@cyanheads/census-mcp-server

Public Hosted Server: https://census.caseyjhand.com/mcp

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Overview

U.S. Census Bureau data — datasets, variables, and geography — via the Census Data API, TIGERweb, and the Census Geocoder. Discover datasets and variables, resolve place names or addresses to FIPS codes, and query or rank demographic, economic, and housing estimates across geographies from any MCP client. Runs as a stdio process, a local Streamable HTTP server, or the public hosted endpoint above.

Tools

Tool	Description
census_list_datasets	Browse available Census Bureau datasets (ACS5, ACS1, Population Estimates, Decennial, County Business Patterns, Economic Census, Nonemployer Statistics) with vintage years and dataset codes.
census_list_geographies	List the geography levels supported by a dataset and year, with parent requirements and example FIPS values.
census_search_variables	Keyword search across variable labels and concept groups. On ACS, returns estimate and margin-of-error codes together.
census_get_variable	Fetch full metadata for one or more variable codes — label, concept, predicate type, universe, MOE sibling.
census_list_predicate_values	List the codes a filter dimension accepts (EMPSZES, LFO, POPGROUP, NAICS2017…), from the dataset dictionary or a live wildcard enumeration.
census_resolve_geography	Convert place names (e.g., "King County, WA") or street addresses to Census FIPS identifiers via TIGERweb and Census Geocoder.
census_query_data	Query a Census dataset for variables at a specific geography. Returns estimates with MOE, suppression codes resolved to readable reasons, and predicate filtering for the business datasets.
census_compare_geographies	Rank and compare variables across multiple geographies — all counties in a state, all states nationally, or a named set. Sorted table output, with the same predicate filtering.

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Capability reference

census_list_datasets tool

• Returns dataset codes, names, descriptions, and available vintage years
• Covers ACS5, ACS5 Data Profiles, ACS5 Subject Tables, ACS1, ACS1 Data Profiles, Population Estimates, Decennial Redistricting (P.L. 94-171), Decennial DHC, County Business Patterns (cbp), Economic Census (ecnbasic), and Nonemployer Statistics (nonemp)
• Each description names the filter predicates the dataset requires and the geography levels it publishes — both vary by dataset
• Accepts an optional keyword filter
• Dataset codes (e.g., acs/acs5) are the values to pass to other tools
• available_years is exhaustive, not a sample: any other year fails with year_not_available before a request goes out, naming the years that do work. It is narrower than what the Census API hosts — pep/charv reaches its 2020-2022 estimates through the YEAR filter inside the 2023 vintage, and the cbp/nonemp vintages left out reject the NAME column every query here sends

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census_list_geographies tool

• Returns one row per geography level — geography_level, whether a parent is required, required_parent_levels, and an example FIPS value
• geography_level values are the exact inputs to geography_level in census_query_data and census_compare_geographies
• year defaults to the dataset's latest available vintage
• dataset_not_found when the dataset code is unrecognized; year_not_available when the dataset has no geography data for the requested year

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census_search_variables tool

• Full-text search across label and concept fields with relevance scoring (exact concept match > label match > partial)
• On ACS datasets, returns estimate (E suffix) and margin-of-error (M suffix) codes together so both can be requested in one query — no other family publishes margins of error, and an E-final code there is an ordinary code
• Also surfaces the predicate codes a dataset filters on, such as NAICS2017 in cbp
• Configurable limit (default 20, max 100); total_matches indicates how many matched before the limit
• Cache-backed: variables.json is fetched once per dataset+year with a configurable TTL (default 24h)

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census_get_variable tool

• Accepts one or more variable codes (case-sensitive) and returns metadata in the same order — label, concept, predicate type, and universe when the dataset publishes one
• On ACS datasets, returns estimate_code/moe_code sibling references; other families publish no margins of error and carry neither field
• Also resolves predicate/filter dimension codes (e.g., NAICS2017, SEX) to confirm a dimension exists in a dataset — census_list_predicate_values lists the values it accepts
• dataset defaults to acs/acs5, year defaults to the dataset's latest available vintage
• variable_not_found when a code isn't defined in the dataset and year

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census_list_predicate_values tool

• Two routes, picked by where the answer lives: a dimension with a published value list is read from the dataset dictionary, one without is enumerated live by wildcarding it on the data endpoint. NAICS* and POPGROUP always publish one (thousands of codes — narrow them with query); on the current vintages EMPSZES, LFO, RCPSZES, TAXSTAT, and TYPOP publish none, so the live route is the only place their codes appear
• A dictionary value list is a classification shared across Census products, not a record of what one dataset serves — dec/ddhca declares 5,543 POPGROUP codes and publishes 2,996, cbp declares 6,694 NAICS2017 codes and publishes 2,003. The declared list is checked against the dataset's own published rows and the dead codes are dropped; source says whether that check ran and the notice says how many were withheld
• Keyword query matches code and label; results are sorted by code and a truncated list is disclosed rather than passed off as complete (default limit 50, max 500)
• ecnbasic publishes TAXSTAT and TYPOP per industry, so within_naics scopes the enumeration — and the notice says the result is complete for that industry alone
• Live enumerations are cached per dataset, year, dimension, industry scope, and probe measure

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census_resolve_geography tool

• Named places (e.g., "King County, WA") resolve via TIGERweb; street addresses resolve to tract level via Census Geocoder
• Auto-detects geography_type for state, county, place, and tract; metropolitan/micropolitan statistical areas, combined statistical areas, and consolidated cities are never auto-detected and need an explicit geography_type, since their names overlap city names
• Optional county_fips scopes resolution to the county and tract levels only — required when a tract name matches more than one county; county_scope_unsupported when paired with any other level or a street address
• Prefers an exactly-named match over a partial one (e.g., "Kansas City, MO" does not resolve to North Kansas City)
• A name matching more than one geography returns ambiguous_name, with every candidate's FIPS code and the state that separates them
• Returns state_fips (→ parent_fips) and fips_summary (→ geography_fips) ready to pass to other tools; a statistical area omits state_fips since it can span several states

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census_query_data tool

• Requires FIPS codes (use census_resolve_geography for place names) and up to 50 variable codes per call; geography_fips: "*" returns every geography at the level within the parent, and each row carries both geography_fips and the nationally-unique geography_geoid
• Level and parent are checked against the dataset's own geography metadata before querying — parent_required and parent_not_accepted name what's missing or unaccepted rather than surfacing a raw Census 400
• Optional predicates map filters the business/pep/dec datasets (e.g., {"NAICS2017": "5112"}); a dimension left unset applies a Census-chosen default — an all-categories total on some datasets, a single category on others — echoed per row in applied_filters
• A dataset that publishes more than one record per geography (pep/charv) returns multiple rows, each carrying a record field; pin one with predicates (e.g., {"MONTH": "7"})
• Suppression codes resolve to human-readable reasons; a null estimate means the value is either suppressed, a text cell (returned under value), or genuinely empty
• Requires CENSUS_API_KEY

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census_compare_geographies tool

• Ranks all geographies at a level, or a named geographies list of GEOIDs/bare level codes, in one call; within/within_county scope to a state/county, omit for a national comparison
• Configurable sort_by variable, sort_dir (default desc), and limit (default 50, max 500); total_count reports how many geographies matched before the limit
• Same predicates map, geography validation, and applied_filters default-echoing as census_query_data, applied to every geography in the ranking
• A dataset that publishes more than one record per geography (pep/charv) fails with ambiguous_rows unless predicates pins one (e.g., {"MONTH": "7"})
• Suppressed values are labeled and sorted to the end rather than passed through as raw sentinels; a text value has no ordering, so sorting on it leaves rows tied
• Requires CENSUS_API_KEY

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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) surfaced as human-readable reasons instead of raw numbers
• Recovery hints on errors — ambiguous geography names include candidate lists; missing API key errors include registration URL

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Getting started

Public Hosted Instance

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

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

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

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Configuration

Variable	Description	Default
CENSUS_API_KEY	Required for data queries. Register free at api.census.gov/data/key_signup.html.	—
CENSUS_DEFAULT_YEAR	Default vintage year when no year is specified.	2024
CENSUS_VARIABLE_CACHE_TTL_HOURS	Hours to cache variables.json per dataset+year in memory.	24
MCP_TRANSPORT_TYPE	Transport: stdio or http.	stdio
MCP_SESSION_MODE	HTTP session mode: stateful, stateless, or auto. The server declares stateless in src/index.ts; set this only to override it.	stateless
MCP_HTTP_PORT	Port for HTTP server.	3010
MCP_AUTH_MODE	Auth mode: none, jwt, or oauth.	none
MCP_LOG_LEVEL	Log level (debug, info, notice, warning, error).	info
OTEL_ENABLED	Enable OpenTelemetry instrumentation.	false

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

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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.

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Project structure

Path	Purpose
src/index.ts	createApp() entry point — registers tools and initializes services.
src/config/server-config.ts	Census-specific env var parsing and validation with Zod.
src/mcp-server/tools/definitions/	Tool definitions (*.tool.ts).
src/services/census-api/	Census Data API client — data queries, suppression code mapping, retry logic.
src/services/geography/	Geography resolution — TIGERweb named-place lookup and Census Geocoder address-to-tract.
src/services/variable-cache/	In-process variables.json cache with TTL and keyword search.
tests/	Vitest tests mirroring src/ structure.

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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

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Contributing

Issues are welcome. Run checks and tests before submitting:

bun run devcheck
bun run test

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License

Apache-2.0 — see LICENSE for details.