Overview

US Census Bureau demographic, economic, and housing data. Find datasets and variables, resolve place names or addresses to FIPS codes, and query or rank estimates across any set of geographies.

Tools

Capability reference

census_list_datasets _tool

• Returns dataset codes, names, descriptions, and available vintage years, with an optional keyword filter. Covers ACS5 and ACS1 (data profiles, subject tables, comparison profiles, and acs/acs1/spp), acs/acsse, Population Estimates, the 2020 Decennial files (Redistricting (P.L. 94-171), dec/dhc, dec/dp, dec/sdhc, dec/ddhca, dec/ddhcb), cbp, ecnbasic, and nonemp
• Each description names the filter predicates the dataset requires and the geography levels it publishes
• Dataset codes (e.g., acs/acs5) are what the other tools take. Case is ignored and a two-part code works by its last part alone (acs5, pl); three-part codes such as acs/acs5/profile must be given in full
• available_years is exhaustive: any other year fails with year_not_available, naming the years that work

census_list_geographies _tool

• Returns one row per geography level and parent hierarchy: geography_level, whether a parent is required, required_parent_levels, and an example FIPS value. totalLevels counts each level once
• geography_level values are the exact inputs to census_query_data and census_compare_geographies; year defaults to the latest vintage
• Fails with dataset_not_found or year_not_available (naming the years served)

census_search_variables _tool

• Whole-word search across label and concept: every query word must match, otherwise the closest matches come back with a notice. A variable or table code in any case returns that variable, or the whole table in code order, first (B19013 returns B19013_001E then B19013_001M)
• A code the dataset does not publish gets a notice naming the sibling dataset that holds it (S1701 in acs/acs5 points to acs/acs5/subject) or census_get_variable for an attribute column such as B19013_001EA
• On ACS datasets, estimate (E suffix) and margin-of-error (M suffix) codes come back together; comparison profiles and the other families publish no margins of error. Also finds predicate codes by keyword (naics) or code (NAICS2017 in cbp)
• limit is an integer from 1 to 100 (default 20), rejected rather than clamped when out of range; totalMatches counts matches before the limit. variables.json is cached per dataset+year (default TTL 24h)

census_get_variable _tool

• Takes one or more variable codes (case-insensitive) and returns label, concept, predicate type, and the table's universe, in the order given. If groups.json can't be fetched, variables come back without universe and a notice says to retry
• Resolves annotation and flag columns such as B19013_001EA and EMP_F, with attribute_of naming the column each belongs to and attribute_type its kind (ANNOTATION, FLAG, MARGIN_OF_ERROR). On ACS datasets it also returns estimate_code/moe_code sibling references
• Confirms predicate dimension codes (NAICS2017, SEX) exist in a dataset; census_list_predicate_values lists their values. dataset defaults to acs/acs5 and year to the latest vintage
• variable_not_found when a code isn't defined, naming the sibling dataset that publishes it when one is found (DP03_0062E in acs/acs5 points to acs/acs5/profile)

census_list_predicate_values _tool

• Lists the codes a filter dimension accepts, read from the dataset dictionary when it publishes a list and enumerated live otherwise. NAICS* and POPGROUP have long lists (thousands of codes; narrow with query); EMPSZES, LFO, RCPSZES, TAXSTAT, and TYPOP are live-only
• Dictionary lists are checked against the rows the dataset actually publishes and unpublished codes are dropped; source says whether the check ran and the notice gives the number withheld
• query matches code and label; results are sorted by code, and truncation is disclosed (limit an integer from 1 to 500, default 50; totalCount counts matches). On ecnbasic, within_naics scopes TAXSTAT and TYPOP to one industry

census_resolve_geography _tool

• Named places (e.g., "King County, WA") resolve via TIGERweb; street addresses resolve to tract level via Census Geocoder, with block_group_fips and place_fips. A 5-digit ZIP (or ZIP+4) resolves to its ZIP Code Tabulation Area (zip code tabulation area)
• Auto-detects geography_type for state, county, place, tract, and ZIP. Metropolitan/micropolitan statistical areas, combined statistical areas, consolidated cities, and economic places need an explicit geography_type; economic place returns the 8-digit ecnbasic code (Seattle 03363000)
• The place level includes census-designated places, which answer a name only when no incorporated place or county matches exactly; use the full name ("Arlington CDP, VA") or geography_type: "place" to reach one. Optional county_fips scopes county and tract resolution (county_scope_unsupported for any other level or an address)
• Matching ignores case, accepts a state abbreviation or full name, and tolerates Saint/St. and accent differences. New York City boroughs resolve to their counties ("Brooklyn, NY" → Kings County 047) with a notice
• A name matching several geographies returns ambiguous_name with every candidate's FIPS code and state. Results carry state_fips (→ parent_fips) and fips_summary (→ geography_fips) for the next tool; statistical areas and ZCTAs omit state_fips

census_query_data _tool

• Requires FIPS codes (use census_resolve_geography for place names). geography_fips: "*" returns every geography at the level within the parent; each row carries geography_fips and the nationally unique geography_geoid. Wildcards return up to limit rows (default 50, max 500) in GEOID order, with offset for paging and totalCount and truncated for the rest
• Up to 49 variable codes per call, fewer on datasets that add label or record columns (too_many_variables states the maximum). Unknown codes fail with variable_not_found; level and parent problems fail with parent_required or parent_not_accepted. Optional tract_fips (6 digits, with a concrete county_fips) scopes a block-group or decennial block query to one tract
• Optional predicates map filters the business, pep, and dec datasets and acs/acs1/spp (e.g., {"NAICS2017": "5112"}). Unset dimensions take a Census default, echoed per row in applied_filters; "*" returns one row per category, labelled in record. pep/charv returns several rows per geography, so pin one (e.g., {"MONTH": "7"})
• ACS sentinel values resolve to their published meanings, and a median in an open-ended interval is flagged open_ended. On cbp, ecnbasic, and nonemp, withheld values are reported as suppressed with the flag's meaning. A null estimate means suppressed, a text cell (returned under value), or empty
• On dec/ddhca and dec/ddhcb, a geography's POPGROUP group sits in only one table of a family; a no_data hint or notice names the sibling tables. Requires CENSUS_API_KEY

census_compare_geographies _tool

• Ranks all geographies at a level, or a named geographies list of GEOIDs/bare level codes; within/within_county scope to a state/county, omit for a national comparison
• Ranks on one variable: sort_by (default the first code; must be a requested code, else sort_by_not_requested), sort_dir (default desc), and limit (1 to 500, default 50); totalCount counts geographies before the limit. A count ranks by size, not rate, so rank a published percentage for a rate (S1701_C03_001E in acs/acs5/subject, DP04_0047PE in acs/acs5/profile)
• Takes the same predicates, variable limit, validation, and applied_filters as census_query_data; multi-record datasets (pep/charv) or a "*" predicate fail with ambiguous_rows unless predicates pins one. Suppressed values sort last with the same reasons, and text values can't be ranked (the notice says so). Requires CENSUS_API_KEY

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.

Census-specific:

• In-process variable cache with configurable TTL — variables.json fetched once per dataset+year, searched client-side
• Three-API backend: Census Data API for data queries, TIGERweb for named-place resolution, Census Geocoder for address-to-tract
• Automatic retry with backoff on all external API calls
• FIPS formatting helpers — zero-padded state, county, and tract codes ready to pass between tools

Agent-friendly output:

• Workflow-oriented tool surface — fips_summary and state_fips return values are ready to pass as geography_fips and parent_fips to the next tool
• Suppression codes decoded — Census negative sentinel values (e.g., -666666666) and business-dataset withholding flags (e.g., D) surfaced as their published meanings instead of raw numbers or false zeros
• Recovery hints on errors — ambiguous geography names include candidate lists; missing API key errors include registration URL

Getting started

Public Hosted Instance

A public instance is available at https://census.caseyjhand.com/mcp — no installation required. It serves Streamable HTTP; each client takes its own configuration shape.

For Claude Code:

claude mcp add --transport http census-mcp-server https://census.caseyjhand.com/mcp

For Cline (cline_mcp_settings.json):

{
  "mcpServers": {
    "census-mcp-server": {
      "type": "streamableHttp",
      "url": "https://census.caseyjhand.com/mcp"
    }
  }
}

For Cursor (mcp.json):

{
  "mcpServers": {
    "census-mcp-server": {
      "url": "https://census.caseyjhand.com/mcp"
    }
  }
}

Claude Desktop adds remote servers under Settings → Connectors.

Self-Hosted / Local

API key: Register a free key at api.census.gov/data/key_signup.html. Variable search and geography resolution work without a key; data queries (census_query_data, census_compare_geographies) require one.

Add the following to your MCP client configuration file:

{
  "mcpServers": {
    "census-mcp-server": {
      "type": "stdio",
      "command": "bunx",
      "args": ["@cyanheads/census-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "CENSUS_API_KEY": "your-census-api-key"
      }
    }
  }
}

Or with npx (no Bun required):

{
  "mcpServers": {
    "census-mcp-server": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@cyanheads/census-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "CENSUS_API_KEY": "your-census-api-key"
      }
    }
  }
}

Or with Docker:

{
  "mcpServers": {
    "census-mcp-server": {
      "type": "stdio",
      "command": "docker",
      "args": [
        "run", "-i", "--rm",
        "-e", "MCP_TRANSPORT_TYPE=stdio",
        "-e", "CENSUS_API_KEY=your-census-api-key",
        "ghcr.io/cyanheads/census-mcp-server:latest"
      ]
    }
  }
}

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

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

Prerequisites

• Bun v1.4.0 or higher (or Node.js v24+).
• A Census API key — register free at api.census.gov/data/key_signup.html. Required for census_query_data and census_compare_geographies; other tools work without it.

Installation

1. Clone the repository:

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

2. Navigate into the directory:

cd census-mcp-server

3. Install dependencies:

bun install

4. Configure environment:

cp .env.example .env
# edit .env and set CENSUS_API_KEY

Configuration

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

Running the server

Local development

# 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 audit
bun run test       # Vitest test suite
bun run lint:mcp   # Validate MCP definitions against spec

Docker

docker build -t census-mcp-server .
docker run --rm -e CENSUS_API_KEY=your-key -p 3010:3010 census-mcp-server

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

Project structure

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 via the barrel in src/mcp-server/tools/definitions/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.