url before any request: a Chronicling America link that names no single page (an issue link without seq-M, an /ocr/ view, a file link), any other host, a path with a dot segment or percent-escape, and a non-numeric sp
• Returns that page's own text (OCR, or a volunteer transcription for some manuscripts) as ocr_text — in the text output, a fenced code block holding the same text, so OCR noise never reads as Markdown — with the issue's newspaper_title, date, place_of_publication, states, edition, the page's sequence, and the page count segment_count; ocr_available: false marks an image-only page, and a notice says why a page with text came back empty — a text service that did not answer (retry), a body that could not be read as text (retrying will not help), or a blank page. LOC's blank-page placeholder ([NO TEXT AVAILABLE FOR THIS PAGE] as the whole text) is returned as empty ocr_text
• item_title is the title of the item the page belongs to, on every page type — the book or manuscript title (the text output's heading for those pages), or a newspaper issue's title with place and date beside the unchanged newspaper_title
• item_id names the item the page belongs to — the newspaper issue (sn83030214/1912-04-16/ed-1), book, or manuscript volume (mm77093930); pass it to libofcongress_get_item to list every page with its page_url and has_text, to follow an article onto a later page or read a book page by page. It comes from the page response LOC already sends, so it costs no extra request
• match returns the passages around a term instead of the whole page — pass the query you searched with. Space-separated words are separate terms and a double-quoted span is one phrase; each matches case-insensitively as literal text, with line breaks read as spaces. matches holds up to 10 passages, about 300 characters either side of each occurrence (overlapping ones merged up to 1,200 characters, never overlapping, in page order; when more would be needed, the rarest terms' occurrences are chosen first, so sinking of the titanic shows the Titanic rather than ten passages of "the"), match_count counts every occurrence, and ocr_text is empty; a notice says when nothing matched or some occurrences fall outside the passages, which the whole page (no match) holds. On a 50,241-character page of the 16 April 1912 New-York Tribune, match: "titanic" returns 2 passages of about 600 characters each — about 3 KB across both response surfaces instead of 104 KB

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libofcongress_search_subjects _tool

• Keyword query, up to 50 results per call (default 10; a larger limit, up to 1000, returns up to 50 with a notice); returns the authorized LCSH label and stable LOC uri, ranked by relevance with an exact match for the query first — pass the label verbatim as libofcongress_search_items's subject filter, since LCSH uses standardized wording ("World War, 1939-1945", "Aerial photography")
• A partial word with no keyword match (aerial photog) falls back to the headings that begin with it; when more headings match than the call returns, the response says how many

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libofcongress_browse_collections _tool

• Optional keyword filter on collection name and description; up to 100 collections per page — a larger limit (up to 1000) is served as a 100-collection page with effectiveLimit and a notice
• Each collection carries a slug for libofcongress_search_items's collection_slug (taken from the loc.gov route, not derivable from the title); item_count is approximate and omitted when LOC doesn't provide it

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libofcongress://item/{+item_id} _resource

• Returns the same full record as libofcongress_get_item, as application/json, with its default page window (the first 50 pages, no files); next_page_offset marks where libofcongress_get_item's page_offset continues
• item_id comes from a libofcongress_search_items result's id field, or from libofcongress_get_item; it is checked like the tool's item_id, so a URL or path that is not an item ID is rejected as invalid_item_id before any request
• Multi-segment newspaper IDs keep their slashes intact (e.g. libofcongress://item/sn95047246/1935-09-05/ed-1); percent-encoded slashes (%2F) also resolve

Features

Built on @cyanheads/mcp-ts-core: stdio and Streamable HTTP transports, pluggable auth (none / jwt / oauth), swappable storage (in-memory, filesystem, Supabase, Cloudflare KV/R2/D1), structured logging with optional OpenTelemetry tracing.

Library of Congress-specific:

• One request queue in front of www.loc.gov: concurrent calls start in arrival order, at least LOC_REQUEST_DELAY_MS apart (default 3100ms, ~19 req/min), and no 60-second window holds more than LOC's documented 20 starts, whatever the delay is set to. A call the queue cannot start within its time budget fails at once as server_busy, with the seconds to wait in retryAfter
• Rate-limit block enforcement: a 429 triggers LOC's 1-hour block, checked again as each queued request is dispatched, so calls already waiting fail without sending anything; the error's recovery hint counts down the minutes left and names the time it lifts
• Soft-block pause: when www.loc.gov answers with a Cloudflare challenge (cf-mitigated: challenge) or an HTML page instead of JSON — LOC's documented CAPTCHA response under heavy load — the call fails as upstream_unavailable with retryAfter: 60, and further www.loc.gov requests, queued ones included, are refused for 60 seconds without being sent. libofcongress_search_subjects (id.loc.gov) is unaffected
• Out-of-range page handling: a page past the end of the results (HTTP 404 on any page after the first) or past the retrieval ceiling (HTTP 400) returns an empty result with a notice, not an error — both point back to page 1 for the real page count, and the ceiling notice also explains how to partition a search that matches more than LOC will page through
• One 50-second budget per tool call covers queue wait, every attempt including its body read, retry backoff, and the newspaper page's OCR fetch, so a slow upstream fails as request_timed_out with a recovery hint before a client's 60-second request timeout. Network drops retry with backoff while budget remains; the 429 rate-limit path is never retried, since a retry would deepen LOC's 1-hour block
• An unreachable or misbehaving upstream — refused or dropped connections once retries run out, a 5xx or 403, or a 200 carrying an empty or truncated body or JSON that is not a response object — fails as upstream_unavailable with a retry hint, on every tool and the item resource. The message names the upstream and any HTTP status, never a URL, host, or transport error text
• Newspaper OCR is surfaced as-is: quality varies by digitization batch and era, and 19th-century or degraded pages may contain garbled text

Agent-friendly output:

• Empty results always include a notice field with recovery hints — echoes the applied filters and suggests how to broaden
• Pagination status on every search response (total, page, pages, has_next), capped at LOC's ~100,000-item retrieval ceiling, with a notice disclosing how to page past it
• ocr_available and is_item discriminator fields let callers branch on data availability without parsing text
• Recovery hints on every typed error contract — actionable next steps for the agent on every failure mode

Getting started

Public Hosted Instance

A public instance is available at https://libofcongress.caseyjhand.com/mcp — no installation required. Point any MCP client at it via Streamable HTTP:

{
  "mcpServers": {
    "libofcongress-mcp-server": {
      "type": "streamable-http",
      "url": "https://libofcongress.caseyjhand.com/mcp"
    }
  }
}

Self-Hosted / Local

Add the following to your MCP client configuration file.

{
  "mcpServers": {
    "libofcongress-mcp-server": {
      "type": "stdio",
      "command": "bunx",
      "args": ["@cyanheads/libofcongress-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_SESSION_MODE": "stateless",
        "MCP_LOG_LEVEL": "info"
      }
    }
  }
}

Or with npx (no Bun required):

{
  "mcpServers": {
    "libofcongress-mcp-server": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@cyanheads/libofcongress-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_SESSION_MODE": "stateless",
        "MCP_LOG_LEVEL": "info"
      }
    }
  }
}

Or with Docker:

{
  "mcpServers": {
    "libofcongress-mcp-server": {
      "type": "stdio",
      "command": "docker",
      "args": [
        "run", "-i", "--rm",
        "-e", "MCP_TRANSPORT_TYPE=stdio",
        "-e", "MCP_SESSION_MODE=stateless",
        "ghcr.io/cyanheads/libofcongress-mcp-server:latest"
      ]
    }
  }
}

For Streamable HTTP, set the transport and start the server:

MCP_TRANSPORT_TYPE=http MCP_HTTP_PORT=3010 bun run start:http
# Server listens at http://localhost:3010/mcp

Prerequisites

• Bun v1.4.0 or higher (or Node.js v24+).
• No API key required — the LOC JSON API and LC Linked Data endpoints are open. LOC recommends a descriptive LOC_USER_AGENT for polite access.

Installation

1. Clone the repository:

git clone https://github.com/cyanheads/libofcongress-mcp-server.git

2. Navigate into the directory:

cd libofcongress-mcp-server

3. Install dependencies:

bun install

4. Configure environment:

cp .env.example .env
# edit .env if you want to set LOC_USER_AGENT or LOC_REQUEST_DELAY_MS

Configuration

All configuration is validated at startup via Zod schemas in src/config/server-config.ts.

Variable	Description	Default
LOC_USER_AGENT	User-Agent header sent with LOC API requests. LOC recommends a descriptive value for polite access.	libofcongress-mcp-server/0.5.1
LOC_REQUEST_DELAY_MS	Delay in milliseconds between LOC API requests to stay under the 20 req/min rate limit.	3100
MCP_TRANSPORT_TYPE	Transport: stdio or http.	stdio
MCP_HTTP_PORT	Port for HTTP server.	3010
MCP_AUTH_MODE	Auth mode: none, jwt, or oauth.	none
MCP_SESSION_MODE	HTTP session mode. This server is explicitly stateless.	stateless
MCP_LOG_LEVEL	Log level (RFC 5424).	info
LOGS_DIR	Directory for log files (Node.js only).	<project-root>/logs
LOG_TOOL_FAILURE_PAYLOADS	Log each failed tool call's arguments and result, redacted by key name and capped at LOG_TOOL_FAILURE_PAYLOAD_MAX_BYTES (default 16384). A secret inside a free-form value is not redacted.	false
STORAGE_PROVIDER_TYPE	Storage backend.	in-memory
OTEL_ENABLED	Enable OpenTelemetry instrumentation (spans, metrics, completion logs).	false

See .env.example for the full list of optional overrides.

Running the server

Local development

• Build and run:

  # One-time build
  bun run rebuild
  
  # Run the built server
  bun run start:stdio
  # or
  bun run start:http
  

• Run checks and tests:

  bun run devcheck   # Lint, format, typecheck, security
  bun run test       # Vitest test suite
  bun run lint:mcp   # Validate MCP definitions against spec
  

Docker

docker build -t libofcongress-mcp-server .
docker run --rm -p 3010:3010 libofcongress-mcp-server

The Dockerfile defaults to HTTP transport, stateless session mode, and logs to /var/log/libofcongress-mcp-server. OpenTelemetry peer dependencies are installed by default — build with --build-arg OTEL_ENABLED=false to omit them.

Project structure

Directory	Purpose
src/index.ts	createApp() entry point — registers tools, resource, and initializes services.
src/config	Server-specific environment variable parsing (LOC_USER_AGENT, LOC_REQUEST_DELAY_MS).
src/mcp-server/tools	Tool definitions (*.tool.ts) — six LOC tools.
src/mcp-server/resources	Resource definitions — libofcongress://item/{+item_id}.
src/services/loc-api	LocApiService wrapping www.loc.gov — search, item fetch, page text, collection browser.
src/services/lc-linked-data	LcLinkedDataService wrapping id.loc.gov — LCSH subject heading suggest.
tests/	Unit and integration tests mirroring src/.

Development guide

See CLAUDE.md for development guidelines and architectural rules. The short version:

• Handlers throw, framework catches — no try/catch in tool logic
• Use ctx.log for request-scoped logging, ctx.state for tenant-scoped storage
• Register new tools and resources via the arrays in src/index.ts
• Wrap external API calls: validate raw → normalize to domain type → return output schema; never fabricate missing fields

Contributing

Issues are welcome. Run checks and tests before submitting:

bun run devcheck
bun run test

License

Apache-2.0 — see LICENSE for details.