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Tools

7 tools covering the full Census data workflow — from dataset discovery and variable search through geography resolution and ranked comparisons:

Tool	Description
census_list_datasets	Browse available Census Bureau datasets (ACS5, ACS1, Population Estimates, Decennial) 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. 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_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.
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.

census_list_datasets

Browse available Census Bureau datasets.

• 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), and Decennial DHC
• Accepts an optional keyword filter
• Dataset codes (e.g., acs/acs5) are the values to pass to other tools

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census_search_variables

Search Census variables by keyword.

• Full-text search across label and concept fields with relevance scoring (exact concept match > label match > partial)
• Returns estimate (E suffix) and margin-of-error (M suffix) codes together so both can be requested in one query
• 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_resolve_geography

Convert place names and addresses to Census FIPS identifiers.

• Named places (e.g., "King County, WA", "California") resolved via TIGERweb MapServer
• Street addresses resolved to tract level via Census Geocoder
• Auto-detects geography type from the name; accepts explicit geography_type override
• Detects ambiguous names (multiple states) and returns candidates with recovery hints
• Returns state_fips (→ parent_fips) and fips_summary (→ geography_fips) ready to pass to other tools

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census_query_data

Query a Census dataset for one or more variables at a specific geography.

• Requires FIPS codes — use census_resolve_geography first for place names
• Use geography_fips: "*" to return all geographies at the level within the parent
• Suppression codes (geography too small, data not collected, etc.) resolved to human-readable reasons
• Variable labels enriched from cache and surfaced alongside estimates
• Requires CENSUS_API_KEY

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census_compare_geographies

Rank and compare variables across multiple geographies.

• Fetches all geographies at a level (e.g., all WA counties) in one API call, then sorts and slices
• Optional within parameter to constrain to a parent FIPS; omit for national comparison
• Optional geographies list to filter to specific FIPS values
• Configurable sort variable, direction, and limit (default 50, max 500)
• Suppressed values sorted to end of results and labeled rather than passed through as negative sentinels
• Requires CENSUS_API_KEY

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Features

Built on @cyanheads/mcp-ts-core:

• Declarative tool definitions — single file per tool, framework handles registration and validation
• Unified error handling — handlers throw, framework catches, classifies, and formats with recovery hints
• Structured logging with optional OpenTelemetry tracing
• STDIO and Streamable HTTP transports

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

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 v1.3.0 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_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 and pull requests 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.