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Zip-Codes.com Location Intelligence: Spatial Analysis, Districts & 14+ Years of Demographics

by com.zip-codes
OverviewDiscussionsAdmin

County reference

county_reference
Read-onlyIdempotent

US counties and county equivalents — all 3,235 of them — with their Census and USPS names, which disagree for over a hundred, plus ANSI code, legal type, land and water area, and USPS delivery footprint. Filter by state to get every county in that state: without a filter only the first 25 are returned, and they are FIPS-ordered, so an unfiltered call answers with Alabama rather than with whatever was asked about. Costs nothing and needs no key.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "county_reference"
  }
}

Input Schema

NameRequiredDescriptionDefault
limitNoCap the number of counties returned. Defaults to 25 when no state is given, or to all of them when one is.
stateNoTwo-letter US state or territory code. Returns EVERY county in that state, so no limit is needed — Texas is the largest at 254. Omit to browse all states.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.4/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds non-obvious behavior beyond that: the 25-row default without a filter, FIPS ordering with its practical consequence, and the fact that the tool is free and needs no key.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three dense sentences, front-loaded with what the dataset is, then the critical filtering caveat, then cost. Every clause carries information, though the parenthetical 'all 3,235 of them' and field litany make it slightly heavier than necessary.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

There is no output schema, but the description itself enumerates the returned fields and explains the row-count behavior, so an agent knows both how to call it and what comes back. For a two-optional-parameter reference tool, nothing material is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so both parameters are already documented, and the description still adds meaning: it explains why the default limit is 25, why no limit is needed when a state is supplied, and scales expectations (Texas at 254). It stops short of describing output field formats, which keeps it at 4.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete resource (US counties and county equivalents, all 3,235) and enumerates the fields returned: Census/USPS names, ANSI code, legal type, land/water area, and delivery footprint. The resource is inherently distinct from the zip-, city-, and metro-focused siblings, so an agent can route here without ambiguity.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives explicit when-to guidance ('Filter by state to get every county in that state') and a strong when-not warning that an unfiltered call returns only the first 25 in FIPS order and therefore answers with Alabama. It does not name a sibling alternative for cross-cutting queries, which keeps it from a 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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